Study of Drell-Yan process in CMS experiment at Large Hadron Collider

The proton-proton collisions at the Large Hadron Collider (LHC) is the begining of a new era in the high energy physics. It enables the possibility of the discoveries at high-energy frontier and also allows the study of Standard Model physics with high precision. The new physics discoveries and the precision measurements can be achieved with highly efficient and accurate detectors like Compact Muon Solenoid. In this thesis, we report the measurement of the differential production cross-section of the Drell-Yan process, $q ar{q} ightarrow Z/gamma^{*} ightarrowmu^{+}mu^{-}$ in proton-proton collisions at the center-of-mass energy $sqrt{s}=$ 7 TeV using CMS experiment at the LHC. This measurement is based on the analysis of data which corresponds to an integrated luminosity of $intmath{L}dt$ = 36.0 $pm$ 1.4 pb$^{-1}$. The measurement of the production cross-section of the Drell-Yan process provides a first test of the Standard Model in a new energy domain and may reveal exotic physics processes. The Drell-Yan process which is understood with high precision, up to next-to-next leading order accuracy, is an important handle for the detector calibration and alignment. Also the Drell-Yan process is an important background for the Higgs searches and other exotica searches i.e. SUSY, large extra dimensions, etc. and hence it is essential to understand the Drell-Yan production very accurately with the LHC data available recently. In the present analysis, the Drell-Yan production cross-section has been measured in the invariant mass range of 15-600 GeV/$c^{2}$. The measured cross-section is corrected for the detector effects, selection efficiencies and effects of Final State Radiation. In order to reduce the systematic uncertainty due to the luminosity, the corrected cross-section is then normalized to the cross-section of the Z-mass peak region defined in the mass window 60-120 GeV/c$^{2}$. In this analysis the cross-section of the Z-mass peak region has been measured to be 923 $pm$ 5 (stat) $pm$ 6 (syst) $pm (4%)$ lumi pb and is consistent with the CMS measurement of the Z-mass peak cross-section which is 974 $pm$0.7 (stat) $pm$0.7 (syst) $pm$ 4% (lumi) pb. Thus the final results in this analysis are presented in the form of this ratio of cross-sections and found to be in good agreement with the theoretical predictions at next-to-leading order. This thesis also reports the measurement of the Underlying Event using the Drell-Yan process, $q ar{q} ightarrow Z/gamma^{*} ightarrowmu^{+}mu^{-}$, in the proton-proton collisions at the center-of-mass energy of $sqrt{s}=$ 7 TeV using CMS experiment at the LHC. This analysis also uses data which corresponds to an integrated luminosity of $intmath{L}dt$ = 36.0 $pm$ 1.4 pb$^{-1}$. The Drell-Yan process provides an excellent way to study the Underlying Event activity by separating the hard interaction from the soft component. Good understanding of the Underlying Event is important for the precision measurements of SM processes and the search for new physics. Some of the examples where the UE plays crucial roles are the determination of the losses of events due to isolation criteria in lepton identification, computation of missing transverse energy, or the computation of reconstruction efficiency for processes, like $H ightarrowgammagamma$ where the vertex is given by the Underlying Event. In the present analysis, Underlying Event activity is measured along, transverse and opposite to the direction of muons pair in a plane transverse to the beam direction. The experimental results are corrected for the detector effects and selection efficiencies and compared with the predictions of various models for soft interactions. The measured Underlying Event activity is not fully described by any of the existing models and hence requires further tuning of the theoretical models.