Physics buzz in Paris

Sixty years ago, particle physics was in its infancy. In 1950 Cecil Powell received the Nobel Prize in Physics for the emulsion technique and the discovery of the charged pions, and an experiment at Berkeley revealed the first evidence for the neutral version. In New York, the first in a new series of conferences organized by Robert Marshak took place at the University of Rochester with 50 participants. The “Rochester conference” was to evolve into the International Conference on High-Energy Physics (ICHEP) and this year more than 1100 physicists gathered in Paris for the 35th meeting in the series.

ICHEP’s first visit to the French capital was in 1982. CERN’s Super Proton Synchrotron had just begun to operate as a proton–antiproton collider and the UA2 collaboration reported on the first observations of back-to-back jets with high transverse momentum. This year, as ICHEP retuned to Paris, jets in a new high-energy region were again a highlight. This time they were from the LHC, one undoubted “star of the show”, together with the president of France, Nicolas Sarkozy.

Given the growth in the field since the first Rochester conference, this report can only touch on some of the highlights of ICHEP 2010, which took place on 22–28 July at the Palais des Congrès and followed the standard format of three days of parallel sessions, a rest day (Sunday) and then three days of plenary sessions. The evening of 27 July saw Parisians and tourists well outnumber physicists at the “Nuit des particules”, a public event held at the Grand Rex theatre (see box). On the rest day, in addition to various tours, there was the opportunity to watch the final stage of the 2010 Tour de France as it took over the heart of Paris.

A tour of LHC physics

The LHC project has had similarities to the famous cycle race – participants from around the world undertaking a long journey, with highs and lows en route to a thrilling climax. In the first of the plenary sessions, Steve Myers, director for accelerators and technology at CERN, looked back over more than a year of repair and consolidation work that led to the LHC’s successful restart with first collisions in November 2009. With the collider running at 3.5 TeV per beam since March this year, the goal is to collect 1 fb^–1 of integrated luminosity with proton collisions before further consolidation work takes place in 2012 to allow the machine to run at its full energy of 7 TeV per beam in 2013. The long-term goal is to reach 3000 fb^–1 by 2030. This will require peak luminosities of 5 × 10³⁴ cm^–2 s^–1 in 2021–2030 for which studies are already underway, for example on the use of crab cavities.

The proposed long-term schedule envisages one-year shutdowns for consolidation in 2012, 2016 and 2020, with shorter periods of maintenance in December/January in the intervening years, and 6–8 month shutdowns every other year after 2020. Heavy-ion runs are planned for each November when the LHC is running, starting this year. Myers also provided glimpses of ideas for a 16.5 TeV version of the LHC that would require 20 T dipole magnets based on NbSn₃, NbAI and high-temperature superconductors.

What many at the conference were waiting for were the reports from the LHC experiments on the first collision data, presented both in dedicated parallel sessions and by the spokespersons on the first plenary day. Common features of these talks revealed just how well prepared the experiments were, despite the unprecedented scale and complexity of the detectors. The first data – much of it collected only days before the conference as the LHC ramped up in luminosity – demonstrated the excellent performance of the detectors, the high efficiency of the triggers and the swift distribution of data via the worldwide computing Grid. All of these factors combined to allow the four large experiments to rediscover the physics of the Standard Model and make the first measurements of cross-sections in the new energy regime of 7 TeV in the centre-of-mass.

The ATLAS and CMS collaborations revealed some of their first candidate events with top quarks – previously observed only at Fermilab’s Tevatron. They also displayed examples of the more copiously produced W and Z bosons, seen for the first time in proton–proton collisions, and presented cross-sections that are in good agreement with measurements at lower energies. Lighter particles provided the means to demonstrate the precision of the reconstruction of secondary vertices, shown off in remarkable maps of the material in the inner detectors.

Both ATLAS and CMS have observed dijet events, with masses higher than that of the Tevatron’s centre-of-mass energy. The first measurements of inclusive jet cross-sections in both experiments show good agreement with next-to-leading-order QCD (The window opens on physics at 7 TeV). In searches for new physics, ATLAS has provided a new best limit on excited quarks, which are now excluded in the mass region 0.4 <M <1.29 TeV at 95% CL. For its part, by collecting data in the period between collisions at the LHC, CMS derived limits on the existence of the “stopped gluino”, showing that it cannot exist with lifetimes of longer than 75 ns.

The LHCb collaboration reported clear measurements of several rare decays of B mesons and cross-sections for the production of open charm, the J/ψ and bb states. With the first 100 pb^–1 of data, the experiment should become competitive with Belle at KEK and DØ at Fermilab, with discoveries in prospect once 1 fb^–1 is achieved.

The ALICE experiment, which is optimized for heavy-ion collisions, is collecting proton–proton collision data for comparison with later heavy-ion measurements and to evaluate the performance of the detectors. The collaboration has final results in charged multiplicity distributions at 7 TeV, as well as at 2.36 TeV and 0.9 TeV in the centre-of-mass. These show significant increases with respect to Monte Carlo predictions, as do similar measurements from CMS. ALICE also has interesting measurements of the antiproton to proton ratio.

While the LHC heads towards its first 1 fb^–1, the Tevatron has already delivered some 9 fb^–1, with 6.7 fb^–1 analysed by the time of the conference. One eagerly anticipated highlight was the announcement of a new limit on the Higgs mass from a combined analysis of the CDF and DØ experiments. This excludes a Higgs between 158–175 GeV/c², thus eliminating about 25% of the favoured region from analysis of data from the Large Electron–Positron collider and elsewhere. As time goes by, there is little hiding place for the long-sought particle. In other Higgs-related searches, the biggest effect is a 2σ discrepancy found in CDF for the decay to bb of the Higgs in the minimal supersymmetric extension to the Standard Model.

Stressing the Standard Model

The strongest hint at the Tevatron for physics beyond the Standard Model comes from measurements of the decays of B mesons. The DØ experiment finds evidence for an anomalous asymmetry in the production of muons of the same sign in the semi-leptonic decays of B_s mesons, which is greater than the asymmetry predicted by CP violation in the B system in the Standard Model by about 3.2σ. While new results from DØ and CDF for the decay B_s→J/ψ+Φ show a better consistency with the Standard Model, they are not inconsistent with the measurement of A^b_sl,.

Experiments at the HERA collider at DESY, and at the B factories at KEK and SLAC, have also searched extensively for indications of new physics, and although they have squeezed the Standard Model in every way possible it generally remains robust. Of course, the searches extend beyond the particle colliders and factories, to fixed-target experiments and detectors far from accelerator laboratories. The Super-Kaminokande experiment, now in its third incarnation, is known for its discovery of neutrino oscillations, which is the clearest indication yet of physics beyond the Standard Model, but it also searches for signs of proton decay. It has now accumulated data corresponding to 173 kilotonne-years and, with no evidence for the proton’s demise, it sets the proton’s lifetime at greater than 1 × 10³⁴ years for the decay to e⁺π⁰ and greater than 2.3 × 10³⁴ years for νK⁺.

The first clear evidence for neutrino oscillations came from studies of neutrinos from the Sun and those created by cosmic rays in the upper atmosphere, but now it is the turn of the long-baseline experiments based at accelerators and nuclear reactors to bring the field into sharper focus. At accelerators a new era is opening with the first events in the Tokai-to-Kamioka (T2K) experiment, as well as the observation of the first candidate ν_τ in the OPERA detector at the Gran Sasso National Laboratory, using beams of ν_μ from the Japan Proton Accelerator Research Complex and CERN respectively.

While T2K aims towards production of the world’s highest intensity neutrino beam, the honour currently lies with Fermilab’s Neutrino beam at the Main Injector, which delivers ν_μ to the MINOS experiment, with a far-detector 735 km away in the Soudan Mine. MINOS now has analysed data for 7.2 × 10²⁰ protons on target (POT) and observes 1986 events where 2451 would be expected without oscillation. The result is the world’s best measurement for |Δm²| with a value of 2.35+0.11/–0.08 × 10^–3 eV², and sin²2θ> 0.91 (90% CL). MINOS also finds no evidence for oscillations to sterile neutrinos and puts limits on θ₁₃. Recently, the experiment has been running with an anti-neutrino beam, and this has proved to hint at differences in the oscillations of antineutrinos as compared with neutrinos. With antineutrinos, the collaboration measures |Δm²|= 3.36+0.45/–0.40 × 10^–3 eV² and sin²2θ = 0.86±0.11. As yet the statistics are low, with only 1.7 × 10²⁰ POT for the antineutrinos, but the experiment can quickly improve this with more data.

The search for direct evidence of dark-matter particles, which by definition lie outside the Standard Model, continues to have tantalizing yet inconclusive results. Experiments on Earth search for the collisions of weakly interacting massive particles (WIMPs) in detectors where background suppression is even more challenging than in neutrino experiments. Recent results include those from the CDMS II and EDELWEISS II experiments, in the Soudan Mine and the Modane Underground Laboratory in the Fréjus Tunnel, respectively. CDMS II presented its final results in November 2009, following a blind analysis. After a timing cut, the analysis of 194 kg days of data yields two events, with an expected background of 0.8±01(stat.)±0.2 (syst.) events. The collaboration concludes that this “cannot be interpreted as significant evidence for WIMP interactions”. EDELWEISS II has new, updated results, which now cover an effective 322 kg days. They have three events near threshold and one with a recoil energy of 175 keV, giving a limit on the cross-section of 5.0 × 10^–8 pb for a WIMP mass of 80 GeV (at a 90% CL).

Higher energies, in nature and in the lab

Looking to the skies provides a window on nature’s own laboratory of the cosmos. The annihilation of dark matter in the galaxy could lead to detectable effects, but the jury is still out on the positron excess observed by the PAMELA experiment in space. Back on Earth, the Pierre Auger Observatory and the High-Resolution Fly’s Eye (HiRes) experiment in the southern and northern hemispheres, respectively, detect cosmic rays with energies up to 10²⁰ eV (100 EeV) and more. Both have evidence for the suppression of the highest energies by the Greisen-Zatsepin-Kuzmin (GZK) cut-off. There is also evidence for a change in composition towards heavier nuclei at higher energies, although this may also be related to a change in cross-sections at the highest energies. The correlation of the direction of cosmic rays at energies of 55 EeV or more with active galactic nuclei, first reported by the Pierre Auger collaboration in 2007, has weakened with further data, from the earlier value of 69 + 11/–13% to stabilize around 38 + 7/–6%, now with more than 50 events.

Cosmic neutrinos provide another possibility for identifying sources of cosmic rays. The ANTARES water Cherenkov telescope in the Mediterranean Sea now has a sky map of its first 1000 neutrinos and puts upper limits on point sources and on the diffuse astrophysical neutrino flux. IceCube, with its Cherenkov telescope in ice at the South Pole, also continues to push down the upper limits on the diffuse flux with measurements that begin to constrain theoretical models.

In the laboratory, the desire to push further the exploration of the high-energy frontier continues to drive R&D into accelerator and detector techniques. The world community is already deeply involved in studies for a future linear e⁺e^– collider. The effort behind the International Linear Collider to reach 500 GeV per beam is relatively mature, while work on the more novel two-beam concept for a Compact Linear Collider to reach 3 TeV is close to finishing a feasibility study. Other ideas for machines further into the future include the concept for a muon collider, which would require muon-cooling to create a tight beam, but could provide collisions at 4 TeV in the centre-of-mass. Reaching much higher energies will require new technologies to overcome the electrical breakdown limits in RF cavities. Dielectric structures offer one possibility, with studies showing breakdown limits that approach 1 GV/m. Beyond that, plasma-based accelerators still hold the promise of still greater gradients, as high as 50 GV/m.

Particle physics has certainly moved on since the first Rochester conference; maybe a future ICHEP will see results from a muon collider or the first plasma-wave accelerator. For now, ICHEP 2010 proved a memorable event, not least as the first international conference to present results from collisions at the LHC. Its success was thanks to the hard work of the French particle-physics community, and in particular the members of the local organizing committee, led by Guy Wormser of LAL/Orsay. Now, the international community can look forward to the next ICHEP, which will be in Melbourne in 2012.

• Proceedings of ICHEP 2010 are published online in the Proceedings of Science, see http://pos.sissa.it.