Physics buzz in Paris
The International Conference on High Energy Physics (ICHEP) took place from 22 to 28 July in Paris, and first results from the Large Hadron Collider experiments received top billing.
The buzz about the LHC experiments caught the eye of French President Nicolas Sarkozy, who addressed the conference on Monday 26 July. President Sarkozy exhorted the particle physics community to continue its quest to understand the nature of the Universe, and stated his belief that investment in fundamental research is critical for the progress of mankind. Steve Myers started off the morning with a presentation on the LHC accelerator. He was followed by the spokespersons for ALICE, ATLAS, CMS and LHCb, who summarized the most important results from the performance of their detectors with the first few months of collision data, and the first measurements of proton collisions at 7 TeV.
One of the items of breaking news from the ATLAS and CMS experiments was the first observation of top quark candidates at the LHC. The top, the heaviest elementary particle observed to date, has so far only been produced at Fermilab’s Tevatron collider in the US. ATLAS and CMS also presented their measurements of the W and Z boson cross-sections, or probabilities of being produced. These measurements - made at energies 3.5 times higher than ever before - confirm predictions from the Standard Model. The two experiments have also measured the expected difference between the production of positively and negatively charged W bosons, which may ultimately help physicists better understand the structure of the proton.
The ALICE experiment’s new measurement of the number of charged particles produced from proton collisions at 7 TeV does not agree with predictions from theoretical models, and will send physicists back to their computers to further refine the models so that they better reflect the way the Universe works and better predict new phenomena.
LHCb reported clear measurements of several rare decays of beauty mesons, particles which contain a beauty quark. With the collision data expected to be delivered by the LHC over the next few months, the collaboration expects to make precise studies of the nature of CP violation, the difference in the way Nature treats matter and antimatter. These studies will allow LHCb to confirm or refute unexpected results regarding this matter-antimatter difference recently announced by the experiments at the Tevatron.
And even with a small amount of data, ATLAS and CMS have already presented limits for the possible existence of several exotic particles. ATLAS has placed new, improved limits on the possible existence of excited quarks, excluding their existence with masses of less than 1.29 TeV. The existence of such heavy quarks would imply that quarks are not fundamental, but are made up of even smaller particles. By collecting data from its detector in the period between collisions of LHC beam bunches, CMS has narrowed the limits on the existence of a proposed particle called a stopped gluino, showing that it cannot exist with lifetimes of longer than 75 nanoseconds.
All the experiments noted that the Worldwide LHC Computing Grid, which has met or even exceeded expectations in the first few months of collision data-taking, was greatly aiding their ability to produce first results quickly.
In his presentation, Myers reviewed events at the LHC accelerator since 2008, with special emphasis on the progress since the first collisions in November 2009. He also briefly presented the 10-year plan recently agreed by the CERN Management and LHC leadership. In addition to the known schedule for the next few years—the current LHC run will end in December 2011, and will be followed by a 15-month shutdown–there is also provision for two three-year running periods followed by long shutdowns in 2016 and 2020.
Another hotly anticipated presentation at ICHEP came from the CDF and DZero experiments at the Tevatron. The two experiments have not yet spotted the Higgs boson, but have further limited the territory in which it may be hiding. So the Higgs is still out there, waiting to be found, and the LHC experiments have shown at ICHEP that they are well on the way to joining the hunt.
For those who couldn't make it to Paris, August's LHC Physics Day will review the LHC physics results presented by the experiments at ICHEP.
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For further information:
CERN Press Release - Fermilab's Press Release - Symmetry Breaking article
