“This very timely publication is a milestone for the whole experiment and for all the participating scientists in Switzerland”, says Guenther Dissertori, professor of Physics at the ETH Zurich. “The recently published results are based on the data taken with the CMS detector, one of four experiments of the new accelerator of CERN in Geneva, the LHC. Along with two colleagues, Dissertori coordinates the physics analysis activities of almost 3’000 physicists, all scientists of the CMS collaboration.

Data analysis within hours

The researchers have been waiting 15 years for this moment. This is the time it took to plan and build the detector, to put it in function and to obtain the first measurements. The incident of 2008, which caused the stoppage of the LHC until the autumn of 2009, extended this period of an extra year. However, at the end of November 2009 the LHC started for the first time to accelerate protons and to make them collide, enabling scientists, and among them researchers and students from ETH Zurich (Groups of professors Günther Dissertori, Felicitas Pauss and Christoph Grab), to analyse in a matter of hours the very first data.

Scientists were interested in charged particles resulting from the proton collisions, and quickly found interesting candidates for the sought events. After having analyzed the raw data in record time, scientists could establish how many particles were produced, determine their spatial distribution and average momentum, as well as how such quantities depend upon the centre-of-mass energy. The pixel detector, one of the components of CMS, played a major role in this analysis. It was developed under the leadership of Roland Horisberger, Professor of physics at the Paul Scherrer Institute (PSI), as a joint effort with colleagues from the ETH and the university of Zurich. Vincenzo Chiochia, Professor of physics at the University of Zurich, is currently deputy project manager of the CMS Tracking system. “This part of the detector played a crucial role for analysing the data and reconstructing particle tracks”, explains Dissertori.

The successful operation and fast data analysis of an experiment of unprecedented complexity is really impressive, as pointed out by the referee of the Journal to which the article was submitted. While clearly this is a success for the whole collaboration, the timely and efficient data analysis owes a lot to colleagues from the Hungarian Universities (Budapest and Debrecen) and the Massachusetts Institute of Technology in Boston. The scientists are very proud of the quality of the measurements. So far there are no significant and fundamental discoveries. “However, we did not expect that”, confirms Dissertori. “The fact that our data agree with expectations shows that we have an excellent understanding of the detector and of its simulation”.

Beating of record in sight

The first collisions took place at the LHC with a relatively modest energy of 900 billions of Electron Volts (0.9 TeV). In December 2009, the LHC broke a world record by reaching the energy of 2.36 TeV. The previous record of 1.96 TeV was held since 2001 by a US accelerator, the Tevatron at Fermilab. After this success, the LHC was switched off as planned for maintenance at the end of 2009. It should be put back in service soon, and in March it should be ramped up to 7 TeV, setting a new record. “A higher energy step was originally planned, however it was recently decided to operate the LHC for two years at this energy, to learn and to observe before going further in energy”, explains Dissertori.

In parallel, the competition between Europe and the US for the discovery of new particles goes on. Following the LHC incident in 2008, the European scientists feared that their US colleagues could prove the existence of the long sought Higgs boson before them. The construction of the LHC in Geneva was largely motivated by the hope of such prestigious discoveries. The publication of the CMS results, however, opens an era of competition beyond the one between Europe and the US: the competition between the four LHC experiments. The CMS collaboration is very pleased to have been the first to publish the analysis of the data produced at 2.36 TeV. “The other experiments will follow during the coming weeks”, says Dissertori, continuing: “This is a highly positive competition”.