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04.07.2012 09:19

CMS Experiment, CERN: Observation of a New Particle with a Mass of 125 GeV

In a joint seminar today at CERN and the “ICHEP 2012” conference in Melbourne (1), researchers of the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) presented their preliminary results on the search for the standard model (SM) Higgs boson in their data recorded up to June 2012.

Event recorded with the CMS detector in 2012 at a proton-proton centre of mass energy of 8 TeV. The event shows characteristics expected from the decay of the SM Higgs boson to a pair of photons (dashed yellow lines and green towers). The event could also be due to known standard model background processes.

CMS observes an excess of events at a mass of approximately 125 GeV[2] with a statistical significance of five standard deviations (5 sigma)[3] above background expectations. The probability of the background alone fluctuating up by this amount or more is about one in three million. The evidence is strongest in the two final states with the best mass resolution: first the two-photon final state and second the final state with two pairs of charged leptons (electrons or muons). We interpret this to be due to the production of a previously unobserved particle with a mass of around 125 GeV.

The CMS data also rule out the existence of the SM Higgs boson in the ranges 110-122.5 GeV and 127-600 GeV with 95% confidence level[4] – lower masses were already excluded by CERN’s LEP collider at the same confidence level.
Within the statistical and systematic uncertainties, results obtained in the various search channels are consistent with the expectations for the SM Higgs boson. However, more data are needed to establish whether this new particle has all the properties of the SM Higgs boson or whether some do not match, implying new physics beyond the standard model.

The LHC continues to deliver new data at an impressive rate. By the end of 2012, CMS hopes to have more than triple its total current data sample. These data will enable CMS to elucidate further the nature of this newly observed particle. They will also allow CMS to extend the reach of their many other searches for new physics. 

For interview on UZH News with Prof. Vincenzo Chiochia see: www.uzh.ch 

For more information you can contact the following experts:
University of Zurich, Prof. Vinchenzo Chiochia Tel. 076 487 57 50
ETH Zurich, Prof. Rainer Wallny  Tel. 076 487 88 29
Prof. Günter Dissertori  Tel. 076 487 32 86
PSI, Prof. Roland Horisberger Tel. 079 631 93 62
PD. Michael Spira  Tel. 056 310 36 56

[1] ICHEP is the 36th International Conference on High Energy Physics, Melbourne, Australia from 4-11 July, 2012. The results will be presented jointly: in person at CERN and by real-time video link to ICHEP.
[2] The electron volt (eV) is a unit of energy. A GeV is 1,000,000,000 eV. In particle physics, where mass and energy are often interchanged, it is common to use eV/c2 as a unit of mass (from E = mc2, where c is the speed of light in vacuum). Even more common is to use a system of natural units with c set to 1 (hence, E = m), and use eV and GeV as units of mass. 
[3] The standard deviation describes the spread of a set of measurements around the mean value. It can be used to quantify the level of disagreement of a set of data from a given hypothesis. Physicists express standard deviations in units called “sigma”. The higher the number of sigma, the more incompatible the data are with the hypothesis. Typically, the more unexpected a discovery is, the greater the number of sigma physicists will require to be convinced.
[4] Confidence level is a statistical measure of the percentage of test results that can be expected to be within a specified range. For example, a confidence level of 95% means that the result of an action will probably meet expectations 95% of the time.
[5] http://news.stanford.edu/news/2004/july21/femtobarn-721.html
[6] cms.web.cern.ch/news/cms-search-standard-model-higgs-boson-lhc-data-2010-and-2011 [7] http://cms.web.cern.ch/news/blinding-and-unblinding-analyses

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