Conférence de l'EPS: perspectives de la physique des particules

La Conférence de la Société européenne de physique sur la physique des hautes énergies s'est tenue en 2007 à Manchester. Au programme, plus de 400 sessions parallèles ou plénières, pour un panorama de l'état de la physique des particules en 2007, qui pourrait être une année décisive. Alors que certains secteurs de la discipline travaillent sur l'affinage du modèle standard, d'autres s'efforcent d'accéder à une nouvelle physique grâce au LHC. L'un des thèmes récurrents était le succès du modèle standard, en particulier dans le domaine de la physique des saveurs, ainsi que la nécessité du LHC, qui devrait aider à percer le mystère de la matière noire et de l'énergie sombre.

The English summer, renowned for being fickle, smiled kindly on the organizers of the 2007 European Physical Society (EPS) conference on High Energy Physics (HEP), which was held in Manchester on 19–25 July. In a city that is proud of both its industrial heritage and a bright commercial future, HEP 2007 surveyed the state of particle physics, which also seems to be at a turning point. While certain areas of the field pin down the details of the 20th-century Standard Model, others seek to prise open new physics as the LHC prepares to open a new frontier.

The conference had a packed programme of 12 plenary sessions and 69 parallel sessions. In his opening talk, CERN's John Ellis took a lead from Paul Gauguin's painting Life's Questions, and interpreted the questions in terms of the status of the Standard Model (where are we coming from?), searches beyond the Standard Model (where are we now?) and the search for a "theory of everything" (where are we going?). More than 400 talks covered all three aspects, in particular the status of the Standard Model and the current and future efforts to go beyond it. This report summarizes some of the highlights within these broad themes.

A beautiful model

The success of the Standard Model underpinned the 2007 award of the EPS High Energy and Particle Physics prize to Makoto Kobayashi of KEK and Toshihide Maskawa of the University of Tokyo for their work in 1972 that showed that CP violation occurs naturally if there are six quarks, rather than the original three. Kobayashi was at the conference to receive the prize and to give a personal view of the early work and the current understanding of CP violation. The idea of six quarks began to attract attention with the discovery of the τ lepton in 1976. The rest, as they say, is history, and the Cabibbo–Kobayashi–Maskawa (CKM) matrix describing six quarks is now a key part of the Standard Model.

Moving to the present, Kobayashi pointed to the work of the experiments at the B-factories – BaBar at the PEPII facility at SLAC and Belle at KEK-B. They have played a key role in pinning down the well known triangle that expresses the unitarity of the CKM matrix. The two experiments have shown that the three sides of the triangle really do appear to close – a leitmotif that ran throughout the conference. Measurements of sin2β (sin2φ1) now give a clear value of 0.668 ± 0.028 – a precision of 4% – and even measurements of the angle γ (φ3) are becoming quite good thanks to the performance of B-factories.

Both facilities have provided high beam currents and small beam sizes, leading to extremely high luminosities. With a peak luminosity of 1.21 × 1034 cm–2 s–1 – four times the design value – PEPII has delivered a total luminosity of 460 fb–1 but is now feeling the stress of the high currents. Nevertheless, there are plans to try for still-higher luminosity and deliver the maximum possible before the facility closes down at the end of September 2008. KEK-B, with a peak luminosity of 1.71 × 1034 cm–2 s–1, has reached a total of 715 fb–1 and there are also plans for increasing luminosity in this machine, using the recently tested "crab crossing" technique, to bring the angled beams into a more direct collision (CERN Courier September 2007 p8).

The extra luminosity is important now that the experiments are moving on to a new phase, searching for new physics. This may be manifest in small deviations from the Standard Model at the 1% level, although guidelines from theory are made difficult – not least by uncertainties in QCD. The charmless B decay, B → φ K0 – where at the quark level b → sss – currently shows a small systematic deviation from theory. However, many agree with Kobayashi's opinion that it is premature to derive any conclusion. "Super B-factories", as proposed for example at KEK, will probably be necessary to clarify this and other hints of new physics.

B physics is not only the preserve of the B-factories, nor is interest in heavy flavours restricted only to B physics. The CDF and DØ experiments at Fermilab's Tevatron have measured Bs oscillations for the first time, in a 5 σ effect with ΔMs = 17.77 ± 0.10 ± 0.07 ps–1. This result presents no surprises, but the award of the 2007 EPS Young Physicist prize reflected its importance. This went to Ivan Furic of Chicago, Guillelmo Gomez-Ceballos of Cantabria/MIT, and Stephanie Menzemer of Heidelberg, for their outstanding contributions to the complex analysis that provided the first measurement of the frequency of Bs oscillations. In the physics of the lighter charm particles, the BaBar and Belle experiments have made the first observations of D mixing, at the level of about 4 σ, with no evidence for CP violation. Neither Bs nor D mixing is easy to measure, the first being very fast, the second being very small. Moreover, D mixing is difficult to calculate as the charm quark is neither heavy nor particularly light. On the other hand, the Standard Model clearly predicts no CP violation. Elsewhere in the heavy-flavour landscape, CDF and DØ have found new baryons that help to fill the spaces remaining in the multiplets of various quark combinations.

The electroweak side of the Standard Model has known precision for many years, with the coupling constants α and GF, and more recently the mass of the Z boson, Mz, available as precise input parameters for calculations of a range of observables. Now with a steadily increasing total integrated luminosity in Run II – 2.72 fb–1 in DØ, for example, by the time of the conference – the mass of the W boson, MW, is measured with similar precision at both the Tevatron and LEP, and is known to ± 25 MeV. CDF and DØ also continue to pin down other observations, in particular in the physics of top, with studies of top decays and measurements of the top mass, Mt, with a latest value of 170.9 ± 1.8 GeV/c2. DØ also has evidence for the production of single top – produced from a W rather than a gluon – which gives a handle on ⃒Vtb2 in the CKM matrix. A comparison of Mw and Mt from the Tevatron with the results from LEP and the SLAC Linear Collider provides a powerful check on the Standard Model – Mt is measured at the Tevatron, whereas it was inferred at the e+e colliders – and constrains the mass of the Higgs boson. There will be no beam in the LHC until 2008, so the Tevatron is currently the only hunting ground for the Higgs; with upgrades planned to take its total luminosity to at least 6 fb–1, there are interesting times ahead.

While the Tevatron is still going strong, HERA – the first and only electron–proton collider – shut down for the last time this past summer, having written the "handbook" on the proton. HERA provided a unique view inside the proton through deep inelastic scattering, which is still being refined as analysis continues. Once the final pages are written they will provide vital input, in particular on the density of gluons, for understanding proton collisions at the LHC. This effort continues at the Tevatron, where the proton–antiproton collisions provide a complementary view to HERA, in particular regarding what is going on underneath the interesting hard scatters. Additionally, the HERMES experiment at HERA, COMPASS at CERN and experiments at RHIC are investigating the puzzle of what gives rise to the spin of the proton (or neutron) in terms of gluons or orbital angular momentum.

Measurements at HERA and the Tevatron have challenged the strong arm of the Standard Model by testing QCD with precision measurements that involve hadrons in the initial state, not just in the final state, as at LEP. In particular, they provide a testing ground for perturbative QCD (pQCD) in hard processes where the coupling strength is relatively weak, and show good agreement with theoretical predictions. The challenge now is to apply the theory to the more complex scenario of collisions at the LHC, in particular to calculate processes that will be the backgrounds to Higgs production and new physics.

QCD enters a particularly extreme regime in the relativistic collisions of heavy ions, where hundreds of protons and neutrons coalesce into a hot, dense medium. Results from RHIC at Brookhaven National Laboratory (BNL) are already indicating the formation of deconfined quark–gluon matter in an ideal fluid with small viscosity. Here the anti-de Sitter space/conformal field-theory correspondence offers an alternative view to pQCD, with predictions for the higher energies at the LHC.

Beyond the Standard Model

Experiments at the Tevatron and HERA have all searched for physics beyond the Standard Model and find nothing beyond 2 σ. At HERA, however, the puzzle remains of the excess of isolated leptons, which H1 still sees with the full final luminosity (reported at the conference only three weeks after the shutdown), although ZEUS sees no effect. This excess will have to be seen elsewhere to demonstrate that it is new physics, and not nature being unkind.

While the high-energy collider experiments see no real signs of new physics, at least neutrino physics is beginning to provide a way beyond the Standard Model. Neutrinos have long been particles about which we know hardly anything, but as Ken Peach from the University of Oxford commented in his closing summary talk, at least now we "clearly know what we don't know". Research has established neutrino oscillations, and with them neutrino mass. However, we still need to know more about the amounts of mixing of the three basic neutrino states to give the flavour states that we observe, and about the mass scale of those basic states.

Clarification in one area has come from the MiniBoone experiment at Fermilab, which finds no evidence for oscillations as reported by the LSND experiment (CERN Courier May 2007 p8). However, there are signs of a new puzzle as MiniBoone sees an excess of events at neutrino energies at 300–475 MeV. The Main Injector Neutrino Oscillation Search collaboration presented a new result for mixing in the 23 sector, with Δm223 = 2.38 + 0.20 – 0.16 × 10–3 eV2 and sin223 = 1.00 with an error of 0.08. For the 13 sector, however, there is still a desperate need for new experiments. The Karlsruhe Tritium Neutrino experiment will try to measure directly the electron neutrino (an incoherent sum of mass states) using the classic technique of measuring the endpoint of the tritium beta-decay spectrum, with a sensitivity of 0.2 eV. Neutrinoless double beta-decay experiments provide another route to neutrino mass and could constrain the lightest state in the mass hierarchy. Taking what we already know from oscillations, one or two of the lightest neutrinos (depending on mass hierarchy) should have masses of at least 0.05 eV. Much now depends on experiments to come.

Dark matter in the cosmos seems to be another sure sign of physics beyond the Standard Model. Cosmology indicates that it is composed of non-baryonic particles and is mostly "cold" – low energy – and so cannot consist of the known lightweight neutrinos. Current direct searches for dark-matter particles are reaching cross-sections of around 10–44 cm2, and the next generation of experiments are aiming to reach a factor of 10 lower. Dark-matter annihilation can affect the gamma-ray sky, so the GLAST mission, due to be launched in December, could complement the searches for dark-matter candidates that will take place at the LHC.

The cosmos holds other mysteries for particle physics, in particular the long-standing question of the origin of ultra high-energy cosmic rays. Clues to the location of the natural accelerators lie in the precise shape of the spectrum at high energies: are there particles with energies above the Greisen–Kuzmin–Zatsepin cut-off? The Pierre Auger Observatory has ushered in a new age of hybrid detection based on a combination of scintillation and air fluorescence detectors. Together, the two techniques reveal both the footprint and the development of an extensive air shower, so reducing the dependence on interaction models. Auger now has more events above 10 EeV than previous experiments, and confirms the "ankle" and steepening at the end of the spectrum (and, since the conference, the first evidence for sources of ultra-high-energy cosmic rays, see "Pierre Auger Observatory pinpoints source of mysterious highest-energy cosmic rays"). Understanding thie spectrum depends on determining the mass of the incoming particles. Photons constitute less than 2% of the cosmic radiation at these high energies; is the remainder all protons, or are there heavier components, as the data from Auger hint at?

Back on Earth, the LHC is uniquely poised to go beyond the Standard Model, as Ellis pointed out in his opening talk. So a key question for everyone is: when will the LHC start-up? Lyn Evans, LHC project leader at CERN, brought the latest news, but first reminded the audience just how remarkable the project is. He began by paying homage to Kjell Johnsen, who died on 18 July, the week before the conference. Johnsen led the project to build the world's first proton–proton collider, the Intersecting Storage Rings (ISR) at CERN. The LHC is a magnificent tribute to Johnsen, explained Evans, for without the ISR, there would be no LHC. The idea of storing protons, without the synchrotron radiation damping effects inherent in electron beams, was a leap of faith; respected people thought that it would never work.

Now, the LHC is built and the effort to cool down and power-up is underway. Unsurprisingly in a project so complex, problems arise, but they are being overcome; the schedule now foresees that beam commissioning should begin in May 2008, with the aim for first collisions at 14 TeV two months later. The injection system can already supply enough beam for a luminosity of 1034 cm–2 s–1, but in practice commissioning will start with only a few bunches for each beam, to ensure the safety of the collimation and protection systems.

For the LHC experiment collaborations, commissioning will also start with an emphasis on safety. They will study the first collisions with minimum-bias triggers while they gain full understanding of their detectors, before moving on to QCD dijet triggers to "rediscover" physics of the Standard Model. With 1 fb–1of data collected, there will be the opportunity to begin searching for new physics, with signs of supersymmetry perhaps appearing early. A major goal will of course be to discover the Higgs boson – or whatever mechanism it is that underlies electroweak symmetry-breaking. This is a key issue that the LHC should certainly resolve. Beyond it lie other more exotic questions, concerning extra dimensions and tests of string theory, for example, and even "unparticles" – denizens of a scale-invariant sector weakly coupled to the particles of the Standard Model, as recently proposed by Howard Georgi.

As the LHC nears completion, there is plenty of activity on projects to complement it. The largest is the proposed Inter-national Linear Collider (ILC) to provide e+e collisions at a centre-of-mass energy of 500 GeV. The collaboration released the Reference Design Report in February, putting the estimated price tag for the machine at $6.4 thousand million (CERN Courier April 2007 p4). Like the LHC, it will be a massive undertaking, involving some 1700 cryogenic units for acceleration. To reach still higher energies with an e+e collider, the Compact Linear Collider study is an international effort to develop technology to go up to 3 TeV in the centre-of-mass. The key feature is a double-beam delivery system, with a main beam and a drive beam, and normal conducting structures. It will require an accelerating gradient of more than 100 MV/cm to reach 3 TeV in a total length less than 50 km. The aim is to demonstrate the feasibility by 2010, with a technical design report in 2015.

In other areas, there are proposals for super B-factories and neutrino factories to produce the intense beams needed to study rare and/or weak processes in both fields. The idea behind the neutrino factories is to generate large numbers of pions, which will decay to muons that can be cooled and then accelerated before they decay to produce the desired neutrinos. An important requirement will be a high-intensity proton driver to produce pions in primary proton collisions. Such drivers have, of course, other uses: the Spallation Neutron Source in Oak Ridge, for example, is operating with the world's first superconducting proton linac, currently delivering 0.4 × 1014 protons with each pulse. Other issues for a future neutrino factory are the cooling and acceleration of the muons. The Muon Ionisation Cooling Experiment at the UK's Rutherford Appleton Laboratory will test one such concept, using liquid hydrogen absorbers to reduce the muon momentum in all directions. The subsequent acceleration will have to be fast, before the muons decay, and in this respect researchers are revisiting the idea of fixed-field alternating gradient (FFAG) accelerators, which dates back to the early 1950s. To test the principle, a consortium at the Daresbury Laboratory in the UK plans to build the world's first non-scaling FFAG machine, a 20 MeV electron accelerator.

The design of particle detectors will have to adapt to the more exacting conditions at future machines, to deal with larger numbers of particles, higher densities of particles and higher radiation doses. Issues to consider include: segmentation to deal with the high density of particles; speed to handle large events quickly; and thin structures to keep down the material budget. For the ILC, various collaborations are working on four concepts for the collider detectors; the aim is to select two of these in 2009 and have engineering designs completed by 2010.

The next conference in the series is in Krakow, in 2009. It will be interesting to learn how the new ideas presented at HEP 2007 have advanced, to see the first steps across the new frontier with the LHC and to find out if we can see further towards where we are going.

• HEP 2007 was organized by the universities of Durham, Leeds, Lancaster, Liverpool, Manchester and Sheffield, together with the Cockcroft Institute and Daresbury Laboratory.