Résumé

Interactions à hautes énergies dans les Alpes

Organisées dans la pittoresque station de La Thuile, dans les Alpes italiennes, les « Rencontres de Moriond » désignent l’une des principales conférences d’hiver dans le domaine de la physique des particules. Sous la forme d’une série de séminaires répartis sur deux semaines, il y est question d’interactions électrofaibles, de chromodynamique quantique et d’interactions hautes énergies, de cosmologie, de gravitation, d’astrophysique des particules et de nanophysique. L’article évoque certains résultats présentés lors des quelque 90 conférences données à la session 2011 sur le thème de la chromodynamique quantique et des interactions à hautes énergies, du 20 au 27 mars. La conférence a confirmé que ce sont les expériences LHC qui prennent le relais s’agissant des découvertes en physique des hautes énergies.

Held in the picturesque mountain setting of La Thuile in the Italian Alps, the international Rencontres de Moriond is one of the most important winter conferences for particle physics (CERN Courier July/August 2005 p21). Composed of meetings spread over two weeks, it covers the main themes of electroweak interactions, QCD and high-energy interactions, cosmology, gravitation, astroparticle physics and nanophysics. This article reviews some selected results from the approximately 90 talks presented at the 2011 QCD and high-energy interactions session on 20–27 March.

In the well known spirit of the Moriond meetings, the conference provided an important platform for young physicists to present their latest results. In particular, the sessions this year covered the search for the Higgs boson, the physics of heavy flavours and the top quark, the search for new objects and the first results from the heavy-ion run at the LHC. Lively discussions between theorists and experimentalists followed the presentations and were particularly motivating for the young physicists present.

The LHC had an outstanding first year of operation in 2010, with beam intensity rising systematically over the course of the year. The LHC experiments collected 35–40 pb–1 of proton–proton collision data, of which around 50% were taken during one of the last weeks of proton running. Lead–ion collisions were observed for the first time in November. In 2011 and 2012, most of the run time is planned for physics data-taking, with the aim of collecting 1–3 fb–1 of proton collisions per experiment in 2011.

In the quest for the highest collision energies, the LHC was preceded by the Tevatron at Fermilab in the US. In La Thuile, the collaborations for the CDF and DØ experiments at the Tevatron presented new, combined results, confirming that there is no Standard Model Higgs boson in the mass region between 159 GeV and 173 GeV (95% confidence level). This year, both collaborations also presented exclusion limits within this Higgs-mass region. The Tevatron will end its successful period of data-taking in September. With all of the collected data and improved analyses, the CDF and DØ teams expect to exclude the existence of the Higgs boson in the whole mass region between 114 GeV and 200 GeV – if it does not exist. On the other hand, the experiments will not have enough data to prove discovery if a Higgs does, indeed, exist in this mass region.

The CMS and ATLAS experiments at the LHC cannot yet reach the Tevatron experiments’ level of sensitivity in the search for the Higgs boson. However, within a year and if all goes well and the LHC delivers the expected number of collisions then both CMS and ATLAS will be able to explore the full range between 130 GeV and 460 GeV. If the teams do not see evidence of the Higgs in this wide mass region then they can conclude that no new particle exists with the properties of the Higgs boson and that mass. If a new signal does appear in the data, they will need to wait for more data and improved statistics before confirming any new discovery – but this will happen only in 2012.

The region for a low-mass Higgs, between the 114 GeV limit set by the experiments at the Large Electron–Positron collider and 130 GeV, is more difficult at the LHC. More data time will be needed to exclude or discover the Higgs in this region. The exclusion limits depend on the theoretical calculations of Higgs boson production. The theoretical uncertainties of these calculations formed the subject of a long and interesting discussion between experimentalists and theorists during the Moriond meeting.

One important area of the LHC programme relates to direct searches for new phenomena. The ATLAS and CMS collaborations presented results from the 2010 data-taking period, which show that new physics has not (yet?) been found. However, in many cases the exclusion limits have already surpassed the ones from the Tevatron. The search for new phenomena has always played an important role at the Moriond meetings and is set to become even more so following the increase in luminosity and energy at the LHC.

The LHC experiments are also searching indirectly for new physics. LHCb is doing so through the lens of rare decays of the B particle. This requires high sensitivity of the experimental apparatus and extremely high accuracy in the data analysis. At La Thuile, the LHCb collaboration showed that – after just a few months of operation – their detector has reached a sensitivity that in some cases is already comparable to other detectors that have run for years. These include the measurements of the rare decay of the Bs meson to pairs of muons, where the Standard Model branching ratio is precisely calculated (CERN Courier April 2011 p9), as well as the mixing frequency in the Bs system. By the end of 2011, LHCb may be able to measure, among other things, the production rate of like-sign muon pairs in B decay. This is important to complement the measurement by DØ, which showed an unexpectedly high matter–antimatter asymmetry in the number of pairs from B0 decay (CERN Courier July/August 2010 p6). LHCb should confirm whether or not the observed phenomenon can be associated with new physics.

In early December last year, the first ion–ion collisions at the LHC confirmed the astonishing jet-quenching phenomenon (CERN Courier January/February 2011 p6), one of the possible signatures of quark–gluon plasma. For the first time, the LHC experiments could actually see the disappearance of the energy of the recoiling jet that is interacting with the produced medium, providing new insights into the strong interaction through quantitative studies of the dynamics of jet quenching. The Moriond conference provided a good opportunity to discuss the redistribution of the jet energy, which happens over an unexpectedly wide angle, as observed recently by CMS and ATLAS. This is an important step towards understanding jet quenching, as well as the behaviour of the medium in heavy-ion collisions. In another highlight, the ALICE collaboration has found that the effects of the strongly interacting medium at lower particle momenta are stronger than those observed at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven. These recent findings will give valuable input to theorists and improve understanding of the jet-quenching phenomenon, and the LHC will allow the effects of the medium to be studied at high particle momenta.

The top quark was discovered at the Tevatron in 1995 but it has yet to be explored fully because – with its high mass – it sits astride the border between Standard Model physics and new physics. At La Thuile, the CMS and ATLAS collaborations presented for the first time results of their analyses of the whole 2010 dataset. Their sensitivity in measurements of the top cross-section is approaching that of the Tevatron experiments and they are now ready to study other properties of the particle, for example making a precise measurement of the mass. For the time being, the most precise measurements of the properties come from DØ and CDF, but the LHC experiments have already seen the production of single top quarks, something that it took 14 years to observe at the Tevatron.

CDF and DØ have observed significant forwards–backwards tt asymmetries in the proton–antiproton collisions at the Tevatron, particularly at a tt mass above 450 GeV. This could be interpreted as a sign of new physics. The size of the effect is expected to be smaller in the proton–proton collisions of the LHC, so interesting comparisons with the Tevatron are not expected until the end of 2011.

The search for new physics requires an excellent understanding of Standard Model processes. In this respect, the LHC experiments have shown important progress in jet reconstruction and calibration, while theorists have made improvements in higher-order QCD corrections, discussed in detail at La Thuile. The agreement now achieved between experimental measurements and theoretical calculations is setting an important baseline in the search for new phenomena.

Meanwhile, far from the LHC, the Pierre Auger Observatory (PAO) in South America has opened the window to the study of interactions at far higher energies in the cosmic radiation. The PAO collaboration presented evidence of an unexpected effect: the highest-energy cosmic rays may have an important contribution from iron ions (CERN Courier October 2010 p9). This observation was possible because protons and iron nuclei generate showers of different shapes but confirmation of the effect will require a better understanding of these shower shapes.

During their long history the Rencontres de Moriond meetings have followed advances at the frontier of energy at the Tevatron, the frontier of flavour at the BaBar and BELLE experiments, the frontier in heavy-ions at RHIC and the detailed measurements of structure functions at the HERA electron–proton collider at DESY. This year, the evidence at La Thuile is that these excellent research programmes will all be continued at the LHC and its experiments.