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Getting heavy on Capri

CCfla1_10_08 — Participants in the garden of the Villa Orlandi in Anacapri, where the heavy flavour physics workshop was held.
Image credit: G Ricciardi.

The Second Workshop on Theory, Phenomenology and Experiments in Heavy Flavour Physics took place on 16–18 June in Anacapri, Capri, Italy – the same location as the first meeting in the series in May 2006. The aim of the series is to bring together theoreticians and experimentalists to develop a dialogue on phenomenological issues. The focus is on discussion and interaction among physicists who are active in the field. The topics this year focused on results, especially in B-physics, as well as exploring the potential for heavy flavour physics in both current and future experiments. With participation by invitation only, owing to space problems at the venue, the 60 or so attendees took part in many fruitful and lively discussions after the seminars and during the free time.

During the past decade flavour physics has witnessed unprece-dented experimental and theoretical progress, opening up the era of precision tests of the Standard Model. The quark and lepton sectors of the Standard Model have been subjected to a series of stringent tests, and it has become customary to look for violations of the Standard Model by using unitarity triangles. Updates of the unitarity triangle analyses were presented by Marco Ciuchini from Rome III/INFN and Jérôme Charles from the Centre de Physique Théorique, Marseille, for the UTfit and CKMfitter collaborations, respectively.

Such updates have been possible, not only because of the large amount of data now available, but also through theoretical progress (e.g. in lattice calculations). As Chris Sachrajda from the University of Southampton and Fermilab’s Paul Mackenzie reported, many approximations in typical lattice calculations have been overcome in recent years. Numerous simulations are now being performed that include quark–antiquark pairs, or a pion mass less than or equal to 300 MeV. Moreover, it is now becoming possible to generate configurations on lattices that are sufficiently fine and large to allow direct simulation of the charm quark.

An impressive amount of data has come from the two asymmetric e⁺e^– B factories, PEP-II and KEKB, and their respective detectors, BaBar and Belle. The BaBar experiment concluded data taking in April 2008, having collected a total of 531 fb^–1, of which 433 fb^–1 was on the Υ(4S) peak, corresponding to about 470 × 10⁶ BB pairs. In 2008, BaBar also collected 30 fb^–1 on the Υ(3S) resonance and 14 fb^–1 on the Υ(2S) resonance, with interesting results, such as the first evidence of the η_B – the long-sought bottomonium ground state. By the time of the workshop, Belle had accumulated about 850 fb^–1, with 730 fb^–1 on the Υ(4S) resonance.

The B factories have led the recent progress in knowledge of the unitarity triangle related to the B system, with angles α, β and γ (or φ2, φ1 and φ3). Christoph Schwanda from the Austrian Academy of Sciences and Giuseppe Finocchiaro from the Frascati National Laboratory/INFN reported on measurements of the angles and sides of the unitarity triangle. Paolo Gambino of Torino and Francesca Di Lodovico from Queen Mary, University of London, reviewed the main theoretical problems on the way to the long-sought precise theoretical inclusive determination of ΙV_ubΙ in the Cabibbo–Kobayashi–Maskawa (CKM) matrix.

Golden modes and penguins

The B factories’ golden modes for the extraction of sin2β are b → cc decays, and the latest measurement from BaBar gives sin2β = 0.714 ± 0.032 ± 0.018, in agreement with the results from Belle. Other interesting decays are the b → sqq “penguin dominated” decays, those study of which is motivated not only by the measure of sin2β, but mostly by the search for new physics.

The angle α can be studied in b → uud modes, and its determination is made complicated by the addition of the b → d penguin amplitude to the b → uud tree one. The first measurements of the B⁰ → ρ⁰ρ⁰ decay confirm the indication that the effect of penguin amplitudes is relatively small in ρρ decays, which in fact yield the most stringent constraints on α.

A precise measurement of the unitarity angle, γ, unthinkable when the B factories started, is now becoming possible with the large statistics accumulated by the B factories. Several new measurements of B^± → D⁰ K^± transitions have appeared recently, and strong evidence for direct CP violation in these decays is building up.

With the first runs of the LHC on the horizon, heavy flavour physics is entering a new phase.

Radiative penguin and leptonic B meson decays are another area of interest at B factories because they constitute a powerful probe of new physics. John Walsh of INFN/Pisa reported on recent experimental results in this field.

On the theory side, Luca Trentadue from Parma/INFN discussed the resummation of large logarithmic terms, which otherwise spoil the convergence of the perturbative series in the threshold region, in semileptonic charmed B decays. Cai-Dian Lü of the Institute of High Energy Physics (IHEP), Beijing, analysed charmless two-body decays of the B mesons to light vector and pseudoscalar mesons in the soft-collinear effective theory.

With the first runs of the LHC on the horizon, heavy flavour physics is entering a new phase. Natalia Panikashvili of Michigan, Andrei Starodumov of PSI and Stefania Vecchi of INFN/Ferrara described the role of flavour physics at the LHC for the ATLAS, CMS and LHCb experiments, respectively. Tobias Hurth of CERN/SLAC stressed that a large increase in statistics at LHCb for B_d → K ^*0μ⁺μ^– will make measurements possible with much greater precision, allowing for an indirect search for new physics.

A main goal at the LHC is to measure CP-violating parameters, such as the B_s⁰ mixing phase, which in the Standard Model is predicted to be small, and could be another way of evincing new physics. Luca Silvestrini of INFN/Roma presented a new analysis that claims evidence of new physics through a B_s⁰ mixing phase, that is much larger than expected in the Standard Model. However, this had not been confirmed by independent analyses at the time of the workshop. Future data from the Tevatron or an extended Υ(5S) run of Belle may be of help in assessing the new results. Joaquim Matias of the Institute for High Energy Physics, Barcelona, discussed poss-ible strategies to measure the weak mixing phase of the B_s⁰ system using B mesons decaying into vectors.

CCfla2_10_08 — A boat trip social event offered the opportunity to see more of the island’s spectacular and stunning surroundings.
Image credit: G Ricciardi.

Charm physics, charm spectroscopy, CP violation in charm decays and searches for new physics were all discussed at length during the workshop. Ikaros Bigi of Notre Dame stated that comprehensive and detailed CP studies of charm decays provide a unique window into flavour dynamics. He emphasized the importance of LHCb in achieving the statistics required to find evidence of new physics in the charm sector. There is a powerful programme at LHCb for charm physics, which includes studying D⁰ mixing, observed for the first time at B factories in spring of last year, and CP violation in some specific decays. Pietro Colangelo of INFN/Bari discussed aspects of new charm spectroscopy and David Miller of Purdue presented the latest results from CLEO-c on W annihilation decays of the D⁺ and D_s⁺ mesons. The HERA electron–proton storage ring at DESY had come to the end of its scheduled operation roughly a year before the workshop, but data are still being analysed. Luis Labarga of the Universidad Autónoma de Madrid presented recent results from the H1 and ZEUS collaborations on charm production and fragmentation, in addition to results on beauty production.

The experiment BESII, at the Beijing Electron Positron Collider has accumulated 5.8 × 10⁷ J/ψ events, 1.4 × 10⁷ ψ(2S) events and 35.5 pb^–1 ψ(3770) data. Xiaoyan Shen of IHEP reported on recent results, which include the observation of the Υ(2175) in the J/ψ → ηφf₀(980) decay. In the past few years a wealth of new experimental results on heavy quarkonia and exotic states has become available. Riccardo Faccini of Rome “La Sapienza”/INFN summarized recent developments in the search for excited states of the scalar nonet among the light mesons, and reviewed the experimental evidence for new states. Ruslan Chistov of the Institute of Theoretical and Experimental Physics, Moscow, discussed the experimental status of X, Y and Z states at B factories. More new states, decays and production mechanisms have been discovered in the past few years than in the previous 30 years. Besides the regular quarkonium states (mostly quark–antiquark states), new exotic states have been found the composition of which (molecule, tetraquark, hybrids&ellip;) is currently the centre of a lively debate. It was the subject of a dedicated round table at the workshop, chaired by Nora Brambilla of Milan, with Antonello Polosa of INFN/Rome, Joan Soto of Barcelona and Antonio Vairo of Milan.

Flavour for all

There were many lively discussions on the role of flavour to evince new physics. Andrzej Buras of the Technical University of Munich reviewed several main results on flavour physics beyond the Standard Model, analysing in particular flavour and CP-violating processes in models with supersymmetry, “littlest Higgs” and extra dimensions. Ulrich Nierste of Karlsruhe summarized the role of the decays B⁰ → μ⁺μ^–, B⁺ → τ⁺ν_τ and B → D τ ν_τ in the hunt for new Higgs effects in the minimal supersymmetric Standard Model. Flavour and precision physics in the Randall–Sundrum model were the topics of discussion for Matthias Neubert of the Johannes Gutenberg University, Mainz, while Jernej Kamenik of INFN/Frascati talked about phenomenology in the context of minimal flavour violation.

While waiting for the LHC, Fermilab’s Tevatron has not only demon-strated the possibility of B-physics at hadron machines but also produced measurements that are highly competitive and complementary to those of the B factories. In particular, this was via the unique access that hadron machines have to the B_s sector. Vaia Papadimitriou of Fermilab and the CDF experiment, and Brad Abbott of Oklahoma and D0, presented the latest measurements on the production, spectroscopy, lifetimes and branching fractions for B mesons, B baryons, and quarkonia. Theoretical issues for the measurement of the top mass using jets, and implications for measurements of the top mass at the Tevatron, were discussed by Iain Stewart of the Massachusetts Institute of Technology. Monte Carlo programs have already proved indispensable for making exclusive theoretical predictions at the Tevatron. Christian Bauer from Lawrence Berkeley National Laboratory presented an improved Monte Carlo framework (GenEvA) mainly based on a new notion of phase space.

Tom Browder of Hawaii and Marcello Giorgi of INFN/Pisa made a strong science case for continued heavy flavour physics measurements at future Super-B machines, underlining their complementarity to the LHC programme. One focus for the Super-B factories would be studying, and possibly discovering, new sources of flavour-changing neutral currents and CP-violation. There are plans for a Super-B factory at KEK in Japan based on the existing KEK accelerator and Belle detector, as well as a proposal for a laboratory in Italy, the SuperB project. In both cases the goal is a luminosity of around 1 × 10³⁶ cm^–2s^–1, approximately 60 times as high as achieved at present B factories.

The conference was extremely lively, and most goals of the workshop, such as promoting co-operation and a fruitful exchange of ideas among theoreticians and experimentalists, were fulfilled. Up to now, the data agrees globally with the CKM picture, but there are also hints of discrepancies, which if confirmed could signify new physics. With the advent of the new machines, it will be feas-ible to investigate possible flavour structure and new sources of CP violation beyond the Standard Model through studies of flavour processes. Heavy flavour, and therefore physics, continues to play a fundamental role in particle physics and has an exciting future.

SRF technology comes full circle

Nearly a half-century ago, researchers at Stanford University began investigating superconducting RF (SRF) acceleration. They would not have been surprised to learn that by 1994, SRF had come into large-scale use in Jefferson Lab’s Continuous Electron Beam Accelerator Facility, or that by 2008 it was planned as the enormous, ultra-cold, dynamic-but-delicate heart of the proposed International Linear Collider (ILC). Nor would they be surprised to learn that this complex technology’s challenges nevertheless continue to vex accelerator builders. In my view, it’s time for the accelerator community to go back to where the pioneers at Stanford began, hit the pause button, and take a careful look at more than four decades of SRF R&D.

CCvie1_11_08 — Myneni: “It’s time to go back to where the pioneers of Stanford began.”
Image credit: Jefferson Lab.

Such a renewed learning effort is needed because SRF technology is not only complex and vexing, it’s vital and expensive. Some 16,000 SRF accelerating cavities will be made for the ILC from hundreds of tons of the soft, ductile metal niobium, which becomes superconducting when refrigerated to nearly absolute zero. This is a major component of the ILC’s immense cost.

Niobium-based SRF is also in use or in planning at other projects – for example, Oak Ridge National Laboratory’s Spallation Neutron Source, Fermilab’s Project X, the Facility for Rare Isotope Beams, compact accelerators for university laboratories, accelerator-driven systems for nuclear power production in India, DESY’s XFEL, and energy-recovering linear accelerators driving fourth-generation light sources, such as Jefferson Lab’s free-electron laser.

CCvie2_11_08 — At high magnification, the micrograph shows the many submicron grain boundaries in polycrystalline niobium after expensive modern-day processing.
Image credit: DESY.

Though condensed-matter physicists participated when the field began at Stanford, SRF has long since become highly specialized – maybe even too highly specialized. SRF scientists devote careers to the study of elaborate cavity design and preparation processes. Much effort has gone into development and assessment of techniques that have become standard in SRF, such as buffered chemical polishing, electropolishing and high-pressure rinsing of niobium surfaces. Much effort has been expended to overcome or circumvent the contamination problems introduced in pumping to attain the stringent vacuum conditions needed for superconducting operation.

Many of these efforts have involved, or have even begun with, the issue of the purity of the niobium material. Yet if you look back, you find that during the 1960s, Stanford’s pioneers used niobium of a purity that was not even known. The metal was electron-beam melted into the ingots from which cavities were machined. Without even addressing the purity issue, those early researchers demonstrated high performance and very high quality factors in one type of SRF cavity, the X-band pillbox cavity.

Later, to reduce the cost of larger L-band SRF cavities, researchers at Stanford switched to fine-grain niobium sheets, using 1800 °C annealing to increase the grain size – that is, to enlarge the crystals giving structure to the metal. By reducing the availability of cracks between grain boundaries, this enlargement crucially reduced the potential for hydrogen inclusion in those cracks. Hydrogen, both in the cracks and directly on the material surface, is recognized today as SRF’s major performance limiter.

CCvie3_11_08 — Without magnification, the photo shows the very few grain boundaries in medium-purity niobium simply sliced from an ingot – a decades old technique.
Image credit: CBMM Brazil.

The use of high-purity niobium was not specified until later, although in the 1970s Siemens in Germany used fine-grain niobium of low purity, and demonstrated state-of-the-art peak surface magnetic fields – at levels that would still be impressive today. Researchers at Siemens enlarged the fine-grain structure with 1400 °C annealing, which led to a grain size so large as to have visible boundaries – and thus led also to a reduction in the grain-boundary inclusion of performance-degrading hydrogen.

Thanks to empirical results on three continents, it has now become apparent that SRF can progress using niobium ingot slices of merely moderate purity – that is, niobium with relaxed purity specifications, quite similar to the ingot niobium used originally at Stanford. Optimized and streamlined processes can eliminate or reduce the surface-included hydrogen, resulting in high-performance accelerator structures at reduced cost. This could mean savings of perhaps as much as a few tens of percent on ILC’s SRF cavities, and substantial operational cost savings too.

In other words, SRF’s efforts have now come full circle. The SRF researchers who followed Stanford’s original initiative have done fine work. They have made astute choices based on what they could see. But we now have a half-century of work that we can survey. The time has come to re-assess this entire R&D history. Anything less will fail to do justice to the future of accelerators – and to the future of physics itself.

ALBA: a synchrotron light source for Spain

With the exception of the European Synchrotron Radiation Facility, which serves an international community, all existing synchrotron light sources in Europe are located to the north of an imaginary straight line going from Paris to Trieste. To the south, Spain has only a few accelerators and these have been “turnkey” products, mainly in the medical sector; none belongs to a “big” laboratory. For these reasons, in the early 1990s the Autonomous Government of Catalonia began to consider the construction of a new, third-generation synchrotron light source.

CCalb1_11_08 — A lattice cell being prepared for ALBA’s storage ring, with quadrupoles, dipoles and sextupoles.
Image credit: CELLS.

The Generalitat de Catalunya and the Spanish Government signed a first collaboration agreement in 1995 and then took a final decision in March 2002 to create CELLS, a consortium for the construction, equipping and operation of a synchrotron light laboratory. The aim was to establish a third-generation synchrotron light source in the municipality of Cerdanyola del Vallès, in a technology area to be built next to the campus of the Autonomous University of Barcelona, some 20 km from the city. CELLS came into being at the end of 2003 with the objective of constructing the ALBA light source. The initial plan, based on an existing preliminary design study, was for a storage ring of 3 GeV with five beamlines in the first phase. This was scheduled to start up at the end of 2008, for a total cost of €164 m shared between the two administrations on a 50:50 basis.

Studies of underground characteristics, which took more than a year, led to final agreement on the 60,000 m² site for ALBA. This was followed by project approval for the building and conventional installations, thereby guaranteeing mechanical, electrical and thermal stability. Then in 2006 the governing board of CELLS decided to extend the construction phase to the beginning of 2010, to accept two more beamlines (bringing the total to seven) and to increase the total budget to the end of 2009 to €201 m.

The facility occupies a main building of some 18,500 m². This will host the accelerators and the experimental stations, so it must respect restrictive vibration conditions. It is built on a hard floor floating on a bed of gravel 2 m deep. There will also be peripheral auxiliary laboratories, a technical services building of 7600 m² (including an auxiliary storage building) and an administrative building of 4000 m².

CCalb2_11_08 — ALBA’s 100 MeV llinac works at 3 GHz with a 3 Hz repetition rate.

A high-quality 12 MW power supply will be provided by a natural gas power plant that provides thermal and electrical energy, backed up by a dedicated transformer connected to a 220 kV supply line. A system of static and dynamic uninterruptible power supplies will guarantee the supply to the most critical parts of the facility.

The main elements of ALBA will be a 100 MeV linac working at a frequency of 3 GHz with a repetition rate of 3 Hz; a booster synchrotron of four-fold symmetry with an energy of 3 GeV, a circumference of 249.6 m and less than 20 nm rad emittance; and a storage ring of 268.8 m circumference located in the same tunnel as the booster. The design of the accelerators is based on the latest, but well proven, technologies. To provide as much space as possible for the installation of insertion devices and diagnostics, the design includes an extremely compact double-bend achromatic lattice of 16 cells with four-fold symmetry. It will consist of 16 pairs of combined dipole and quadrupole magnets with a central field of 1.42 T and a central gradient of 5.9 T/m, located on large girders. The lattice will be complemented by 128 quadrupole magnets and 120 sextupole magnets with more than 100 correctors installed in the sextupoles. The light source will have an emittance of 4.3 nm rad, a current of 400 mA and a critical energy of about 4.8 keV.

ALBA’s ultra-high vacuum system is made of stainless steel. To cope with the synchrotron radiation, there are antechambers all around, where water-cooled copper and Glip Cop absorbers stop the synchrotron radiation so as to avoid heating the vacuum chamber.

The radiofrequency (RF) system is based on inductive output tube (IOT) amplifiers, running at 80 kW with 67% efficiency. There will be 13: one in the booster to feed a five-cell PETRA-type cavity; and 12 in the storage ring to feed six higher-order, mode-free cavities. A new cavity combiner designed for ALBA will combine the power of the IOTs.

To obtain submicron beam stability there will be a diagnostic system based on 120 beam-position monitors and digital electronics distributed around the storage ring. In addition, synchrotron radiation monitors, current transformers, fluorescent and optical-transition-radiation screens, strip lines and annular electrodes will determine the characteristics of the electron beam. In general, standardization, modularity and robustness have been the main concerns in achieving an accelerator with high reliability and easy maintenance.

The ring will have a capacity for more than 30 beamlines. There will be 16 of these in bending magnets, three in insertion devices in long (8 m) straight sections, 12 in medium (4.4 m) straight sections and two in short (2.6 m) straight sections. The remaining straight sections will be dedicated to injection and RF.

CCalb3_11_08 — ALBA’s main building is built on a hard floor floating on a bed of gravel.

The Asociación de Usuarios de Sincrotrón de España proposed the first phase of beam lines, seven of which CELLS accepted after consultation with the Scientific Advisory Committee. These beamlines, which are now under construction, will be dedicated as follows: one to non-crystalline diffraction and one to macromolecular crystallography, both based on in-vacuum undulators; one to photoemission spectroscopy and microscopy and one to X-ray circular magnetic dichroism, both based on normal undulators; one to X-ray absorption spectroscopy based on a multipole wiggler; one for high-resolution powder diffraction based on a superconducting wiggler; and one for X-ray microscopy based on a bending magnet. A call for a second phase of beamlines is now under way and is scheduled for approval by the end of 2009.

At present, the ALBA project is developing roughly according to budget and schedule, with a delay of only a few months, mainly owing to problems related to civil engineering and conventional services. The linac has been installed and its commissioning has finished. The civil engineering and the conventional installations will be completed in December this year. Almost all of the accelerator components have been designed, produced, tested and delivered, and installation will start soon in the main tunnel. The commissioning phase of the booster will start at the beginning of the second half of 2009 and the storage ring and beamlines will start commissioning progressively in spring 2010. By the second half of the same year the first beam lines of ALBA should be open to users.

The Higgs and the LHC

• 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.

CChig1_11_08 — Martinus Veltman contemplates what the Higgs sector might reveal.

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.

CChig2_11_08 — Radiative corrections to the W and Z masses arise through self-energy diagrams like this.

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.

CChig4_11_08 — The “Δχ²” curve derived from high-Q² precision electroweak measurements, performed at LEP and by SLD, CDF, and D0, as a function of the Higgs-boson mass, assuming the Standard Model to be the correct theory of nature. The yellow area is excuded by the non-production of the Higgs.
Image credit: LEP Electroweak Working Group.

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.

LHC first beam: a day to remember

At a little before 10.30 a.m. on 10 September, two dots on a colour screen in the CERN Control Centre (CCC) marked the successful first complete turn of protons clockwise round the LHC. It was less than an hour since the operations team began injecting Beam 1 into the machine – under the watchful eyes of the world. Lyn Evans, LHC project leader, was more than satisfied: “It was beyond my wildest dreams to get beam so quickly.”

By the end of the day, not only had the anticlockwise beam, Beam 2, also completed its first circuit but it had made some 300 turns of the machine. It was a heady experience, which was followed by a few more days of steady progress, until a breakdown in a magnet interconnection brought commissioning to a halt until next spring. However, those few days have already demonstrated that, in Evans’ words, “the machine works beautifully”.

“Mountain range” plots. With no RF (top left), there is debunching after 25 × 10 turns. With the wrong phase (top right), the bunch becomes split. The correct injection phasing and frequency (left) lead to capture of the bunch for many turns.

The operations team had been preparing the SPS for injection since 8.00 a.m. and the start-up procedure with Beam 1 began promptly. At 9.30 a.m. they were ready to start, turning on the kickers to direct the beam onto a beam stop just before the interaction point at Point 2. The plan was to send in the beam one bunch at a time and open up the LHC ring step by step. At each of the four points occupied by the LHC experiments, the beam would initially be stopped by closed collimators to allow corrections to be made, if necessary. The collimators would then be opened to allow the subsequent beam shots to proceed through the detector and farther round the ring.

The sequence worked like clockwork: beam to collimators, collimators open, beam to next collimators and so on. Each of the major LHC detectors – ALICE (Point 2), CMS (Point 5), LHCb (Point 8) and ATLAS (Point 1) – lit up in turn with the first beam-related particles as bunches in Beam 1 hit the nearby collimators, creating particle “debris”.

A “Champagne moment” for Lyn Evans on 11 September – a perfect longitudinal beam profile.

The procedure with Beam 2 (in the anticlockwise direction) was almost as smooth. Minor problems with cryogenics delayed the start until 1.30 p.m. and slowed progress from injection at Point 8 to Point 6, where beam arrived at 1.55 p.m. Small difficulties with the beam meant that it did not reach CMS at Point 5 for another 30 minutes. However, Beam 2 had made its first complete turn by 3 p.m.

After a well deserved pause, to let the day’s achievements sink in and to gather thoughts, at 4 p.m. work in the CCC turned to the more earnest matters of studying the properties of Beam 2, and setting it up for multiple turns. Measurements over the next few hours of the kick response and dispersion showed a truly well behaved beam. By 9.30 p.m., Beam 2 orbited the LHC for at least 300 turns, just 12 hours after the first injection with Beam 1.

As the start-up of 10 September came to a close, the real work for the operations team was only just beginning. A bunch of particles travelling round the ring is a major step. However, for an accelerator the key lies in capturing the particles with the RF system that provides the accelerating electric fields – and keeping the bunches in time with the RF on the thousands of turns per second that occur during normal operation.

An essential step on the day after first beam was to turn on the RF and investigate the bunch lifetime. This was, according to Evans, one of the many worries that he had from the days when CERN gained valuable experience with its pioneering conversion of the SPS into a proton–antiproton collider. Noise from the klystrons that provide the RF power can propagate and debunch the particles.

The first tests on 11 September showed that Evans need not have worried, because the so-called “mountain range” plot revealed clearly the effects on an individual bunch on successive turns in Beam 2. Without the RF, the bunch simply broadens as particles stray from the perfect orbit round the machine; the mountain range rapidly broadens and flattens out. With the RF at the correct phase and frequency, the bunches are captured and the mountain range becomes a long, continuous narrow ridge as turn after turn the whole bunch of particles passes the same point at the right time. The result by the end of the day was what Evans called the “real champagne moment” – a perfect longitudinal bunch profile, only a day after first beam.

Timeline

The plan for the day: Beam 1 is to go clockwise from injection just before Point 2. Then it will be the turn of Beam 2, travelling in the anticlockwise direction from close to Point 8.

It’s 9 a.m. and the Silicon Pixel Detector in ALICE lights up with particle “debris” created as beam in the transfer line from the SPS hits the beam stop before Point 2.

At 9.50 a.m. CMS records the first beam-induced events as Beam 1 hits the collimator at Point 5, leading to particles flying into the detectors. Energy deposits are visible in the electromagnetic (pink) and hadronic (blue) calorimeters, as are hits in the resistive plate chamber (green) and drift-tube (red) muon systems.

At 10.12 a.m. the operations team begins to allow Beam 1 through to Point 8. During these first manoeuvres, LHCb’s sensitive tracking detectors are not switched on, but the muon detectors, calorimeters and the ring imaging Cherenkov detectors record particles produced by the beam.

At Point 1, the ATLAS detector records this event at 10.19 a.m. This is the final interaction point before the beam gets back to where it started.

It’s 10.26 a.m. and the screen in the CCC shows the two spots that indicate that beam has for the first time made a complete circuit of the LHC.

At 9.30 p.m. Beam 2 completes at least 300 circuits of the machine. The beam appears to decay as it becomes debunched because there is, as yet, no capture by the RF systems.

Exclusive events give new window on LHC physics

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.

CCTev1_11_08 — Diagrams corresponding to studies in exclusive physics at the Tevatron, showing various types of gluon–gluon “fusion” with a third “screening” gluon, creating a photon-pair (a), a Higgs boson (b), a χ_c meson (c), or a jet-pair (d). (This interaction is also termed double pomeron exchange.)

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.

CCtev2_11_08 — Diagrams for a) photoproduction and b) photon–pomeron fusion. Both processes have been observed at the Tevatron.

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 χ_c⁰, 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.

CCtev3_11_08 — The invariant mass distribution of exclusive di-muon pairs observed by CDF in the mass range 3–4 GeV/c². The larger left peak is the J/ψ, the smaller peak is the ψ(2S).
Image credit: CDF collaboration.

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 χ_c⁰ 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.

CCtev4_11_08 — An exclusive di-jet candidate from CDF.
Image credit: CDF.

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.

Gamma-ray astronomers convene in Heidelberg

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.

CCgam1_11_08 — About 300 experts in astroparticle physics attended the γ 2008 symposium in Heidelberg.
Image credit: Christian Föhr, MPI for Nuclear 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.

CCgam2_11_08 — This year’s symposium took place at the prestigious Heidelberg Convention Centre.
Image credit: Christian Föhr, MPI for Nuclear Physics.

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.

CCgam3_11_08 — A preliminary H.E.S.S. gamma-ray map of the SN 1006 supernova remnant (left) beside an X-ray image of the remnant as seen by the X-ray satellite Chandra (right). The circle in the left image indicates the size of the remnant as seen at radio wavelengths. The gamma-ray emission regions line up with the regions of intense X-ray emission.
Image credit: H.E.S.S. and NASA/CXC/Rutgers/J.Hughes *et al.*

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.

NuPNET looks to future nuclear physics in Europe

At a meeting in Paris on 27 March, representatives from the Nuclear Physics European Collaboration Committee (NuPECC), the EU Commission and 18 national funding agencies launched a network in nuclear physics to enable the community to pilot joint transnational activities.

CCnup1_11_08 — Members of the NuPNET consortium surround the co-ordinator, Sydney Galès (sixth from left, front row) and the co-ordination manager, Dorothée Peitzmann (fifth from left, front row) at the kick-off meeting in Paris on 27 March 2008.
Image credit: X Pierre/CNRS-IN2P3.

The idea to create a European network in nuclear physics arose two years ago, when more than 15 representatives of nuclear physics funding agencies and/or similar organizations, a NuPECC delegation and EU officers met in Paris to discuss the possibility of co-ordinating the existing national funding procedures through a new tool of the European Commission. The tool – the European Research Area Network, or ERA-Net – would focus on networking, mutual opening, development and implementation of joint activities. The participants at the meeting unanimously agreed to prepare a proposal, based on the scientific recommendations made by NuPECC in its latest long-range plan with a view to the ERA-Net scheme, for submission as soon as the EU Commission launched the appropriate call within the Framework Programme for European Research and Technology.

The proposal took the name of NuPNET, for Nuclear Physics Network. Under the scientific co-ordination of the French partner, the co-ordination committee composed of members of funding agencies from France, Germany, Italy and Spain, had the responsibility of working out the full proposal. Thanks to the excellent collaboration between the co-ordination committee and the managers of the 18 European institutions that agreed to be part of this new venture, the final proposal was submitted in May 2007 at the first call of the Seventh Framework Programme for European Research and Technology (FP7). Evaluated during the summer of 2007, the NuPNET proposal was accepted by the European Commission in September 2007. Contract negotiations were completed by 11 March 2008 and a budget of €1.3 m has been granted for three years, from March 2008 to February 2011.

The NuPNET project comprises 18 regular members representing 14 countries (see figure 1). NuPECC is an associated member and acts as the Scientific Advisory Body of the NuPNET consortium to provide independent views on the direction of nuclear physics within Europe through its long-range plans, to give advice on scientific issues, and to inform NuPNET on the views of the scientific community.

On 27 March 2008, the founding member institutions of NuPNET, the representatives from NuPECC and the EU Commission came together for the traditional “kick-off” meeting. Organized by CNRS/IN2P3, the co-ordinator of the NuPNET project, this first official meeting took place in Paris. The participants agreed that NuPNET’s programme will have an important impact on the future of nuclear physics, especially since the ERA-Net proposal – as adopted by the partners and as accepted by the EU Commission – aims, for the first time in the history of nuclear physics, to co-ordinate the various national funding agencies in order to organize better the financing of nuclear physics infrastructures at a European level.

Implementation and governance

The NuPNET project has outlined a stepwise approach to project implementation in the form of four goals. The first is to compare reviewing and funding systems in participating funding agencies; provide a census of resources and agents in nuclear physics and infrastructures that paves the way to common decisions; and liaise with Integrated Infrastructure Initiatives and design studies in FP7 and other European and international initiatives, in particular the European Strategy Forum on Research Infrastructures and the Organisation for Economic Co-operation and Development. This work package is led by Germany.

The second goal is to propose a set of joint transnational activities (based on the science priorities set in the long-range plan of NuPECC) that can be launched by funding agencies thanks to NuPNET co-ordination. Italy leads this work package. The third goal is to launch one or more of those proposed joint transnational activities in the field of nuclear physics infrastructures, in a work package led by Spain. The fourth and final goal is to provide Europe with a sustainable scheme beyond the project duration.

The project is managed by the co-ordinator (CNRS/IN2P3); the governing council (NuPNET member institutions); the co-ordination committee (CNRS/IN2P3) and work package leaders from France, Germany, Spain and Italy; and the Scientific Advisory Body (NuPECC). All parties are involved at the relevant level; however, the governing council is the main decision-making body of the consortium, where only authorized members can vote in the name of the represented member institution. Public bodies interested in joining NuPNET may be invited to attend a meeting of the governing council. The co-ordinator, together with the co-ordination manager, ensures the overall management of the project, whereas the co-ordination committee implements the decisions taken by the governing council and supports the co-ordinator. Now, the work has started. NuPNET has its own logo, a website is being constructed and the first session of Open Days (see figure :2) took place in Athens on 8 September.

Fermi Gamma-ray Space Telescope sees first light

CCgla1_11_08 — Launch of the Fermi Gamma-ray Space Telescope on 11 June from Cape Canaveral Air Station in Florida. Image credit: NASA.

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.

CCgla2_11_08 — Fermi’s “first light” all-sky gamma-ray map showing major sources detected. Image credit: the International Fermi LAT Collaboration.

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.

CCgla3_11_08 — Schematic diagram showing the construction of the Large Area Telescope, showing the conversion of an incoming gamma ray into an electron-positron pair. (Michelson et al. 2008.)

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.

INTEGRAL pinpoints acceleration

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.

CCast1_11_08 — Gamma-ray polarization direction with grey error area superimposed on a composite image of the Crab Nebula in X-rays (blue) and in optical (red) taken by the Chandra and Hubble space telescopes.
Image credit: NASA/CXC/ASU/J Hester *et al.* (for the Chandra image); NASA/HST/ASU/J Hester *et al.* (for the Hubble image)

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.

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