• Dedicated to the memory of Francisco (Paco) Ynduráin, a good friend and excellent physicist (1940–2008).
The LHC is gearing up to do real physics, and after all the astrophysical nonsense about the Big Bang and black holes we now face the cold reality of experiment. In this context, it may be useful to summarize our knowledge of the Higgs system to date, which is the purpose of this article.
First, let me make a clear statement. Our present knowledge of the Standard Model is of course way beyond the knowledge of, say, 1959, when the PS at CERN started up. Clearly there have been many unanticipated discoveries, not to mention theoretical evolution. The Standard Model is chock full of facts crying out for explanation, such as the existence of three generations of quark–lepton families, or the many unexplained parameters of the model, such as the particle masses. That latter problem has, in today’s Standard Model, shifted to the many different particle–Higgs couplings, and is still not totally understood. Consider this: between the neutrino masses (10–3 eV) and the top quark mass (1.75 × 1011 eV) there is a difference of 14 unexplained orders of magnitude. Why? How? We can say nothing meaningful about these things and we have no idea if the LHC will illuminate the problem; at the very least, realizing all this, we should not have the arrogance to think that we know what is going to happen.
That being said, let’s see what we know. Our knowledge of the Higgs sector derives from the measurement of radiative corrections (plus the lower limit on the Higgs mass from direct experimentation at LEP), and the only quantities that depend on the details of the Higgs sector are radiative corrections to the masses of the vector bosons, including the photon. The masslessness of the photon is not automatic within the Standard Model, which provides a serious constraint.
Thus the measured radiative corrections are those affecting the W and Z masses, which come about – theoretically – through self-energy diagrams such as illustrated in figure 1. We really do not know what the X-line in the figure represents. It could be the propagator of one or more particles of distinct mass, or even some smeared-out mass (if the Higgs is heavy and strongly interacting), or of some continuous distribution. These various possibilities have been scrutinized for quite some time, but no definite view has emerged. Even so, it is useful to take a specific model, namely the simplest Higgs sector with one physical Higgs.
In the first instance, in a renormalizable theory, masses are free parameters – to be renormalized and taken from experiment – and therefore radiative corrections are invisible. Nonetheless there are two facts that allow us to come to some conclusions.
The first fact is that the photon mass is zero. Such a mass is not subject to renormalization and we may thus ask under what circumstances the Higgs sector produces a photon mass of zero. It happens that the simplest Higgs model (with just one physical Higgs) produces a massless photon, while adding degrees of freedom to the Higgs sector in general destroys this prediction. To get a zero photon mass in more complicated Higgs systems requires tuning of parameters, in other words, the prediction of zero photon mass is lost. This, in my opinion, is a strong argument against more complicated Higgs systems. To abandon a prediction that agrees with experiment is not something one should do lightly. However, this is not without some nasty consequences.
Here I must mention the strong CP violation contained in the strong interactions in QCD. This effect, which indeed is not observed, is commonly explained away (in the Peccei–Quinn approach) by using two Higgs systems. While it is easy within such a model to tune the photon mass to zero, it is nonetheless a fact that the prediction of zero mass is lost. On top of that, in these models there is normally a particle of very small mass (the axion) of which there is no evidence experimentally. This is a worrying problem, for which there is no generally accepted solution, although there are some attempts at resolving it.
In addition, there is supersymmetry. In supersymmetry one inevitably has more than one Higgs system, so a priori that ruins the prediction of zero photon mass of the simplest Higgs model. The simplest supersymmetric model accidentally escapes this problem and predicts a zero photon mass; however there are other difficulties with this model.
The second fact concerns the vector-boson masses. The simplest Higgs model predicts a certain ratio between the W and Z masses, which is not subject to renormalization. The associated parameter is the r parameter and, assuming that the only things that change the ratio arise from radiative corrections, one obtains a prediction for the Higgs mass (assuming the simplest model). This is the source of the predictions on the limits on the Higgs mass that are commonly quoted (figure 2). It should be pointed out that the corrections to the mass ratio also contain a prediction for the top quark mass that agrees very well with the observed value. So, indeed, we must assume that the Higgs sector is such that the prediction for the mass ratio of the W and Z bosons is that given by the simplest Higgs sector. This puts severe limits on theoretical models for the Higgs sector.
It is clear that our knowledge of the Higgs sector is scanty, and in particular a Higgs system with a very heavy Higgs is quite possible. The latter would probably produce a wide resonance for the X in figure 1, and it would be hard to make precise statements on the decays of such a resonance. Well, let us hope that the LHC clarifies the matter.
Most reactions at Fermilab’s Tevatron occur when the colliding proton and antiproton break apart into quarks and gluons that hadronize to form the particles that fly off into the detector. In exclusive interactions, however, the proton and antiproton avoid the breakup, glancing off each other in a process where the underlying interaction involves some combination of photons and/or gluons.
In 2006, the CDF collaboration at the Tevatron obtained the first clear evidence for exclusive interactions at a proton–(anti)proton collider, when they observed high-energy photon pairs in the central rapidity (barrel) region of the detector, but with nothing else, down to an angle of around 0.1° from the beam (±7.4 units of pseudorapidity). They found only three events initially, against a small background predicted to be at most 0.2 events (Aaltonen et al. 2007). These events were consistent with being produced via gluon–gluon “fusion” via a quark “box” where the gluons originate from the beam particles, as shown in figure 1a. An additional “screening” gluon is exchanged to cancel the colour of the interacting gluons and allow the leading hadrons to stay intact. The collaboration has since observed more exclusive two-photon final state candidates in new data.
The search for this unusual two-photon process at the Tevatron was originally proposed in 2001, when CDF physicists first explored the possibility that the Higgs boson could be produced by gluon–gluon fusion as described in figure 1b (Albrow et al. 2005). The idea is that if the Higgs field fills the vacuum, it should be possible to “excite the vacuum” into a real Higgs particle in a glancing collision of a proton and antiproton. Theorists had various estimates for the probability of this happening, but their predictions varied widely.
The two-photon process measured in the CDF detector is produced in much the same way as the Higgs would be, but much more prolifically, so making it a “standard candle” for the production of Higgs bosons. Theorists from the Centre for Particle Theory at the University of Durham predicted that there should be only about one clean two-photon event of this kind in data corresponding to 532 pb–1 of integrated luminosity taken by CDF in Run II at the Tevatron (Khoze et al. 2006). The three events that the CDF collaboration found confirmed this prediction. Thus, the similar process that could produce the elusive Higgs boson must also happen, and could be measured at the LHC, thereby providing measurements of the particle’s mass, spin and other properties.
In the process of checking this measurement, the CDF physicists came across another exclusive physics process that had never been seen before at a proton–(anti)proton collider. They found 16 events that are consistent with the QED prediction that photons travelling with the proton and antiproton can interact to produce only an electron–positron pair (γγ → e+e–) without breaking up the proton and antiproton (Abulencia et al. 2007). In this case the Tevatron acts as a photon “collider”. As the backgrounds to this process are similar to the final state discussed above, the CDF team gained further confidence in their exclusive two-photon final state analysis. To date, they have found many more exclusive electron–positron candidate events. QED two-photon processes such as this, which have previously been observed in electron–positron, electron–proton and nuclear collisions, should provide a means of calibrating the momentum scale and resolution of forward proton spectrometers proposed for the ATLAS and CMS experiments at the LHC.
The CDF team then reasoned that they should also see exclusive muon-pair events produced by the same QED interaction, as in figure 2a. Apart from an indication of exclusive pair production at the ISR at CERN (Antreasyan et al. 1980), this would be another “first” at a proton–(anti)proton collider. In 2007 their supposition was confirmed, but with an added bonus. The expected process, γγ → μ+μ–, was indeed detected according to QED expectations.
In addition, the CDF physicists recorded, for the first time in hadron-hadron collisions, exclusive photoproduction of the J/ψ and ψ (2S) decaying to a pair of muons (figure 2b). Figure 3 shows the clear, clean signals observed. The team also detected the contribution from exclusive production via gluon–gluon fusion of the χc0, decaying to a muon pair and a soft photon (figure 1c). Evidence for this state in CDF data had also been reported earlier, in 2003 (Wyatt 2003).
An analysis aimed at higher muon-pair masses also revealed the upsilon (Υ). The Υ(1S) and Υ(2S) have been clearly seen in CDF, with the Υ(3S) emerging, to be revealed by the higher statistics that are now available. In the case of the photoproduction of these bottomonium (Υ(1S), Υ(2S)) and the charmonium (ψ(1S), ψ(2S)) states, the Tevatron is acting as a “photon–pomeron collider” (figure 2b). The pomeron is well known from diffractive reactions, which are characterized by the exchange of a quark/gluon construct – the pomeron – with the quantum numbers of the vacuum. Because the exchange is colourless, in these reactions a large region in pseudorapidity space is left empty of particles (the “rapidity gap”). In perturbative QCD, the lowest order prototype of the pomeron is a colour-neutral system of two gluons.
This photoproduction of charmonium and bottomonium was previously studied in collisions at DESY’s electron–proton collider, HERA, with similar kinematics (√s = 100 GeV) and the cross sections are in agreement. A comparison of the J/ψ and ψ(2S) cross sections with predictions from HERA data is sensitive to a possible contribution from the elusive and enigmatic odderon. This is a partner of the pomeron with charge conjugation odd (C-odd) and in QCD is formed from three gluons in a colour-neutral state. Unfortunately these predictions have a spread, weakening the sensitivity of CDF’s search for odderon exchange, but still allowing a useful limit to be set on the production of odderons in this mode.
After publishing results on exclusive lepton-pair and photon-pair production, the CDF collaboration scored a hat-trick in 2008 when it published results on exclusive di-jet production, as in figure 1d (Aaltonen et al. 2008). Using a Roman Pot deployed tracker some 66 m from the interaction point to tag the unbroken antiproton in conjunction with a large rapidity gap on the deflected proton side, the team defined a sample of potentially exclusive events. The greater the share of the mass of the central system that the two jets enjoyed, the “more exclusive” the events were expected to be. This expectation was borne out by the Monte Carlo simulation (Monk and Pilkington 2005) for central exclusive production and in agreement with the predictions of the Durham Group (Khoze et al. 2007). Figure 4 shows an event display of an exclusive di-jet candidate. Also, as the di-jet fractional share of the overall central mass of the event tended to one – and the exclusive di-jet sample became purer and purer – evidence for b-jet suppression was seen, as theoretically expected. As in the case of exclusive gamma-gamma and χc0 production, this is an example of the Tevatron acting as a gluon–gluon collider. The detection at the Tevatron of these exclusive processes, resulting from gluon–gluon interactions, strongly suggests that exclusive production of the Higgs boson by the similar process would be detected at the LHC.
Although forward proton detectors have been used to study Standard Model physics for a couple of decades, the new landscape revealed by exclusive physics at hadron colliders has been fully realized only in the past few years. In this arena, the LHC is not only preparing to take the baton from the Tevatron, but also to enter the race with greatly improved tools. The FP420 R&D project is planning to provide the means to measure the displacement and angle of the outgoing protons from exclusive interactions by deploying high precision “edgeless” silicon trackers less than a centimetre from the beam, at ±420 m from the beam intersection points of the ATLAS and CMS experiments at the LHC (Albrow et al. 2008). This gives these experiments the ability to calculate the proton momentum loss and transverse momentum, allowing the mass of the centrally produced system to be reconstructed with a resolution of a few GeV/c2 per event whatever the central system. Broadly speaking then, in the exclusive physics arena, the LHC becomes a “multi-collider”, where the gluon–gluon, photon–photon, or photon–pomeron beam energy is known.
The ability of the FP420 detectors to measure intact protons from an exclusive interaction, in conjunction with the associated centrally produced system using the current ATLAS and/or CMS detector, will provide rich new perspectives at the LHC on studies in QCD, electroweak physics, the Higgs sector and beyond Standard Model physics. In some scenarios, these detectors may be the primary means of discovering new particles at the LHC, with unique ability to measure their quantum numbers. The addition of the FP420 detectors will thus, for a relatively small cost, significantly enhance the discovery and physics potential of the ATLAS and CMS experiments. The existence proof provided by the exclusive physics results from the Tevatron shows that such a programme is feasible.
Over the past few years, the quality and diversity of data from modern imaging atmospheric Cherenkov telescopes (IACTs) has revolutionized gamma-ray astronomy. With ground-based instruments, detailed imaging of the gamma-ray sky at 100 GeV to 100 TeV has become a reality and a wealth of information is currently being gathered about the universe. The 4th Heidelberg International Symposium on High-Energy Gamma-Ray Astronomy (γ 2008) was a timely opportunity to review the status and perspectives of this young field of astroparticle physics.
The Heidelberg Symposium is a well established series of conferences organized by the Max-Planck-Institute for Nuclear Physics (MPIK) in Heidelberg, a leading institute of the H.E.S.S. collaboration, which operates an array of four IACTs in Namibia. After fruitful meetings in 1996, 2000 and 2004, the 4th symposium, which took place in July this year, celebrated an important breakthrough in gamma-ray astronomy. More than 50 very-high-energy (VHE) gamma-ray sources – with energies above 100 GeV – have been discovered since the last symposium, when only about 20 such sources were known.
This tremendous progress was reflected in the high-quality contributions at γ 2008. Twenty-six invited speakers reviewed the status of the field and related disciplines, and discussed the perspectives for gamma-ray astronomy and astroparticle physics in general. In addition, 56 spoken contributions and some 200 poster presentations addressed a range of topics. The number of abstracts submitted to the conference was significantly higher than for the 2004 symposium, reflecting again the growing interest in gamma-ray astronomy round the world. Talks were given in plenary sessions, allowing for lively discussions among the 300 experts from different fields of astroparticle physics. A significant amount of time was also devoted to the poster sessions, which took place in the relaxing atmosphere of “coffee and cake”, a typical German tradition.
VHE gamma-ray astronomy is currently being driven by four large installations of Cherenkov telescopes: the MAGIC telescope (La Palma, Canary Islands) and the VERITAS telescope array (Arizona, US) in the northern hemisphere; and the H.E.S.S. (Khomas Highlands, Namibia) and CANGAROO-III (Woomera, Australia) arrays in the southern hemisphere. While the northern instruments focus mainly on the observation of extragalactic objects and transient phenomena such as gamma-ray bursts, the southern arrays provide an excellent view of the inner Milky Way and are therefore also devoted to observations of Galactic objects.
As testified in short status reports at the symposium, MAGIC, VERITAS and H.E.S.S. are operating successfully. However, as Ryoji Enomoto of Tokyo University pointed out, the CANGAROO-III array is currently operating only two of its four telescopes, owing to severe mirror deterioration and lack of funding. There were also reports on results from joint observation campaigns on various targets, such as the nuclei of the active galaxies Mkn 421 and M 87. These campaigns provide a way of cross-calibrating the instruments and result in an enhanced energy coverage. Upgrades of MAGIC (with the installation of a second 17 m telescope) and H.E.S.S. (with the installation of a single 28 m telescope in the centre of the existing four 12 m telescopes) to increase their sensitivity are well underway, and first light is expected in late 2008 and 2009, respectively.
After almost a decade of successful operation, the Milagro experiment – a 2000 m2, large field-of-view water Cherenkov detector in New Mexico – has stopped data taking after mapping the northern gamma-ray sky at multi-tera-electron-volt energies. Compared to the Cherenkov telescopes that point to regions of the sky, Milagro’s wide field of view allowed it to monitor the sky continuously, albeit at a higher energy threshold and with rather worse angular resolution. Although energy estimation is difficult for Milagro, Petra Hüntemeyer of the Los Alamos National Laboratory reported on the experiment’s recent success in measuring the energy spectra of sources up to 100 TeV. Plans to replace the instrument by the High Altitude Water Cherenkov (HAWC) project, which would be 10 times more large and more sensitive, were presented in a special session dedicated to future instruments. This session also included discussion of the science issues related to the next generation of gamma-ray instruments.
The key European future project in VHE gamma-ray astronomy is the Cherenkov Telescope Array (CTA). Several tens of medium-sized Cherenkov telescopes will form the core of the CTA observatory, providing a 10-fold boost in sensitivity in the tera-electron-volt energy range compared with current instruments, as well as improved angular resolution. Additional large telescopes at the centre of the array will extend the energy range down to some tens of giga–electron-volts and a widespread halo of telescopes should add enough detection area to reach well into the 100 TeV range. CTA sites in the northern and southern hemispheres should allow full-sky coverage. In this context, the symposium served to foster the already intense communication between CTA and the project for the Advanced Gamma-ray Imaging System in the US, which has similar goals.
Just a few weeks before the conference, the astrophysics community celebrated the successful launch of the Fermi Gamma-ray Space Telescope (formerly GLAST) satellite, a gamma-ray observatory that will provide data in the energy range of approximately 10 MeV to 10 GeV (Fermi Gamma-ray Space telescope sees first light). Together with future ground-based instruments, this instrument will enable gamma-ray observations over seven decades of energy and a direct cross-calibration of ground-based and space-borne instruments for the first time. The perspectives for joint observations between Fermi and the Cherenkov telescopes was an important topic at the meeting, which was discussed by Stefan Wagner of the Landessternwarte Königstuhl in Heidelberg and Stefan Funk of SLAC, among others.
Physics highlights at γ 2008 included the discovery by the H.E.S.S. collaboration of the remnant of the historical supernova SN 1006 in VHE gamma rays. After more than 100 hours of observing time, H.E.S.S. now sees the remains of a massive stellar explosion, which Chinese astronomers reported in 1006, with a statistical significance of six standard deviations above the background. As Melitta Naumann-Godo of the Laboratoire Leprince-Ringuet pointed out, the preliminary image of the object seen by H.E.S.S. resembles the morphology of non-thermal X-ray filaments in the north-west and south-east part of the supernova remnant shell (see figure 1). Because these filaments are produced by synchrotron radiation of electrons that have been accelerated to an energy of about 100 TeV, SN 1006 has long been a prime target for Cherenkov telescopes; it is only the improved sensitivity of the current instruments that has made its discovery possible.
The detection of pulsed emission from the Crab pulsar by the MAGIC collaboration provided another highlight at the symposium. Many of the VHE gamma-ray sources in our galaxy can be identified with pulsar wind nebulae, but no VHE gamma-ray emission had previously been observed from a pulsar itself. The search for pulsed emission – which is well established at energies up to the giga-electron-volt range – matches the continuous efforts to minimize the energy threshold of Cherenkov telescopes. Using a special trigger setup, the MAGIC collaboration succeeded in lowering the threshold of their telescope to 25 GeV, making the detection of pulsed emission possible. Thomas Schweizer of the Max-Planck-Institute for Physics in Munich presented a VHE gamma-ray phasogram from 22 hours of observations of the Crab pulsar, which shows two distinct peaks corresponding to the main pulse and the interpulse. The data are in phase with observations at lower energies and with simultaneous measurements in the optical waveband carried out by the MAGIC collaboration.
Overall, the symposium showed that the stage is set for a bright future in gamma-ray astronomy. As Felix Aharonian of MPIK said in his concluding remarks: “Gamma-ray astronomy has evolved into a new astronomical discipline. Our observations meet all the key features usually attributed to astronomy: imaging, energy spectra, light curves, surveys…”. The community is now looking forward to seeing many new results at the next symposium, which will take place around 2012.
On 26 August, NASA and the US Department of Energy announced the first-light results of the Gamma-Ray Large Area Space Telescope (GLAST). At the same time GLAST changed its name to the Fermi Gamma-ray Space Telescope. Built in an international collaboration of astrophysicists and particle physicists with important contributions from research institutions in France, Germany, Italy, Japan, Sweden and the US, Fermi is expected to discover thousands of new sources of different classes, thus tackling many unresolved questions of fundamental physics, astronomy and cosmology. The telescope is already detecting high-energy gamma-rays from a wealth of cosmic sources – including super-massive black holes in active galactic nuclei, supernova remnants, neutron stars, galactic and solar system sources, and gamma-ray bursts (GRBs) – with more than 30 times the sensitivity of its successful predecessor, the Energetic Gamma Ray Experiment Telescope (EGRET).
Shedding light on many fundamental physics questions
Gamma-rays are produced by the interaction of high-energy charged particles with local matter, magnetic fields or ambient photons, and thus give insight into the physical conditions prevailing in these exotic sources. The physics of the particle acceleration mechanisms believed to be operational in many of these objects was first proposed by Enrico Fermi, who is now honoured with the new name of the telescope. Through investigation of the most extreme places in the universe, Fermi will shed light on many fundamental physics questions, such as, the nature of the ubiquitous dark matter. Dark matter particles could decay or annihilate into gamma-rays and possibly give rise to unambiguous signatures in gamma-ray spectra, which could be used to infer or constrain the properties of the original particles. In understanding dark matter, observations with Fermi will therefore be an essential complement to searches for new particles at CERN.
The main instrument on board Fermi is the Large Area Telescope (LAT), which detects gamma-rays between 20 MeV and 300 GeV. The addition of the secondary GLAST Burst Monitor (GBM) – an instrument primarily dedicated to the detection of GRBs between 8 keV and 30 MeV – gives Fermi a total coverage of seven decades in energy. The aspect ratio of the LAT allows for a large field of view, observing 20% of the whole visible sky at any instant, while the GBM provides complete sky coverage for the detection of GRBs.
Fermi was launched by NASA on 11 June from the Cape Canaveral Air Station in Florida, for a 5–10 year long mission. The first 60 days of data taking constituted the commissioning phase, which went smoothly thanks to the thorough preparatory work undertaken by the whole international Fermi Mission team. During this period, teams undertook the calibration and verification of the performance of the different subsystems. Background rejection, a key element to the success of the mission, proved very satisfactory. Then, on 14 August Fermi entered the phase of nominal science operations, surveying the complete gamma-ray sky every three hours.
The figure below shows the LAT all-sky image released on 26 August. Created using only 95 hours of “first light” observations from the early commissioning phase, this corresponds in source sensitivity to a whole year of observations by EGRET. The map shows gas and dust in the plane of the Milky Way emitting gamma rays owing to collisions with cosmic rays. Other clearly visible sources include the Crab, Geminga and Vela pulsars in our own Galaxy as well as the blazar 3C454.3, an active galaxy located 7.1 billion years away. This source appears particularly bright in the map as it was in a flaring state at the time of the acquisition, as the Fermi/GLAST collaboration announced through the Astronomer’s Telegram.
Fermi has since witnessed the intrinsically dynamic nature of the gamma-ray sky with the detection of another three active galactic nuclei in a high flaring state and the detection of two GRBs with giga-electron-volt energy emission. These bursts were detected by the LAT in coincidence with the GBM, which has also detected another 30, lower-energy bursts since its turn-on on 25 June.
The LAT is a pair-conversion telescope, which consists of an array of 4 × 4 towers, each comprising a precision converter/tracker and a calorimeter. Each tracker module has 18 x-y tracking planes, which contain single-sided silicon strip detectors (400 μm thick with a 228 μm pitch) interleaved with a high-Z converter material (tungsten). The tracker has an active surface of 70 m2 (comparable to the Inner Tracker of the ATLAS detector at CERN, with just over 60 m2) and 900,000 digital channels.
Each calorimeter module consists of 96 CsI(Tl) crystals, which are 2.7 cm × 2.0 cm × 2.6 cm in size and are arranged in eight layers of 12 crystals, each forming a hodoscope (x-y) array. The total depth of the calorimeter is 8.6 radiation lengths (out of 10.1 radiation lengths for the whole instrument). The dimensions of the crystals are comparable to the CsI radiation length (1.86 cm) and Moliere radius for electromagnetic showers (3.8 cm). The segmentation allows for spatial imaging of the shower profile and accurate reconstruction of the shower direction, thus making possible the high energy reach of the LAT and improving background rejection.
The tracker is surrounded by an anticoincidence detector (ACD), consisting of 89 plastic scintillator tiles of different sizes, which are read out by wavelength-shifting fibres coupled to photomultiplier tubes. The ACD is used to reject charged cosmic rays and therefore must have a high efficiency for charged particle detection (<0.9997). The segmentation is optimized to limit the effect of “backsplash” (secondaries produced in the interaction of high-energy photons with the heavy calorimeter, giving a signal in the ACD), which reduced the efficiency of EGRET by at least a factor of two at energies above 10 GeV. The calibration of the LAT is based on a combination of in-orbit and ground-based cosmic-ray data, together with beam tests performed at CERN (at the PS and SPS) and GSI and Monte Carlo simulations using Geant 4.
Opening new observational windows often yields completely unanticipated discoveries
The GBM, which is dedicated to the detection of GRBs, includes 12 sodium iodide (NaI) scintillation detectors and two bismuth germanate (BGO) scintillation detectors. The NaI detectors cover the lower part of the energy range, from a few kilo-electron-volts to about 1 MeV, and provide burst triggers and locations. The BGO detectors cover the energy range from about 150 keV to around 30 MeV, providing a good overlap with the NaI at the lower end, and with the LAT at the high end.
Within only a few days of turn-on, using data originally planned for observatory calibration, Fermi has already corroborated many of the great discoveries both of EGRET and of AGILE. The LAT instrument is already finding new sources. Such spectacular results have only been achieved thanks to an advanced design for the observatory, which makes use of state-of-the-art particle-physics instrumentation that gives Fermi exceptional resolution and sensitivity. As a result, understanding of the high-energy universe is sure to grow tremendously, but even more exciting could be the unexpected, as history shows that opening new observational windows often yields completely unanticipated discoveries.
The institutions participating in the collaboration built and qualified the LAT subsystems which then were integrated at SLAC. The detectors for the GBM were produced at the Max-Planck-Institute for Extraterrestrial Physics in Garching, and were integrated at the Marshall Space Flight Center in Huntsville, Alabama. Both instruments were integrated with the spacecraft at General Dynamics, in Phoenix, Arizona, to form the Fermi observatory. Environmental testing was then performed both at General Dynamics and at the Naval Research Lab in Washington DC.
Several sources of very high-energy gamma-rays are associated with pulsars, revealing that these spinning neutron stars are extremely powerful particle accelerators. The discovery with ESA’s International Gamma-ray Astronomical Laboratory (INTEGRAL) satellite that the gamma-ray emission of the Crab Nebula is strongly polarized along the direction of its spin axis locates the acceleration site in the close vicinity of the pulsar.
The Crab Nebula is the aftermath of a supernova explosion witnessed by Chinese and Arab astronomers in the year 1054. The core of the dying star collapsed to form a neutron star while the outer layers were expelled; their on-going interaction with the interstellar medium produces the beautiful remnant seen nowadays. A neutron star can be thought of as a giant atomic nucleus about 20–30 km across, in which each cubic millimetre weighs about 100,000 tonnes. The neutron star at the centre of the Crab Nebula is actually a pulsar sending radiation pulses 30 times per second, each time the magnetic pole of the spinning neutron star points towards the Earth.
The high-resolution X-ray image of the Crab Nebula obtained by NASA’s Chandra satellite revealed a complex geometry with a collimated jet, thought to be aligned with the spin axis of the neutron star surrounded by a toroidal, doughnut-shaped structure. However, the much lower resolution of current hard X-ray and gamma-ray instruments cannot locate precisely the site of high-energy emission within the Crab Nebula.
A possible clue comes from the study of the polarization properties of the high-energy radiation, a difficult task that has now been achieved for the first time by European astronomers analysing data from the INTEGRAL’s spectrometer. The study, led by Anthony Dean of the University of Southampton, is based on more than 600 individual observations of the Crab taken between February 2003 and April 2006.
The polarization of a gamma-ray photon can be derived if it is scattered off an electron from one detector element to another. This Compton-scattering has a preferred direction related to the polarization angle of the incoming photon. About half a million such events were detected from the Crab Nebula during the quiescent phase of the pulsar cycle, with photon energies between 0.1 and 1 MeV. These data were then fitted to the results of intensive Monte Carlo simulations using GEANT4. The best fit was obtained for a polarization of 46 ± 10% and a polarization angle of 123° ± 11°, closely aligned with the direction of the pulsar spin and the X-ray jet.
This large fraction of polarized photons implies that the high-energy electrons emitting them are accelerated with a high degree of order in a structure apparently closely linked to the spin axis of the pulsar. By considering either synchrotron radiation or curvature radiation, Dean and colleagues estimate a typical electron energy of 1014 to 1015 eV. This is about 1000 times the energy reached by CERN’s LEP collider and is enough to explain the production – by interactions with visible or microwave photons within the Crab Nebula – of the very high-energy gamma-rays detected by Cherenkov telescopes.
The D0 collaboration at Fermilab’s Tevatron has made the first observation of the Ωb, consisting of two s quarks and a b quark. This follows the discovery at Fermilab of the strange b baryon, Ξb, in 2007, and echoes that of the original Ω– particle.
The prediction of the original Ω– dates back to the early 1960s, when assigning the known baryons to symmetry groups according to properties including spin, isospin and strangeness hinted at the existence of a new, triply strange spin–3/2 baryon with a charge of –1. In a triumphant interplay between experiment and theory, the particle was discovered in 1964 in a photograph made at the 80 inch bubble chamber at Brookhaven National Laboratory. Subsequent events turned up soon after at CERN. The success of the symmetry group structure led to the quark model, with three initial types or “flavours” of quark, u, d, and s, where the s quark endows the property of strangeness. The Ω– is a baryon, consisting of three quarks, sss.
The subsequent decades revealed three additional flavours of quark, c, b and t, and the quark model now predicts the existence of baryons made of quarks of all flavours but t. (The heavy top quark, t, decays too quickly to form bound states.) This leads to new multiplets of spin–1/2 and spin–3/2 baryons of u, d, s and b quarks. The newly discovered Ωb baryon is a heavy cousin of the Ω–, with a b quark replacing one of the s quarks occupying the position indicated in figure 1 for the spin–1/2 baryons.
Sifting through the data collected at the proton–antiproton collisions at the Tevatron during 2002–2006, the D0 collaboration identified 18 Ωb candidate events at a mass of 6.165 ± 0.017 GeV/c2, approximately six times as great as the proton mass (Abazov et al. 2008). This makes it the heaviest baryon observed so far. The Ωb candidates were reconstructed from decay daughter particles: Ωb → J/ψΩ–, J/ψ → μ+μ–, Ω– → ΛK– and Λ → pπ–. While the Ω– and Λ have decay lengths of a few centimetres, the Ωb travels only a millimetre or so before decaying. The analysis uses a sample of events with muon pairs from J/ψ decays, followed by successive reconstructions of Λ and Ω– particles from charged tracks before a final combination of J/ψ and Ω– candidates. Figure 2 shows the effective mass spectrum of the J/ψ and Ω– combinations, with a peak of more than 5 σ significance and the observation of the Ωb–.
The Ωb now joins the σb± and Ξb baryons recently observed at the Tevatron. These new states allow detailed study of the strong force, which holds quarks together to form all baryons, and the weak force, which is responsible for their decays.
Will the LHC surprise us? I hope so. Having failed to find any completely unexpected new physics for more than 30 years, we clearly need nature’s help to progress, and the case is good.
The last really big surprise in particle physics was the discovery of the third charged lepton (the tau) in 1975. There have of course been many extremely important discoveries since then, and our understanding of particle physics has advanced enormously. But the only real surprises have been how well the Standard Model has worked, the accuracy with which experiments have been able to check its predictions, and the failure to find its missing ingredient (the mechanism that gives particles their masses: Higgs?), or any other physics beyond the Standard Model, apart from the major discovery of neutrino masses (which, however, was not a huge surprise as no principle required zero mass).
By the time of the major LEP summer study in 1978 the Standard Model was accepted by many, but by no means all, theorists and gaining supporters among experimenters. It was thought that “the (CERN) proton–antiproton collider [which had just been launched] should discover the Z, but apart from measuring its mass (with considerable errors) it will not allow us to investigate its properties in detail (it may also discover the W but this looks more difficult)”. It was argued that LEP1 would be needed to study the Z in detail (or, if it did not exist, discover what else damps the rising weak cross section at LEP energies, where the phenomenological low energy theory had to be wrong), and measure the number of neutrinos into which it can decay; LEP2 would be needed to study the W, and find the Higgs boson (or whatever else generates masses) if it had not been found at LEP1. The surprises (at least for theorists like me) were how easy it was to detect the W (which was discovered in 1983, shortly before the Z) and the accuracy of the LEP results, which led to the exciting discovery that the strengths of the electromagnetic and strong forces converge at high energies, supporting the idea that they are different manifestations of a single “grand unified” force.
At the 1978 LEP summer study the importance of insisting on a relatively long tunnel in order not to compromise the energy of a later proton accelerator or LHC was discussed, and this argument was used when LEP was approved in 1981. The first serious discussion of LHC physics took place in 1984. It was obvious that the time had come to launch R&D on LHC magnets but “less clear whether it is sensible to discuss (LHC) physics…without more complete results from the SPS collider, let alone data from LEP, SLC and HERA…crystal gazing is unusually hazardous following recent tantalizing hints of new discoveries from UA1 and UA2”. These hints, which turned out to be spurious (along with other hints of non-standard physics, from Fermilab neutrino experiments, LEP, and other experiments), remind us of the difficulty of exploring the frontier: we should not be surprised if there are false dawns at the LHC.
In 1984 it was stressed that the physics of mass generation was almost certain to be discovered at the LHC, if the question had not been settled at LEP, and that there are good reasons for expecting physics beyond the Standard Model in the LHC energy range – perhaps supersymmetry, which was discussed in some detail (it was only mentioned briefly at the 1978 summer study, although in the event a huge effort went into unsuccessful searches for supersymmetry at LEP). The case for the LHC was developed in more detail during the 1980s, but its essence has not changed.
The formal proposal to build the LHC presented to the CERN Council in 1993 was introduced with the statement that it will “provide an unparalleled ‘reach’ in the search for new fundamental particles and interactions between them, and is expected to lead to new, unique insights into the structure of matter and the nature of the universe”. The LHC will take us a factor of 10 further in energy (at the level of the proton’s constituents) or equivalently to a tenth of the distance scale that has been explored so far. This alone is enough to whet scientific appetites. But pulses are really set racing by the knowledge that the LHC has a good chance of finding what generates masses (a single elementary Higgs field? Multiple or composite Higgs fields?…?) and may cast light on other mysteries, including: why the mass of the W is so small compared to the scale of the proposed grand unification of electroweak and strong interactions, the magnitude of the asymmetry between matter and anti-matter in the universe, the number of quarks and leptons, and the origin of the dark matter and dark energy that pervade the universe.
What do I expect? I am fairly confident that Higgs, in some form, will show up. If the LHC finds the standard Higgs boson and nothing else I would be extremely disappointed as we would learn essentially nothing. (The biggest surprise would be to find nothing, which would take us nowhere, while making the case for going to much higher energies compelling but probably impossible to sell.) I think there is a reasonable probability that supersymmetry will be found, and I hope this happens: the most convincing arguments are that it is the only possible symmetry allowed by quantum field theory (the mathematical language of particle physics) that has not been found (why would nature utilise all possibilities but one?); “local” supersymmetry (and all the other “continuous” symmetries are local) requires the existence of gravity; and the idea of connecting matter (fermions) with force carriers (bosons) is very appealing, although against this must be set the extravagant proliferation of particles (none found, yet?) that this implies. I am somewhat less impressed by the fact that supersymmetry would stabilize the mass of the W, which is one of the arguments that could put supersymmetry in reach of the LHC.
Thanks to the dedication of the CERN staff the LHC is now starting, and thanks to the community of users around the world, the experiments are ready to take data. It is a fantastic project. I am confident that it will work superbly. I am almost certain that it will make important discoveries, and I hope they will include surprises.
There’s a famous photograph of a young Nepalese climber standing on top of Everest in 1953. It’s the only picture there is, but Tenzing Norgay was not alone. Edmund Hillary, who declined to be photographed, accompanied him to the top. Who got there first? For a while, the two climbers refused to be drawn, saying that what matters is the achievement. And so it is with a mechanism developed in the 1960s to account for the difference between long and short-range interactions in physics.
In the early 1960s, particle physics had a problem. Long-range interactions, such as electromagnetism and gravity, could be explained by the theories of the day, but the short-range weak interaction, whose influence is limited to the scale of the atomic nucleus, could not. The idea that the carriers of the weak force must be heavy, while the carriers of long-range forces would be massless could account for the difference. Conceptually it made sense, but theoretically it couldn’t be done: where would the heavy carriers get their mass? There was no way to reconcile massive and massless force carriers in the same theoretical framework.
Inspired by the new theory of superconductivity put forward in the late 1950s by John Bardeen, Leon Cooper and John Schreiffer, theorist Yoichiro Nambu paved the way to a solution by postulating the idea that a broken symmetry could generate mass. In doing so he in turn inspired three young physicists in Europe to take the next step.
A modest beginning
I met one of those physicists, Peter Higgs, in autumn 2007 in his apartment on the top floor of a walk-up block in Edinburgh new town with views over a leafy square. A slice from an LHC magnet greets visitors to the apartment, where the style is 1970s chic. Copies of Physics World and Scientific American are piled high on the coffee table, topped off with a copy of the satirical paper Private Eye. Bound copies of The Gramophone line the shelves, and the living room’s prominent feature is a chair, optimally placed to make best use of the audiophile Leak hi-fi system.
A few months later, I met Robert Brout and François Englert in a spartanly furnished office, of the kind frequently occupied by professors emeriti, at the Université Libre de Bruxelles. Do we speak English or French was my first question. “Robert will be happier with English,” came the reply. I hadn’t realised that Brout was a naturalized Belgian, and that the two had first worked together in 1959 when he’d hired Englert to join him in his work at Cornell University in statistical mechanics.
As is so often the way with good ideas, the concept of the generation of particle mass through symmetry breaking was developed in more than one place at around the same time, two of those places being Brussels and Edinburgh. It was a modest beginning for a scientific revolution: just two short pages published on 31 August 1964 by Brout and Englert, and little more than a page from Higgs on 15 September. But those two papers were set to influence profoundly the development of particle physics right to this day.
All three scientists are careful to attribute credit to their forerunners, Nambu most strongly. Hints of other influences come from the fact that Higgs has been known to call spontaneous symmetry breaking in particle physics the relativistic Anderson mechanism, a reference to the Nobel prize-winning physicist Philip Anderson who published on the subject in 1963; and in lectures at Imperial College London students are told about the Kibble–Higgs mechanism, in a reference to a later paper published by Gerald Guralnik, Carl Hagen and Tom Kibble.
Brout’s inspiration goes back much further, to another place that symmetry is broken spontaneously in nature with macroscopic effects. “Ferromagnetism was a puzzle in 1900,” he told me, and was solved by French physicist Pierre Weiss in 1907. Essentially, symmetry is broken by the Brout–Englert–Higgs (BEH) mechanism because the ground state of the vacuum is asymmetric, rather like the alignment of the electrons’ magnetic moments in a ferromagnetic material. In the case of the BEH mechanism, however, it’s structure in the vacuum itself that gives rise to particle masses. In the words of CERN’s Alvaro de Rújula: “The vacuum is not empty, there is a difference between vacuum and emptiness.”
The thing that fills the vacuum is a scalar field commonly known as the Higgs field. Some particles interact strongly with this field, others don’t, and it is the strength of the interaction with the field that determines the masses of certain particles. In other words, the carriers of the weak interaction, the W and Z particles, are sensitive to the structure of empty space. This is how the BEH mechanism can accommodate short and long-range interactions in a single theory. The long-awaited confirmation of the mechanism is expected in the form of excitations of the field appearing as scalar bosons (Higgs particles).
Esoteric as this may seem, there are potential astronomical implications, since what particle physicists call the Higgs field, cosmologists call the cosmological constant, or dark energy. A substance that appears to make up some 70% of the universe’s matter and energy, dark energy made itself apparent as recently as 2003 in observations of the farthest reaches of the universe.
Renormalization
Despite the emergence of the BEH mechanism, particle physics still had a problem in the mid-1960s, because the underlying theory was literally not normal. It predicted abnormal results, such as probabilities of more than 100% for given outcomes. It needed to be renormalized, and that would take the best part of a decade. Brout and Englert toyed with the idea in 1966, but a rigorous renormalization had to wait until 1971, when Gerardus ‘t Hooft, a student of Martinus Veltman at Utrecht University, published the first of a series of papers by student and supervisor that would rigorously prove the renormalizability of the theory. They were rewarded with a trip to Stockholm in 1999 to collect the Nobel Prize in Physics.
If Brout, Englert and Higgs had provided a cornerstone of the Standard Model, ‘t Hooft and Veltman gave it its foundations. From then, theoretical and experimental progress was rapid, and accompanied by a rich harvest of Nobel Prizes. In 1973, a team at CERN led by André Lagarrigue found the first evidence for heavy carriers of the weak interaction. In 1979, Sheldon Glashow, Steven Weinberg and Abdus Salam received the Nobel Prize for Physics for their work on unifying the electromagnetic and weak interactions, the theory in which the BEH mechanism plays its crucial role. Then in 1984, Carlo Rubbia and Simon van der Meer received the Nobel Prize for their decisive contributions to the programme that discovered the carriers of the weak force, the W and Z particles, at CERN in 1982–1983.
“The experimental discovery of the W and Z particles confirmed both the validity of the electroweak model,” explained François Englert “and of the BEH mechanism.” There remained, however, a missing ingredient. A machine was needed that could shake the scalar boson of the BEH mechanism out of its hiding place in the vacuum of space. That machine is the LHC. Many scientists would, and indeed have, bet on the discovery of the particle, but however elegant and enticing the work of Brout, Englert and Higgs, no-one can be sure it is right until the scalar boson has been seen. Nature might have chosen to endow particles with mass in a different way, so until the particle is found, the BEH mechanism remains no more than speculation. Whatever the case, the LHC will give us the answer.
There are many stories as to how the BEH mechanism and its associated particle came to be named after Higgs. The one Higgs told me involves a meeting that he had with fellow theorist Ben Lee at a conference in 1967, at which they discussed Higgs’s work. Then along came renormalization, making field theory fashionable, and another conference. “The conference at which my name was attached to pretty well everything connected with spontaneous symmetry breaking in particle physics was in ’72,” explained Higgs. It was a conference at which Lee delivered the summary talk.
Brout, Englert and Higgs have rarely met, but they have much in common. All came to a field, unfashionable with particle theorists at the time, from different areas of science. “Sometimes you do things in a domain in which you are not an expert and it plays a big role,” explained Englert. “We had no reason to dismiss field theory because people didn’t use it.” The three also agree on many things – their inspiration for one. “What was interesting me back in the early 1960s was the work of Nambu, who was proposing field theories of elementary particles in which symmetries were broken spontaneously in analogy to the way that it happens in a superconductor,” said Higgs. Englert said it slightly differently: “We were very impressed by the fact that Nambu transcribed superconductivity in terms of field theory,” he said. “That’s a beautiful paper.”
The three are in agreement about the results that the LHC might bring. “The most uninteresting result would be if we find nothing other than that which we’re most expecting,” said Englert. According to Higgs: “The most uninteresting result would be if they found the Higgs boson and nothing much else.” “If the Standard Model works, then we’re in trouble,” said Brout. “We’ll have to rely on human intelligence to go further,” said Englert completing the thought. And the most interesting direction for physics? Gravity, they all concur. “Any crumbs that fall off it would have major effects on the world of elementary particles,” said Brout, “in my heart, gravity is the secret to everything.”
Physicists and mountaineers have much in common. They are on the whole fiercely competitive, yet collaborative at the same time, and they can be magnanimous to an extraordinary degree. “I was delighted to discover that we are sharing the prize,” Higgs said on being informed that the European Physical Society had awarded him a prestigious prize in 1997. “I get a lot of publicity for this work, but (Brout and Englert) were clearly ahead of me.”
So who did get there first? At Everest, it turns out to have been Hillary who put his foot on the summit first. In physics Brout and Englert were first to publish, but that’s not what matters. In physics, as in mountaineering, it’s the achievement that counts.
The principal goal of the experimental programme at the LHC is to make the first direct exploration of a completely new region of energies and distances, to the tera-electron-volt scale and beyond. The main objectives include the search for the Higgs boson and whatever new physics may accompany it, such as supersymmetry or extra dimensions, and also – perhaps above all – to find something that the theorists have not predicted.
The Standard Model of particles and forces summarizes our present knowledge of particle physics. It extends and generalizes the quantum theory of electromagnetism to include the weak nuclear forces responsible for radioactivity in a single unified framework; it also provides an equally successful analogous theory of the strong nuclear forces.
The conceptual basis for the Standard Model was confirmed by the discovery at CERN of the predicted weak neutral-current form of radioactivity and, subsequently, of the quantum particles responsible for the weak and strong forces, at CERN and DESY respectively. Detailed calculations of the properties of these particles, confirmed in particular by experiments at the LEP collider, have since enabled us to establish the complete structure of the Standard Model; data taken at LEP agreed with the calculations at the per mille level.
These successes raise deeper problems, however. The Standard Model does not explain the origin of mass, nor why some particles are very heavy while others have no mass at all; it does not explain why there are so many different types of matter particles in the universe; and it does not offer a unified description of all the fundamental forces. Indeed, the deepest problem in fundamental physics may be how to extend the successes of quantum physics to the force of gravity. It is the search for solutions to these problems that define the current objectives of particle physics – and the programme for the LHC.
Higgs, hierarchy and extra dimensions
Understanding the origin of mass will unlock some of the basic mysteries of the universe: the mass of the electron determines the sizes of atoms, while radioactivity is weak because the W boson weighs as much as a medium-sized nucleus. Within the Standard Model the key to mass lies with an essential ingredient that has not yet been observed, the Higgs boson; without it the calculations would yield incomprehensible infinite results. The agreement of the data with the calculations implies not only that the Higgs boson (or something equivalent) must exist, but also suggests that its mass should be well within the reach of the LHC.
Experiments at LEP at one time found a hint for the existence of the Higgs boson, but these searches proved unsuccessful and told us only that it must weigh at least 114 GeV. At the LHC, the ATLAS and CMS experiments will be looking for the Higgs boson in several ways. The particle is predicted to be unstable, decaying for example to photons, bottom quarks, tau leptons, W or Z bosons (figure 1). It may well be necessary to combine several different decay modes to uncover a convincing signal, but the LHC experiments should be able to find the Higgs boson even if it weighs as much as 1 TeV.
While resolving the Higgs question will set the seal on the Standard Model, there are plenty of reasons to expect other, related new physics, within reach of experiments at the LHC. In particular, the elementary Higgs boson of the Standard Model seems unlikely to exist in isolation. Specifically, difficulties arise in calculating quantum corrections to the mass of the Higgs boson. Not only are these corrections infinite in the Standard Model, but, if the usual procedure is adopted of controlling them by cutting the theory off at some high energy or short distance, the net result depends on the square of the cut-off scale. This implies that, if the Standard Model is embedded in some more complete theory that kicks in at high energy, the mass of the Higgs boson would be very sensitive to the details of this high-energy theory. This would make it difficult to understand why the Higgs boson has a (relatively) low mass and, by extension, why the scale of the weak interactions is so much smaller than that of grand unification, say, or quantum gravity.
This is known as the “hierarchy problem”. One could try to resolve it simply by postulating that the underlying parameters of the theory are tuned very finely, so that the net value of the Higgs boson mass after adding in the quantum corrections is small, owing to some suitable cancellation. However, it would be more satisfactory either to abolish the extreme sensitivity to the quantum corrections, or to cancel them in some systematic manner.
One way to achieve this would be if the Higgs boson is composite and so has a finite size, which would cut the quantum corrections off at a relatively low energy scale. In this case, the LHC might uncover a cornucopia of other new composite particles with masses around this cut-off scale, near 1 TeV.
The alternative, more elegant, and in my opinion more plausible, solution is to cancel the quantum corrections systematically, which is where supersymmetry could come in. Supersymmetry would pair up fermions, such as the quarks and leptons, with bosons, such as the photon, gluon, W and Z, or even the Higgs boson itself. In a supersymmetric theory, the quantum corrections due to the pairs of virtual fermions and bosons cancel each other systematically, and a low-mass Higgs boson no longer appears unnatural. Indeed, supersymmetry predicts a mass for the Higgs boson probably below 130 GeV, in line with the global fit to precision electroweak data.
The fermions and bosons of the Standard Model, however, do not pair up with each other in a neat supersymmetric manner. The theory, therefore, requires that a supersymmetric partner, or sparticle, as yet unseen, accompanies each of the Standard Model particles. Thus, this scenario predicts a “scornucopia” of new particles that should weigh less than about 1 TeV and could be produced by the LHC (figure 3).
Another attraction of supersymmetry is that it facilitates the unification of the fundamental forces. Extrapolating the strengths of the strong, weak and electromagnetic interactions measured at low energies does not give a common value at any energy, in the absence of supersymmetry. However, there would be a common value, at an energy around 1016 GeV, in the presence of supersymmetry. Moreover, supersymmetry provides a natural candidate, in the form of the lightest supersymmetric particle (LSP), for the cold dark matter required by astrophysicists and cosmologists to explain the amount of matter in the universe and the formation of structures within it, such as galaxies. In this case, the LSP should have neither strong nor electromagnetic interactions, since otherwise it would bind to conventional matter and be detectable. Data from LEP and direct searches have already excluded sneutrinos as LSPs. Nowadays, the “scandidates” most considered are the lightest neutralino and (to a lesser extent) the gravitino.
Assuming that the LSP is the lightest neutralino, the parameter space of the constrained minimal supersymmetric extension of the Standard Model (CMSSM) is restricted by the need to avoid the stau being the LSP, by the measurements of b → sγ decay that agree with the Standard Model, by the range of cold dark-matter density allowed by astrophysical observations, and by the measurement of the anomalous magnetic moment of the muon (gμ–2). These requirements are consistent with relatively large masses for the lightest and next-to-lightest visible supersymmetric particles, as figure 4 indicates. The figure also shows that the LHC can detect most of the models that provide cosmological dark matter (though this is not guaranteed), whereas the astrophysical dark matter itself may be detectable directly for only a smaller fraction of models.
Within the overall range allowed by the experimental constraints, are there any hints at what the supersymmetric mass scale might be? The high precision measurements of mW tend to favour a relatively small mass scale for sparticles. On the other hand, the rate for b → sγ shows no evidence for light sparticles, and the experimental upper limit on Bs → μ+μ– begins to exclude very small masses. The strongest indication for new low-energy physics, for which supersymmetry is just one possibility, is offered by gμ–2. Putting this together with the other precision observables gives a preference for light sparticles.
Other proposals for additional new physics postulate the existence of new dimensions of space, which might also help to deal with the hierarchy problem. Clearly, space is three-dimensional on the distance scales that we know so far, but the suggestion is that there might be additional dimensions curled up so small as to be invisible. This idea, which dates back to the work of Theodor Kaluza and Oskar Klein in the 1920s, has gained currency in recent years with the realization that string theory predicts the existence of extra dimensions and that some of these might be large enough to have consequences observable at the LHC. One possibility that has emerged is that gravity might become strong when these extra dimensions appear, possibly at energies close to 1 TeV. In this case, some variants of string theory predict that microscopic black holes might be produced in the LHC collisions. These would decay rapidly via Hawking radiation, but measurements of this radiation would offer a unique window onto the mysteries of quantum gravity.
If the extra dimensions are curled up on a sufficiently large scale, ATLAS and CMS might be able to see Kaluza–Klein excitations of Standard Model particles, or even the graviton. Indeed, the spectroscopy of some extra-dimensional theories might be as rich as that of supersymmetry while, in some theories, the lightest Kaluza–Klein particle might be stable, rather like the LSP in supersymmetric models.
Back to the beginning
By colliding particles at very high energies we can recreate the conditions that existed a fraction of a second after the Big Bang, which allows us to probe the origins of matter. Experiments at LEP revealed that there are just three “families” of elementary particles: one that makes up normal stable matter, and two heavier unstable families that were revealed in cosmic rays and accelerator experiments. The Standard Model does not explain why there are three and only three families, but it may be that their existence in the early universe was necessary for matter to emerge from the Big Bang, with little or no antimatter.
Andrei Sakharov was the first to point out that particle physics could explain the origin of matter in the universe by the fact that matter and antimatter have slightly different properties, as discovered in the decays of K and B mesons, which contain strange and bottom quarks, members of the heavier families. These differences are manifest in the phenomenon of CP violation. Present data are in good agreement with the amount of CP violation allowed by the Standard Model, but this would be insufficient to generate the matter seen in the universe.
The Standard Model accounts for CP violation within the context of the Cabibbo–Kobayashi–Maskawa (CKM) matrix, which links the interactions between quarks of different type (or flavour). Experiments at the B-factories at KEK and SLAC have established that the CKM mechanism is dominant, so the question is no longer whether this is “right”. The task is rather to look for additional sources of CP violation that must surely exist, to create the cosmological matter–antimatter asymmetry via baryogenesis in the early universe. If the LHC does observe any new physics, such as the Higgs boson and/or supersymmetry, it will become urgent to understand its flavour and CP properties.
The LHCb experiment will be dedicated to probing the differences between matter and antimatter, notably looking for discrepancies with the Standard Model. The experiment has unique capabilities for probing the decays of mesons containing both bottom and strange quarks. It will be able to measure subtle CP-violating effects in Bs decays, and will also improve measurements of all the angles of the unitarity triangle, which expresses the amount of CP violation in the Standard Model. The LHC will also provide high sensitivity to rare B decays, to which the ATLAS and CMS experiments will contribute, in particular, and which may open another window on CP violation beyond the CKM model.
In addition to the studies of proton–proton collisions, heavy-ion collisions at the LHC will provide a window onto the state of matter that would have existed in the early universe at times before quarks and gluons “condensed” into hadrons, and ultimately the protons and neutrons of the primordial elements. When heavy ions collide at high energies they form for an instant a “fireball” of hot, dense matter. Studies, in particular by the ALICE experiment, may resolve some of the puzzles posed by the data already obtained at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven. These data indicate that there is very rapid thermalization in the collisions, after which a fluid with very low viscosity and large transport coefficients seems to be produced. One of the surprises is that the medium produced at RHIC seems to be strongly interacting . The final state exhibits jet quenching and the semblance of cones of energy deposition akin to Machian shock waves or Cherenkov radiation patterns, indicative of very fast particles moving through a medium faster than sound or light.
Experiments at the LHC will enter a new range of temperatures and pressures, thought to be far into the quark–gluon plasma regime, which should test the various ideas developed to explain results from RHIC. The experiments will probably not see a real phase transition between the hadronic and quark–gluon descriptions; it is more likely to be a cross-over that may not have a distinctive experimental signature at high energies. However, it may well be possible to see quark–gluon matter in its weakly interacting high temperature phase. The larger kinematic range should also enable ideas about jet quenching and radiation cones to be tested.
First expectations
The first step for the experimenters will be to understand the minimum-bias events and compare measurements of jets with the predictions of QCD. The next Standard Model processes to be measured and understood will be those producing the W- and Z-vector bosons, followed by top-quark physics. Each of these steps will allow the experimental teams to understand and calibrate their detectors, and only after these steps will the search for the Higgs boson start in earnest. The Higgs will not jump out in the same way as did the W and Z bosons, or even the top quark, and the search for it will demand an excellent understanding of the detectors. Around the time that Higgs searches get underway, the first searches for supersymmetry or other new physics beyond the Standard Model will also start.
In practice, the teams will look for generic signatures of new physics that could be due to several different scenarios. For example, missing-energy events could be due to supersymmetry, extra dimensions, black holes or the radiation of gravitons into extra dimensions. The challenge will then be to distinguish between the different scenarios. For example, in the case of distinguishing between supersymmetry and universal extra dimensions, the spectra of higher excitations would be different in the two scenarios, the different spins of particles in cascade decays would yield distinctive spin correlations, and the spectra and asymmetries of, for instance, dileptons, would be distinguishable.
What is the discovery potential of this initial period of LHC running? Figure 5a shows that a Standard Model Higgs boson could be discovered with 5 σ significance with 5 fb–1 of integrated and well-understood luminosity, whereas 1 fb–1 would already suffice to exclude a Standard Model Higgs boson at the 95% confidence level over a large range of possible masses. However, as mentioned above, this Higgs signal would receive contributions from many different decay signatures, so the search for the Higgs boson will require researchers to understand the detectors very well to find each of these signatures with good efficiency and low background. Therefore, announcement of the Higgs discovery may not come the day after the accelerator produces the required integrated luminosity!
Paradoxically, some new physics scenarios such as supersymmetry may be easier to spot, if their mass scale is not too high. For example, figure 5b shows that 0.1 fb–1 of luminosity should be enough to detect the gluino at the 5 σ level if its mass is less than 1.2 TeV, and to exclude its existence below 1.5 TeV at the 95% confidence level. This amount of integrated luminosity could be gathered with an ideal month’s running at 1% of the design instantaneous luminosity.
We do not know which, if any, of the theories that I have mentioned nature has chosen, but one thing is sure: once the LHC starts delivering data, our hazy view of this new energy scale will begin to clear dramatically.
Unlike the general-purpose detectors, the geometry of the LHCb experiment does not cover the full solid angle, but is developed along the forward direction with respect to the collision point. For 20 m a series of detector planes collects information on the particles coming from the collision point. This design is optimized for the study of B mesons, which, given their relatively small mass compared with the high energy of the LHC collisions, fly mostly in the forward direction.
B mesons have received increasing attention from theorists and experimentalists alike over recent years because their behaviour seems linked to various quantum phenomena that could shed light on new physics. “Today’s Standard Model of particle physics leaves many unanswered questions,” says Andrei Golutvin, spokesperson of the LHCb collaboration. He has recently taken over this role from Tatsuya Nakada who was the first spokesperson and a founder of the experiment. “A lot of physicists expect new physics to be just around the corner and already accessible at the LHC,” he continues. “General-purpose detectors like ATLAS and CMS will look for direct evidence of the existence of new particles. We have a different strategy. We focus on the study of B mesons, where some of their behaviour is very precisely predicted by the Standard Model. However small, a deviation from these predictions would indicate the existence of new phenomena.”
In recent years, two experiments at B-factories – BaBar at SLAC and Belle at KEK – have shown that the B particles are a key element in the process of understanding CP violation – the subtle asymmetry between matter and antimatter within the Standard Model. However, this does not seem to be enough to generate the absence of antimatter in the universe. “We will study with an unprecedented precision how CP violation takes place in the B-system,” explains Golutvin. “The yet undiscovered heavy particles could be a new source of CP violation that could affect the decays of B particles. The Bs mesons seem particularly interesting,” he continues. “Their loop-dominated decays are potentially very sensitive to new particles that could ‘enter’ in the loop virtually and cause observable effects. For example, if we find that the decay rate of the Bs to a particular final state, such as two muons, is higher than predicted by the Standard Model, it could be an indication of a contribution coming from Higgs bosons or supersymmetric particles.”
The LHC, with its high luminosity and high energy, will provide the LHCb collaboration with a particularly rich harvest of beauty particles
The LHC, with its high luminosity and high energy, will provide the LHCb collaboration with a particularly rich harvest of beauty particles, hundreds of times more than those made available by other accelerators to previous experiments. “Both BaBar and Belle, as well as CDF and D0 at the Tevatron proton–antiproton collider, made big contributions to flavour physics, the physics of processes that involve the transformation of quark flavours,” says Golutvin. “Now we know that the indirect contribution of new physics in CP violation is not big, certainly below the 10% level for the most of the decay modes. Thanks to the LHC performance, LHCb will be able to study very rare events and show possible new avenues to physics.”
In its 15-year history, the LHCb detector underwent one major layout modification. The modification – known as the “LHCb light” option – reduced the amount of material in the layers the particles cross, thus reducing the background produced by the interaction of primary particles with the material of the detector. “We work out the momentum of charged particles by measuring the bending angle after the dipole magnets. The original idea was to have additional detectors to follow the trajectory of particles inside the magnet, which means of course a more complicated detector,” Golutvin explains. “After an idea by Nakada and with the help of computer simulations, we understood that we could have very robust pattern recognition even without all those chambers.” The results was that about six years ago the LHCb collaboration decided to simplify detector a little by having no chambers in the magnet. “This minimizes the amount of material along the trajectories of particles and also simplifies the operation of the detector,” says Golutvin. “Besides that, there were a few other minor changes. For example, we decided to use a beryllium beam pipe also to minimize the background.”
LHCb is designed to run at a luminosity of a few times 1032 cm–2s–1, much smaller than the nominal LHC luminosity, 1032 cm–2s–1
During normal running of the LHC, one of the most beautiful and delicate subdetectors of LHCb, the VErtex LOcator (VELO), sits only 5 mm away from the beam. Its mission is to identify the vertices where the B mesons are produced and decayed. Given the number of particles that will be produced closed to the beam direction, the VELO will receive a great deal of radiation in a short time. “The current VELO will have to be replaced after 3 to 4 years of nominal operation,” confirms Golutvin. “The work on the replacement VELO modules started in July this year and should be completed by April 2010. As for the rest of the detector, it is designed to withstand the radiation during the initial physics programme.”
LHCb is designed to run at a luminosity of a few times 1032 cm–2s–1, much smaller than the nominal LHC luminosity, 1032 cm–2s–1. This will be achieved by focusing the beams less at the LHCb collision point. The collaboration is considering a possibility for a major upgrade to work at an order of magnitude higher luminosity, after the initial physics programme is completed in about five to six years from now.
As with the other experiments at the LHC, the LHCb collaborations will use the first run to understand and calibrate the various parts of the detector. After that, it will start physics analysis at the same time as ATLAS and CMS. So just what does the collaboration expect? “As expressed by many people, the following three possible situations would be very exciting for particle physics,” says Golutvin. “The first one is that ATLAS and CMS see some new physics and we don’t. This will be very exciting for them and may be not too much for us. Still, the physics community will have to explain why the new physics does not seem to affect the quantum loop, in order to understand the exact nature of the new physics. Then there is the second option: ATLAS and CMS don’t see new physics while we see a clear deviation from the Standard Model. This might happen if the new particles are very heavy. We would see their virtual effect but they could not be directly produced at the LHC energies in the other experiments. Of course, the best case is if all the experiments see new physics effects and a coherent scenario can be built for this new physics.”
Nature alone knows which of these scenarios will eventually occur, but it could be that new physics might emerge quickly in LHCb, so Golutvin and the LHCb collaboration remain very optimistic.
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