Measurement of the Transverse Momentum Spectra of Weak Vector Bosons Decaying to Muon Channels in Proton-Proton Collisions at the Centre-of-Mass Energy of 8 TeV with the CMS Detector

by Hammid Yusupov (Ph.D Candidate)

Monday Nov 7, 2016, 4:00 PM → 5:00 PM Asia/Seoul

201-4호 (제1과학관)

The CMS is a general purpose detector designed to study proton-proton collisions delivered by the LHC at the CERN. In this thesis we performed the measurement of the W boson cross section as a function of transverse momentum. This is the first measurement of W production cross section as a function of transverse momentum at the centre-of-mass energy of 8 TeV using the data collected with the CMS detector. The analysis is based on a data sample collected in 2012 during the low pileup runs corresponding to an integrated luminosity of 18.4 pb-1.
The W boson cross section measurements are performed with a study of their decay channel to a muon and a neutrino. Well reconstructed muons within the de- tector fiducial volume of |η| < 2.1 with at least 20 GeV transverse momentum on the plane perpendicular to the beam line are used. Studies on muon selection crite- ria, selection efficiencies and effects of momentum scale and resolution of muons are performed. The muon efficiencies are measured as functions of the muon pseu- dorapidity and transverse momentum based on the Tag-and-Probe method using the Z bosons with an invariant mass window of 60 to 120 GeV. Measured efficiencies in data and Monte Carlo are compared and differences are corrected in the analysis. Muon momentum scale and resolution are studied and corrections are applied to match the Monte Carlo and data.
The W boson transverse momentum spectrum is split into bins defined as the result of a compromise between the detector resolution and the statistics available. The number of W signal candidates are extracted in each of the W boson pT bins using a binned maximum likelihood fit to the transverse energy distribution, which is composed of several physical processes; W signal, QCD background and the dif- ferent electroweak backgrounds. A data driven technique is performed to estimate the background from QCD multijet events. Estimation of the backgrounds from other electroweak and top backgrounds are based on Monte Carlo simulations.
A two step unfolding procedure is performed to correct for detector resolution and final state radiation effects. Two response matrices were built and used to unfold the observed W transverse momentum spectrum to the truth level using a technique called Singular Value Decomposition unfolding. The unfolded spectra are corrected for inefficiencies and the obtained charged and charge independent W boson trans- verse momentum shapes are compared with several theoretical expectations like RESBOS, POWHEG and FEWZ. 
The differential cross section spectra as a function of transverse momentum are then normalized to the cross sections from [0-600] GeV. The normalized distributions are also compared with theoretical predictions. The ratios of W^− → μ^ ν-bar to W^+ → μ^+ ν and Z → μ^+ μ^- to W → μν normalized differential cross sections as a function of transverse momentum are also presented in this thesis. The measured differential cross sections and ratios are compared with theoretical predictions up to next-to-next leading order in QCD. The results are important for various reasons such as testing perturbative and nonperturbative theoretical calculations and understanding the weak boson signatures as backgrounds to other physics processes. Measured distributions are in agreement with the theoretical expectations within their uncertainties.

 

Thesis Advisor: Prof. Dongchul Son