The baton passes to the LHC

Held in the picturesque mountain setting of La Thuile in the Italian Alps, the international conference “Rencontres de Moriond” showed how the baton of discovery in the field of high-energy physics is definitely passing to the LHC experiments. In the well-known spirit of Moriond, the conference was an important platform for young students to present their latest results. The Higgs boson might well be within reach this year and the jet-quenching phenomenon might reveal new things soon…


New physics discussed over the Italian Alps during the "Les rencontres de Moriond" conference.  (Photographer: Paul Gerritsen. Adapted by Katarina Anthony)

Known by physicists as one of the most important winter conferences, “Les rencontres de Moriond” are actually a series of conferences spread over two weeks covering the main themes of electroweak interactions, QCD and high-energy interactions, cosmology, gravitation, astroparticle physics and nanophysics. The Bulletin attended the high-energy interactions session that saw presentations on, in particular, the search for the Higgs boson, the physics of the top quark and the search for new objects. “This is one of the rare conferences where there are still vibrant discussions between theorists and experimentalists. It is particularly motivating for the young students who come here to present their latest results,” says Bolek Pietrzyk, main organizer of the high-energy interactions session.

In the race for the highest collision energy, the Tevatron in the US preceded the LHC. In La Thuile, the Tevatron experiments presented new results confirming that there is no Higgs boson in the mass region between 159 GeV and 173 GeV (the result comes with a 95% confidence level – a number that underlines the statistical reliability of the figures). “CMS and ATLAS can’t yet reach the Tevatron experiments’ sensitivity in the search for the Higgs in this mass region,” says Greg Landsberg, member of the CMS Collaboration. “However, within a year, if all goes well and if the LHC delivers the number of collisions we all expect, both CMS and ATLAS will be able to fully explore the mass region between 130 GeV and 460 GeV.” If physicists do not see any evidence of the famous boson in that mass region, they will be able to conclude that no new particle exists with the properties of the Higgs boson and that mass. On the other hand, if a new signal appears in the data, the strict laws of statistics will force them to wait for more data before confirming any new discovery and this will only happen in 2012.

Conference participants at La Thuile.

The Higgs boson is not the only new object that physicists are chasing. Experiments like LHCb are looking for new physics through the lens of rare decays of the B particle, which requires a very high sensitivity of the experimental apparatus and an extremely high accuracy in the data analysis. “In La Thuile, we showed that, after just a few months of operation, our detector has reached a sensitivity that in some cases exceeds that of other detectors that have been running for years,” says Rob Lambert from the LHCb collaboration. “By the end of the year, we hope to be able to measure, among other things, the number of muons arising from the B decay. This is a very important measurement that complements the famous measurement performed by the D0 experiment. Last year, they showed for the first time an unexpected asymmetry in the number of muons. With our data, we will be able to confirm whether or not the observed phenomenon can be associated with any new physics.”

In early December last year, the first ion-ion collisions at the LHC confirmed the astonishing jet-quenching phenomenon, one of the possible signatures of the quark-gluon plasma. “For the first time, in the LHC experiments we can actually see the disappearance of the energy of the recoiling jet that is interacting with the quark-gluon matter. At LHC we can gain new insights into the strong interaction by doing quantitative studies of the dynamics of jet quenching,” says Frank Ma from MIT and member of the CMS Collaboration. “The conference was a good opportunity for us to discuss the energy redistribution of the jet energy that happens over an unexpected wide angle. This important observation was recently made by CMS and ATLAS. In another highlight from the ALICE experiment, it was shown that the effects of the strongly interacting medium at lower particle momenta are stronger than those observed at RHIC. The LHC allows us to study the behaviour of the medium effects at high particle momenta. These recent findings will give valuable inputs to theorists to fully understand the jet quenching phenomenon.”

Discovered in 1995 at the Tevatron, the Top quark has yet to be fully explored as, with its very high mass, it sits astride the border between Standard Model physics and new physics. “CMS and ATLAS presented here for the first time the result of their analysis on the whole 2010 data set. Their sensitivity in measurements of the top cross section is getting similar to that of Tevatron and the experiments are now ready to study other properties of the particle, such as the precise measurement of the mass. We will use the 2011 data to look for new physics in the production and decay mechanism of the top quark,” says Meenakshi Narain, from Brown University in the US and member of the D0 and CMS Collaborations. “For the time being, the most precise measurement of the top quark mass and properties come from D0 and CDF but at the LHC we have already observed single top quark production, something that took us 14 years to observe at the Tevatron.”

by CERN Bulletin