The LHCf collaboration has measured the production spectrum of photons using the highest-energy accelerator beams in the world, at CERN’s LHC machine. With proton beams at 3.5 TeV the total collision energy is equivalent to when protons of 2.5 × 1016 eV strike a stationary target, which is an energy region that is of interest to cosmic-ray physicists.
The LHCf experiment consists of two independent calorimeters installed on either side of the ATLAS interaction point at the LHC. Using data obtained in 2010 during proton runs at 7 TeV in the centre-of-mass, the collaboration has measured the photons emitted into two very forward regions, that is, close to zero degrees to the beam direction, in the pseudo-rapidity ranges from 8.81 to 8.99 and from 10.94 to infinity (Adriani et al. 2011). To minimize contamination from beam-gas background and pile-up events, the team chose a limited but best dataset corresponding to an integrated luminosity of 0.68 nb–1. After selecting single photon-like events in common pseudo-rapidity ranges, they obtained consistent energy spectra from two detectors.
The collaboration has compared its data with the predictions from various hadron interaction models used in the study of cosmic-ray air showers, together with PYTHIA 8.145, which is popular in the high-energy-physics community. As the figure shows, there is significant deviation between the data and model above 2 TeV in the higher rapidity region. Three well known models – DPMJET 3.04, QGSJET II-03 and PYTHIA 8.145 – predict significantly higher photon yields than the experiment finds above 2 TeV, but agree reasonably well with the data at 0.5–1.5 TeV. The other models – SIBYLL 2.1 and EPOS 1.99 – do not predict such high photon yields but predict a smaller yield over the whole energy range. The difference is less marked in the lower rapidity region, but nevertheless none of the models shows perfect agreement with data.
The energy spectra of collision products at high-rapidities are crucial to understand correctly the development of cosmic-ray-induced air showers. Following recent notable improvements in observations of ultra-high-energy cosmic rays (UHECR), it is becoming increasingly important to reduce the uncertainty. The impact of the current LHCf results on cosmic-ray physics is now under study as the collaboration works together with theorists on further analyses of the data on neutral pions and neutrons. The data will also cast light on the energy dependence of hadron interactions and the extrapolation into the UHECR energy range. At the same time, the collaboration is studying the feasibility of data-taking during ion collisions (ion–ion and/or proton–ion), which would give a better simulation of cosmic-ray-air collisions.
The top quark was first observed in the mid-1990s by the CDF and DØ experiments at the Tevatron collider at Fermilab. These were produced and observed as top-antitop pairs, but it was not until 2009 that the two experiments reported the observation of single-top quarks. The ATLAS and CMS experiments at the LHC reported the first signs of top-antitop last summer, just a few months after the first collisions at a centre-of-mass energy of 7 TeV. Now, CMS has completed two complementary single-top analyses using the full data sample of 2010; that is, an integrated luminosity of 36 pb–1.
Such single tops are much more difficult to observe experimentally because they are produced at a lower rate and have a less distinctive signature compared with top-antitop pairs. This makes it more difficult to distinguish single-top events from the background physics processes.
In their recent analyses, the CMS collaboration focused on the production of single top via the so-called “t-channel W boson exchange” process in which the top quark emerges from the exchanged W together with a light quark. They observed the top quark through its decay into a W boson and a b-quark. The W boson was detected in turn through its decay to a charged lepton (electron or muon) plus a neutrino, while the jet from the b-quark was tagged by the high-precision silicon tracking detectors in CMS.
The two analyses establish the observation of single-top production by CMS with a statistical significance of about 3.5 σ. One analysis exploited the angular characteristics between the light quark jet and final-state lepton, shown in the figure, while the other used a multivariate analysis technique to separate the signal from the background. Data-driven background estimates were used in both these analyses. The two analysis methods were combined to yield a cross-section for single-top production in proton–proton collisions at 7 TeV of 83.6± 29.8 (stat+syst.)± 3.3 (lumi.) pb. This result agrees well with the rate predicted by the Standard Model.
Such a rapid detection of the elusive single top, despite the challenging background conditions, shows that the experiments are well prepared to detect and measure signals of new physics. These may soon manifest themselves as the LHC continues to produce ever more data at the high-energy frontier.
The ATLAS collaboration has announced its latest cross-section measurements of inclusive jet and dijet production, which involve final states containing at least one or two jets, respectively. Each jet is the result of a parton (quark or gluon) that emits radiation through the strong force, creating a collimated spray of hadrons.
These high-pT jet measurements confront QCD, the theory of the strong force, in a large and previously unexplored kinematic region in jet transverse-momentum and dijet invariant-mass. The measurements constitute one of the most stringent tests of QCD ever performed. They probe predictions of perturbative QCD, constrain the density of partons within the proton and are sensitive to new physics scenarios, such as quark compositeness, which may become apparent at very short distance scales.
The analysis uses the full data sample collected in LHC proton–proton collisions at 7 TeV during 2010, corresponding to an integrated luminosity of 37 pb–1. The results extend far beyond the kinematic reach achieved at the Tevatron, as do recent results from CMS (CMS collaboration 2011). The ATLAS results extend to 1.5 TeV in jet transverse-momentum (as in figure 1) and to 4.1 TeV in dijet invariant-mass. These jet measurements also provide unprecedented coverage out to forward rapidities of ΙyΙ < 4.4. Next-to-leading order perturbative QCD predictions are found to be in good agreement with the measured data across 10 orders of magnitude in cross-section (figure 2).
The jet cross-section measurements have been corrected for detector effects, and the analysis exploits a greatly improved understanding of the detector performance. The dominant source of systematic uncertainty is in the calibration of the jet energy scale, which has been determined to within 2.5% for central jets with pT above 60 GeV.
A publication is currently in preparation. Work is on-going to reduce the systematic uncertainties further and the collaboration will extend the kinematic reach of these exciting high-pT jet measurements with much larger datasets in 2011–2012.
Members of the international STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory have observed antihelium-4. This is the heaviest antinucleus detected so far, following the discovery of the first antihypernucleus (an antiproton, an antineutron and a Λ) by the same collaboration just a year ago. After sifting through 0.5 × 1012 tracks in data for 109 gold–gold collisions at centre-of-mass energies of 200 GeV and 62 GeV per nucleon–nucleon pair, the STAR collaboration found 18 events with the signature of the antihelium-4 nucleus, which is distinguished by its mass together with its charge of -2.
While the curvature of the tracks in the magnetic field of the STAR detector provide a momentum measurement, key information also comes from the mean energy-loss per unit track length, 〈dE/dx〉, in the gas of the TPC and from the time of flight of particles arriving at the time-of-flight barrel that surrounds the TPC. The 〈dE/dx〉 information helps in identification by distinguishing particles with different masses or charges, the time of flight being needed for identification at higher momenta, above 1.75 GeV/c. The figure shows the identification of isotopes based on energy loss and mass calculated from momentum in the region of helium-3 and helium-4 for both positive and negative particles, with 18 counts for antihelium-4.
The team used this observation to calculate the antimatter yield at RHIC and found that the production rate falls by a factor of 1.6 +1.0/–0.6 × 103 (1.1 +0.3/–0.2 × 103) for each additional antinucleon (nucleon). This is in line with the expectations from coalescent nucleosynthesis models, as well as from thermodynamic models.
The finding ties in with the scientific goals of the Alpha Magnetic Spectrometer launched on 16 May (AMS takes off), which will search for antimatter in space. It also nicely marks the centenary of the paper by Ernest Rutherford in which he analysed the scattering of helium nuclei (alpha particles) on gold and first established the existence of the atomic nucleus.
The goal of ALICE (A Large Ion Collider Experiment) is to measure the properties of strongly interacting matter generated in heavy-ion collisions at the LHC at CERN. On 7 November 2010, the LHC became the world’s most energetic heavy-ion accelerator when lead nuclei collided at a centre-of-mass energy √(sNN) = 2.76 TeV per colliding nucleon pair. This is an energy more than 10 times higher than that of the previous record holder, the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory in New York.
Quantum chromodynamics (QCD), the theory of strong interactions, predicts that at a temperature of about 170 MeV (2 × 1012 K), nuclear matter undergoes a phase transition from its normal hadronic state to a deconfined partonic phase, the quark-gluon plasma (QGP). This is about 100,000 times hotter than the core of the Sun, and such extreme conditions occur only under special circumstances. One such circumstance is the early universe, where the QGP filled all space a few microseconds after the Big Bang; another is the head-on collision of heavy ions at the LHC and RHIC, where a QGP may be created for a fleeting instant.
RHIC has now been running for a decade. One of its spectacular findings was that the matter generated in heavy-ion collisions flows like a liquid with very low internal resistance to flow, almost at the limit of what is allowed for any material in nature. This tells us that the constituents of this matter are quite different from freely interacting quarks and gluons. This almost-perfect fluid has been found to be opaque to even the most energetic partons (quarks and gluons), which appear as “jets” of particles from the collisions – an effect known as jet quenching. The physical mechanisms underlying these phenomena are not well understood. One of the first tasks of heavy-ion studies at the LHC is to “rediscover” these effects and probe them further with new tools as the basis for a much broader and deeper study of the QGP in the coming years. So what have we learnt at the LHC from heavy ions so far?
“Calibrating” at the LHC
To explore the features of hot QCD matter we have to calibrate our tools. Interpretation of the complex interaction of heavy-ions relies on theoretical modelling, beginning with the initial conditions of the hot system – the fireball – at the instant after the collision. One of the crucial inputs for calibrating the models is the distribution of the multiplicity (total number) of particles produced in a collision. This tells us a great deal about how the quarks and gluons in the incoming nuclei transform into the particles (pions, kaons, and so on) observed in the detector.
The number of generated particles is correlated with the impact parameter of the collision; that is, the distance between centres of the colliding nuclei. Small impact parameters, in which the colliding nuclei hit each other nearly head-on so that the largest number of incoming protons and neutrons “participate” in the collision, generate the most particles. Thus, ordering the ensemble of measured collisions according to their multiplicity allows them to be sorted into different classes of impact parameter. The number of created particles can also tell us about the energy density reached within the collisions and the temperature of the fireball.
Multiplicity measurements by the ALICE experiment show that the system created at the LHC initially has much higher energy density and is at least 30% hotter than at RHIC, resulting in about double the particle multiplicity for each colliding nucleon pair (Aamodt et al. 2010a). Figure 1 shows the energy dependence of particle production with the new measurement obtained at the LHC.
Perhaps surprisingly, despite their vastly different collision energies, the growth in particle multiplicity from RHIC to the LHC is similar at all impact parameters, as figure 2 shows (Aamodt et al. 2011). These measurements by ALICE also show that various predictions driven either by phenomenological extrapolation from the lower energies or by colour-charge density-saturation models are inadequate at the LHC.
A perfect liquid at the LHC?
Off-centre nuclear collisions, with a finite impact parameter, create a strongly asymmetric “almond-shaped” fireball. However, experiments cannot measure the spatial dimensions of the interaction (except in special cases, for example in the production of pions). Instead, they measure the momentum distributions of the emitted particles. A correlation between the measured azimuthal momentum distribution of particles emitted from the decaying fireball and the initial spatial asymmetry can arise only from multiple interactions between the constituents of the created matter; in other words it tells us about how the matter flows, which is related to its equation of state and its thermodynamic transport properties.
The measured azimuthal distribution of particles in momentum space can be decomposed into Fourier coefficients. The second Fourier coefficient (v2), called elliptic flow, is particularly sensitive to the internal friction or viscosity of the fluid, or more precisely, η/s, the ratio of the shear viscosity (η) to entropy (s) of the system. For a good fluid such as water, the η/s ratio is small. A “thick” liquid, such as honey, has large values of η/s. Comparison of the elliptic flow measured in heavy-ion collisions at RHIC with theoretical models suggests that the hot matter created in the collision flows like a fluid with little friction, with η/s close to its lower limit – the theoretical limit for a perfect fluid limit – given by η/s = ħ/4πkB, where ħ is Planck’s constant and kB is the Boltzmann constant.
In heavy-ion collisions at the LHC, the ALICE collaboration found that the elliptic flow of charged particles increases by about 30% compared with flow measured at the highest energy at RHIC of 0.2 TeV (figure 3). However, hydrodynamic calculations tuned to reproduce the results at RHIC – when recalibrated to the LHC energy regime – reproduce the new measurements well. The hot and dense matter at the LHC also behaves like a fluid with almost zero viscosity. With these measurements, ALICE has just begun to explore the temperature dependence of η/s and we anticipate many more in-depth flow-related measurements at the LHC that will constrain the hydrodynamic features of the QGP even further.
A basic process in QCD is the energy loss of a fast parton in a medium composed of colour charges. This phenomenon, “jet quenching”, is especially useful in the study of the QGP, using the naturally occurring products (jets) of the hard scattering of quarks and gluons from the incoming nuclei. A highly energetic parton (a colour charge) probes the coloured medium rather like an X-ray probes ordinary matter. The production of these partonic probes in hadronic collisions is well understood within perturbative QCD. The theory also shows that a parton traversing the medium will lose a fraction of its energy in emitting many soft (low energy) gluons. The amount of the radiated energy is proportional to the density of the medium and to the square of the path length travelled by the parton in the medium. Theory also predicts that the energy loss depends on the flavour of the parton.
Jet quenching was first observed at RHIC by measuring the yields of hadrons with high transverse momentum (pT). These particles are produced via fragmentation of energetic partons. The yields of these high-pT particles in central nucleus–nucleus collisions were found to be a factor of five lower than expected from the measurements in proton–proton reactions. ALICE has recently published the measurement of charged particles in central heavy-ion collisions at the LHC. As at RHIC, the production of high-pT hadrons at the LHC is strongly suppressed. However, the observations at the LHC show qualitatively new features (see box). The observation from ALICE is consistent with reports from the ATLAS and CMS collaborations on direct evidence for parton energy loss within heavy-ion collisions using fully reconstructed back-to-back jets of particles associated with hard parton scatterings. The latter two experiments have shown a strong energy imbalance between the jet and its recoiling partner (G Aad et al. 2010 and CMS collaboration 2011). This imbalance is thought to arise because one of the jets traversed the hot and dense matter, transferring a substantial fraction of its energy to the medium in a way that is not recovered by the reconstruction of the jets.
With the first findings on hydrodynamic features of the medium created at the LHC and its opaqueness to energetic partons, the LHC has, to a large extent, reproduced measurements at RHIC. The measurements at the LHC will, however, profit from the denser medium and its longer lifetime. The vast kinematic reach provided by the higher-energy collision system enables qualitatively new measurements of the QGP.
On 23–28 May, Quark Matter, a key conference in heavy-ion physics, takes place in Annecy. The most recent experimental results and theoretical state-of-the-art concepts and calculations will be presented, targeted at the detailed understanding of QGP at RHIC and at the LHC. The ALICE collaboration will report on the observations discussed here and will also present new, in-depth studies of the elliptic flow with respect to the type of particle and its mass. Also, the first studies addressing the interplay between collective features of the medium and jet production at the LHC will be shown. Moreover, ALICE will present its first insight into the energy loss of heavy flavour (charm and bottom quarks) in the hot QCD medium. In the coming years, all of these crucial measurements will help to uncover the key properties of the QGP at the LHC.
The 13th International Conference on B-Physics at Hadron Machines (Beauty 2011) was held at the Felix Meritis building in the historic centre of Amsterdam on 4–8 April. Hosted by Nikhef, the National Institute for Subatomic Physics of the Netherlands, the meeting attracted about 100 participants, including experts from Europe, America and Asia. There were 60 invited talks.
The main topic was the physics of Bq mesons, which consist of a b (“beauty”) quark and an anti-q quark, where q can be an up, down, strange or charm quark. These particles offer interesting probes for precision tests of the Standard Model. In this context, asymmetries between decay rates of B and B mesons, which violate the charge-parity (CP) invariance of weak interactions, play a key role. Such observables and various strongly suppressed rare decays of B mesons show a sensitivity to “new physics”, thanks to the possible impact of the contributions of new particles to virtual quantum loops.
The search for these indirect footprints of physics beyond the Standard Model through high-precision measurements is complemented by the search for direct signals of new particles at high-energy colliders. Here, physicists aim to produce new particles (such as supersymmetric squarks or new gauge bosons) and to study their decays in general-purpose detectors – ATLAS and CMS, in the case of the LHC at CERN. The exploration of heavy flavours, and the B-meson system in particular, is the target of the LHCb experiment, which is exploiting the many B mesons that are produced in the proton–proton collisions at the LHC.
Studies of CP violation
The Beauty conferences traditionally have a strong focus on studies of B mesons at hadron machines. In the previous decade, this field was the domain of the CDF and DØ experiments at the Tevatron, the proton–antiproton collider at Fermilab. The electron–positron B factories at SLAC and KEK, with the BaBar and Belle detectors respectively, were the first to establish CP violation in the B-meson system, while the Tevatron experiments have extended the measurements into the Bs-meson sector, which is still poorly explored. These studies have shown that the Cabibbo-Kobayashi-Maskawa matrix is the dominant source of flavour and CP violation, in accordance with the Standard Model.
However, there is evidence that this model is not complete and recent studies of Bs-decays by the CDF and DØ collaborations give a hint of new sources of CP violation in a quantum-mechanical phenomenon, Bs–Bs mixing – although the uncertainties are still too large to draw definite conclusions. In specific scenarios for physics beyond the Standard Model (such as supersymmetry and models with extra Z’ bosons), it is actually possible to accommodate the effect of new physics of this kind.
The first physics results from the LHC experiments were the main highlight of Beauty 2011. It was impressive to see the wealth and high quality of the data presented. The LHCb collaboration’s presentation of the first analysis of the CP-violating observables of the Bs → J/Ψφ decay was particularly exciting. Although the experimental errors are still large, it is intriguing that the data seem to favour a picture similar to the results from CDF and DØ, mentioned above. Fortunately, the LHCb experiment should be able to reduce the uncertainties significantly within a year, with the prospects of revealing new phenomena in Bs–Bs mixing.
Quantum loops
Another exciting decay in which to search for new physics is the rare decay Bs → μ+μ–, which originates from quantum-loop effects in the Standard Model. New particles running in the loops or even contributing at the tree level may significantly enhance the decay rate. So far, this decay has been the domain of the CDF and DØ experiments; they have put upper bounds on the branching ratio that are still about one order of magnitude above the Standard Model prediction. Now LHCb has entered the arena, presenting a first upper bound that is similar to the results from the Tevatron. The constraints from LHCb, and soon those from ATLAS and CMS, will quickly become stronger and it will be interesting to see whether eventually a signal for Bs → μ+μ– will emerge that is significantly different from the predictions of the Standard Model.
In addition to these key channels that are facilitating the search for new physics in B decays in the early phase of the LHC, the conference covered a range of other topics. Results on heavy-flavour production were presented with the first LHC data collected in the ATLAS, CMS, LHCb and ALICE experiments. Another interesting topic was charm physics, with results from the BES III experiment, CDF and the first analyses from LHCb. A summary was given of B-factory results on the measurement of CP violation and the unitarity triangle parameters and the status of lepton-flavour violation and models of physics beyond the Standard Model was also presented. Moreover, the potential of upcoming B-physics experiments – SuperB, SuperKEKB and the LHCb upgrade – was discussed.
The many experimental presentations were complemented by theoretical review talks. Theory also figured in the conference summaries, in which Andrzej Buras of the Technische Universität München developed a vision for theory for 2011 and beyond, while the outgoing LHCb spokesperson, Andrei Golutvin, highlighted the experimental results. The discussions about physics also continued in an informal way during a tour on historic boats through the canals of Amsterdam, with people enjoying the spectacular weather and a visit to the Hermitage museum where the conference dinner was held.
Beauty 2011 showed that these are exciting times for B physics, with plenty still happening at the Tevatron and the first physics results from the LHC. It will be interesting to see whether the data collected by LHCb and the general-purpose detectors in 2011 will already reveal new physics in the B-meson sector. Flavour physics is moving towards new frontiers and is a fascinating part of the LHC adventure. Correlations between various flavour-physics observables and the interplay with the direct searches for new particles will play a key role in obtaining insights into the physics lying beyond the Standard Model.
For further information and the slides of the presentations, visit the conference webpage www.beauty2011.nikhef.nl.
The ALICE collaboration has measured the size of the pion-emitting system in central lead–ion collisions at the LHC at a centre-of-mass energy of 2.76 TeV per nucleon pair. The radii of the pion source were deduced from the shape of the Bose-Einstein peak in the two-pion correlation functions.
In hadron and ion collisions, Bose-Einstein quantum statistics leads to enhanced production of bosons that are close together in phase space, and thus to an excess of pairs at low relative momentum. The width of the excess region is inversely proportional to the system size at decoupling, i.e. at the point when the majority of the particles stop interacting.
An important finding at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven was that the QCD matter created there behaved like a fluid, with strong collective motions that are well described by hydrodynamic equations. The collective flow makes the size of the system appear smaller with increasing momentum of the pair. This behaviour is also clearly visible for the radii measured at the LHC in the ALICE experiment. Figure 1 shows the results for measurements of the radius of the pion source in three dimensions: along the beam axis, Rlong; along the transverse momentum (kT) of the pair, Rout; and in a direction perpendicular to these two, Rside.
The similarity between the values for Rout and Rside indicates a short duration for the emission, hence an “explosive” emission. The time when the emission reaches its maximum – measured with respect to the first encounter – can be derived from the dependence of the longitudinal radius on the transverse momentum, Rlong(kT). ALICE has found this to be 10–11 fm/c, which is significantly longer than it is at RHIC. Moreover, the product of the three radii at low pair-momentum – the best estimate of the homogeneity volume of the system at decoupling – is twice as large as at RHIC (figure 2).
These results, taken together with those obtained from the study of the multiplicity and the azimuthal anisotropy, indicate that the fireball formed in nuclear collisions at the LHC is hotter, lives longer and expands to a larger size than at lower energies. Further analyses, in particular including the full dependence of these observables on centrality, will provide more insights into the properties of the system – such as initial velocities, the equation of state and the fluid viscosity – and strongly constrain the theoretical modelling of heavy-ion collisions.
Measurements with leptons are an important tool for physics studies at the LHC. While electrons and muons – being the easiest to detect and identify – are used for many analyses, studies that include τ leptons are important for searches and for electroweak measurements in particular. It is a sign that experimental analyses are reaching maturity when physics results on τ leptons become available, as they are now doing with CMS.
The lifetime of the τ is of the order of 10–13 s, so it decays shortly after production, complicating its identification and use in physics analyses. It decays most often leptonically, into an electron or muon plus two neutrinos, or hadronically to either one or three charged particles together with neutral hadrons and a neutrino. The hadronic decays of the τ thus contain collimated low-multiplicity jets, a feature that is used experimentally to select τ decays, while reducing background from QCD jets.
CMS recently published two physics papers studying decays into τ leptons. The first presents a study of the decay of Z bosons into τ pairs, using both leptonic and hadronic decays of the τ (CMS collaboration 2011a). The τ leptons are identified via isolated groups of particles, found through the CMS particle-flow event reconstruction, that are compatible with the possible τ decays. Figure 1 shows the visible invariant mass of the two τ candidates for a τ pair, where one decays leptonically to a muon and the other decays hadronically. Because of the escaping neutrinos in the τ decays the reconstructed Z boson mass is not at its known value, but the result of the measurement agrees well with the expectation from the Monte Carlo simulation.
This yields a cross-section for Z → ττ, in proton–proton collisions at 7 TeV, of 1.00 ± 0.05 (stat.) ± 0.08 (syst.) ± 0.04 (lumi.) nb. This agrees well with similar cross-sections measured in the electron and muon decay modes of the Z – as is expected from the lepton universality in Z decays that was established in precision measurements by experiments in the 1990s at the Large Electron Positron collider.
More interestingly, the τ can be used to search for new particles, for the Higgs boson in particular. Higgs particles in the minimal supersymmetric extension of the Standard Model (MSSM) are expected to show a large decay-rate to τ pairs, especially for large values of the parameter tanβ, which is the ratio of the vacuum expectation values of the two members of the Higgs doublet.
CMS has carried out such an analysis with the full data sample of 2010 and found no excess of τ pair production above the expected background (CMS collaboration 2011b). The resulting excluded region in the plane of tanβ and the mass of pseudoscalar Higgs boson in the MSSM, for a benchmark scenario called mhmax, is shown in figure 2.
The surprise is that the search already goes well beyond the reach of the searches at the Tevatron, in part thanks to the high efficiency and high quality of the detection and reconstruction of the τ leptons in CMS. Clearly, the τ has now become an important tool for the collaborations in exploring the new energy region at the LHC.
The CDF collaboration at Fermilab’s Tevatron has published two measurements that hint at the existence of physics beyond the well tested Standard Model of particles and their interactions. The first measurement revealed an unexpected asymmetry in the production of top/anti-top (tt) quark pairs. The second analysis unveiled surprising evidence for an excess of events that contain a W boson accompanied by two hadronic jets. The excess cannot be due to the long sought-after Higgs boson but could perhaps be explained by new physics ideas.
While both measurements rely on the Tevatron’s unique ability to produce proton–antiproton collisions, if the new physics hinted at in these results does exist, it will manifest itself in some other form in the particle collisions at the LHC at CERN.
The Tevatron has been producing tt pairs since the early 1990s. In first-order Standard Model calculations, the direction of flight of tt pairs produced in proton–antiproton collisions should be independent of the colliding particles’ charge, thus there should be equal numbers of t and t quarks emitted along either beam direction. More detailed, next-to-leading-order calculations predict an asymmetry of 9 ± 1% at large rapidity, favouring the proton beam’s direction.
CDF announced in March that it measured a tt production asymmetry of 48 ±11% for an invariant mass of the tt pair (Mtt) larger than 450 GeV/c2, which is three standard deviations above the Standard Model expectation. The result is based on the analysis of 5.3 fb–1 of collision data, about half of the number of collisions that CDF has recorded to date. The asymmetries were observed in both the laboratory frame of reference and the tt rest frame. A number of theoretical models predict such asymmetries, including models with a Z’ or large extra dimensions.
The analysis was repeated more recently on events where the t and t quarks decay to a different final state. The asymmetry was again measured at close to a 3 σ level with a value of 0.42 ± 0.15 ± 0.05, averaged over all masses, compared with a 6% Standard Model expectation (T Aaltonen et al. 2011a). This confirms the earlier result with a completely independent data sample.
The second surprising result from CDF started out as a routine Standard Model measurement of collisions, where a W boson was detected in coincidence with two hadronic jets. The team found an unexpected peak in the spectrum of the invariant mass of the pair of jets. The excess of approximately 250 events appeared as a bump around 144 GeV/c2 (T Aaltonen et al. 2011b).
The analysis required the presence of a high-transverse-momentum, isolated lepton; a significant amount of missing energy; and two hadronic jets. The invariant mass spectrum of the jet pair shows a clear peak at 80–90 GeV from a W or Z boson decaying into a jet pair. The surprising peak shows up at a higher mass (figure 2). It has a width compatible with the CDF detector resolution and its significance is 3.2 σ, which takes into account systematics and trial factors. If the peak is from a single particle, the particle would have a production cross-section of approximately 4 pb–1.
The peak cannot result from the Higgs boson predicted by the Standard Model. If a Higgs boson had a mass of 140 GeV/c2 and such a large production rate, both the CDF and DØ experiments at the Tevatron would have seen its decay into pairs of W bosons a long time ago. Furthermore, such a Higgs would decay mainly into bottom-quark jets, which are not observed in an appreciable amount in the CDF data peak. There are, however, new physics ideas that predict the appearance of resonances with the observed features, such as technicolour-based models. If the peak does not originate from a new particle, particle physicists will need to reconsider how the Standard Model is used to make precise predictions for the production of a W boson and two jets.
Physicists from CDF and DØ are in the process of analysing larger data samples, up to 10 fb–1, to either refute or confirm these two results. At the same time, they may find even more interesting signals.
“Why still do experimental quantum electrodynamics, isn’t everything known?” This provocative question is often heard by the collaborators at one of the smaller CERN experiments, NA63. Their answer is almost as short as the question: it is precisely the fact that everything is supposed to be known that makes it interesting. This understanding enables the exploration of physics in regimes of strong electromagnetic fields, for example as a function of interaction times or in studies of scattering. The results cast light on phenomena in various branches of physics.
Take, as an example, the emission of beamstrahlung, which is expected in the next generation of electron–positron linear colliders, such as the Compact Linear Collider (CLIC) currently under conceptual design at CERN. Particles in a bunch of particles in one beam “see” the electric field in the opposing bunch as boosted by 2γ2–1, where γ is the Lorentz factor. This appears as a strong electric field in the bunch’s rest frame and leads to the emission of intense synchrotron-like radiation, which is known as beamstrahlung. The electric field seen by the particles is comparable to the so-called critical field, which depends only on the reduced Planck’s constant, ħ–, the speed of light, c, and the mass, m, and charge, e, of the electron – m2c3/h–e – and is equivalent to 1.32 × 1016 V/cm and a corresponding magnetic field of 4.41 × 109 T. In such fields, quantum corrections to the emission of synchrotron radiation become important in determining the emission spectrum. They lead to a strong suppression when compared with the classical calculations that are applicable in most other contexts for synchrotron radiation emission.
Into the laboratory
The effects of strong fields are also relevant in many other branches, ranging from the so-called “bubble-regime” in plasma wakefields used for extremely high-gradient particle acceleration, through astrophysical objects such as magnetars, to intense lasers and heavy-ion collisions. The concept even applies in a gravitational analogue – Hawking radiation. Therefore, further investigation of the underlying phenomena is of broad interest.
Clearly, electric fields of the order the critical field are inaccessible in the laboratory. However, by replacing the opposing bunch in the example of a linear collider by a crystalline target, processes linked to the critical field can be studied with relative ease because the crystalline electric fields are orders of magnitude higher. At small angles of incidence to a crystallographic axis or plane, the strong electric fields of the nuclear constituents add coherently to form a macroscopic, continuous field with a peak value around 1011 V/cm. In the rest frame of an ultra-relativistic electron with γ around 105, the field encountered by the incident particle thus becomes comparable to the critical field.
Applications of these strong crystalline electric fields are widely known, in particular in “channelling”, where a beam of charged particles is steered by the fields within a crystal. This has been used, for example in the NA48 experiment at CERN, to deflect a well defined fraction of the main proton beam for the generation of kaons.
The NA63 experiment, following on from its predecessor, NA43, focuses on fundamental investigations of the strong fields themselves. The results have already shown that the emission of synchrotron radiation in the quantum regime is, indeed, well understood, being strongly suppressed as expected. These results mean that reliable estimates based on QED of beamstrahlung in future machines can now be made. In addition, the spin-flip component of the synchrotron-like radiation that is emitted as the beam passes through the crystal is many orders of magnitude higher in energy and intensity than that of a storage ring, with corresponding polarization times of femtoseconds instead of hours.
Strong scattering effects
The suppression in the emission of radiation arises loosely speaking because the field becomes so strong that the particle is deviated out of the formation zone necessary for the generation of the photon – in effect before it has time to generate the radiation. It is equivalent to a shortening of the formation zone. Although the concept of the formation zone was introduced more than 50 years ago by the Armenian physicist Mikhail Ter-Mikaelian, it is still a surprise to many that it can take time corresponding to macroscopic travel distances for a relativistic electron to emit a photon. This is the basis of the Landau-Pomeranchuk-Migdal (LPM) effect, where multiple scattering within the formation length leads to a reduction in radiation emission.
Figure 1 illustrates the suppression mechanism at play. It depicts the electric field from a particle, incident along the dashed line, that has scattered twice (at locations marked by crosses). Outside a radius given by the time since the scattering event, the field points towards the location that the particle would have had if it had not scattered. This is a result of the finite propagation time of information; inside the corresponding sphere, the field follows the particle. The transverse components correspond to radiation and, because of the short time between the scattering events, they are closely spaced and pointing in opposite directions. A distant observer looking at low frequencies will see two electric field lines that mutually cancel – and, therefore, less radiation. It is as if a “semi-bare” electron is interacting.
However, as the NA63 collaboration has recently shown, if a particle impinges on a target that is so thin that the formation zone extends beyond the target, then the LPM suppression is alleviated. To study this effect the collaboration measured the radiation emission from ultra-relativistic electrons in targets consisting of a number of thin foils of tantalum corresponding to 0.03%–5% radiation lengths. They found that, for the thinnest targets, the radiation emission agrees with expectations from the Bethe-Heitler formulation of bremsstrahlung, with the target acting as a single scatterer. Only as the thickness increases does the distorted Coulomb field resulting from the first scattering lead to a suppression of radiation emission in subsequent scattering such that the radiation yield becomes a logarithmic function of the thickness, eventually to become LPM suppression (Thomsen et al. 2010).
The NA63 collaboration has also studied higher-order processes, such as “trident production”, in which an electron impinging on an electromagnetic field produces a positron–electron pair directly through the emission of a virtual photon. The process is illustrated in figure 2 in a reference frame close to the rest frame of the incident electron, in which the field has the critical value. In the laboratory frame, the original particle plus the pair are all directed forwards in a three-prong pattern, giving rise to the name “trident”. The effect is reminiscent of a phenomenon studied by Oskar Klein and Fritz Sauter 80 years ago – the so-called Klein paradox. Klein was one of the first to do calculations using the celebrated equation of Paul Dirac. In 1929 Klein looked at the probability of reflection of an electron from the steep potential barrier provided by an electric field and found that the probability for transmission into a potential of infinite height approached the velocity of the incident electron in units of the speed of light, i.e. that transmission into a “forbidden” region approaches certainty. Soon after, Sauter found that the process takes place for electric fields beyond the critical field, i.e. when the field is so high that an electron transported over a Compton wavelength produces its rest mass, mc2. Today, this process is understood in terms of pair production at the boundary, but without knowledge of the positron this was an impossible conclusion for Klein, hence the name “Klein paradox”.
Studies by NA63 of trident production, with crystals of germanium a few hundred micrometres thick, have shown a similar phenomenon: that when the crystal is turned to an axial direction along the beam, giving rise to a critical field in the particle’s rest frame, the trident process increases significantly (Esberg et al. 2010). Recent calculations have shown that trident production is an important factor in the design of the collision zone at CLIC, underlining the relevance of these experimental investigations.
A suppression mechanism also occurs in the case of pair production. In this case mutual screening of the charges in the pair substantially reduces the energy deposition in matter in the vicinity of the creation vertex. Because of the directionality of the pair, at high energies this internal screening – the King-Perkins-Chudakov effect – takes place over a distance of several tens of micrometres. This is a distance comparable to the sensitive layers in a CCD or a silicon vertex detector (figure 3), which can be used to study the effect.
Finally, as Allan Sørensen of Aarhus University has recently calculated, bremsstrahlung from relativistic heavy ions is expected to show a peak-structure connected to the finite size of the nucleus. The detection of this effect is among the future plans of NA63.
So QED still presents challenges, even for the otherwise well known case of radiation emission. In the words of one of the originators of the quantum theory of beamstrahlung, Richard Blankenbecler: “It is surprising that there is so much more to learn about such a well understood process.”
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