Search for the Standard Model Higgs Boson decaying to tau leptons with the CMS experiment at LHC

In this thesis, I present my work in the CMS experiment on a searchfor the standard model Higgs boson decaying into a pair of tau leptons.The original goal and the final aim of this research is a contribution to thediscovery of the Higgs boson at the LHC.After a summary of the theoretical framework and of the experimentalapparatus, I will describe the particle–flow technique developed for the offlineCMS event reconstruction, which I contributed to commission with the firstLHC collisions delivered at 900 GeV and 7 TeV center–of–mass energy. Theparticle–flow reconstruction gives optimal performances for the measurementof jets, missing transverse energy and taus, and is therefore ideally suited forthe search of the Higgs boson decaying to tau leptons.Tau reconstruction and identification in CMS are described. Particularemphasis is devoted to the discrimination between the electrons and tauleptons decaying semi–leptonically (giving hadrons and neutrinos), an aspectof the tau identification where I brought original improvements.A crucial and challenging aspect in the search for the Higgs boson decaying into tau leptons is the reconstruction of the di–tau mass, given thatan unknown fraction of the tau momenta is carried away by the undetectedneutrinos. Existing and novel di–tau mass reconstruction techniques aredescribed. On this subject, I have provided original contributions in the development of a new likelihood–based technique, called SVfit, which is thenused for the analysis.Finally, a search for the Standard Model Higgs boson in the di–tau chan√nel, based on 4.9 fb−1 of pp collisions at s = 7 TeV, is presented. A firstversion of this analysis was completed for the CMS publications in Spring2012. This analysis was then further improved and included in the com√bination using an additional 5.3 fb−1 collected at s = 8 TeV in the firsthalf of 2012. This 2012 version of the analysis is presented here. Events areselected where one tau decays semi–leptonically and the other decays fully–leptonically into neutrinos and lighter charged leptons (electrons or muons).The sensitivity of the analysis is enhanced by separating the events intoexclusive categories with different signal purity. One of the category is optimized for the vector–boson fusion production mechanism, a process that isintimately related to the very nature of the spontaneous symmetry breaking.No significant excess over the Standard Model backgrounds is observed.The statistical interpretation of the results is then translated into 95% CLupper limits on the signal cross–section.