Measurement Of The Wγ/Zγ Production Cross Section In Proton-proton Collisions With The ATLAS Detector

In the last fifty years, the knowledge of elementary particles and their in-teractions has been described with the so-called Standard Model (SM) particle physics. The Standard Model theory describes all observed sub-atomic phe-nomena, and has been hugely successful in describing the elementary particles. However, it contains a number of crucial missing pieces, particularly the Higgs boson, which is predicted in the Standard Model theory particularly has not been observed yet, and the accuracy of the theory in describing phenomena at much higher energy scales has yet to be verified.The Large Hadron Collider (LHC) at CERN (European Organization for Nuclear Research) near Geneva is the biggest and most powerful collider for high energy physics research in the world. The designed center of mass energy of the LHC is 14 TeV with a peak instantaneous luminosity of 1034 cm-2s-1. The main physics goals of the LHC arc to verify the Standard Model theory, to search for the Higgs boson and other new particles predicted by various theories beyond the Standard model, such as heavy W' and Z' particles, super-symmetric particles and extra-dimension.The ATLAS (A Toroidal LHC ApparatuS) is one of two general purpose detectors, which is located at Point 1 of the LHC. The ATLAS detector is cylin-drical, approximately 7000 tonnes in mass, with a total length about 42 m and a radius of 11 m. There are four major components in the ATLAS detector: the Inner Detector, the Calorimeters, the Muon Spectrometer and the Magnet System.This thesis contains three parts. Part 1 is for theory and experimental setup. Part 2 discusses a new tool for ATLAS analysis—missing transverse momentum calculation based on tracks (pTmiss). The measurement of the Wγ/Zγ3 production cross section in proton-proton collisions at (?)= 7 TeV with the ATLAS detector is presented in Part 3. The cross section measurement is only performed in the electron channel.A precise measurement of the missing transverse energy (ETmiss) is essential for many physics studies at the LHC. It is particularly crucial for some new physics searches, such as supersymmetry or extra dimensions, where a large ETmiss is a key final state signature. However, several effects could either fake large ETmiss or degrade its resolution leading to an increase of the background from physics processes with no real intrinsic transverse missing energy.This thesis presents a method to estimate the missing transverse momentum (pTmiss) as a complement to the existing calorimeter measurement of ETmiss, which is essential for many physics studies at the LHC. The independent measurement of pTmiss could then be used to validate the quality ETmiss measured with the calorimeters as well as an extra tool for background rejection. The performance of missing transverse momentum have been measured based on tracks reconstructed by the ATLAS Inner Detector in both 900 GeV and 7 TeV proton proton collisions data. The measurement of pTmiss in collision data is presented and comparisons of its performance to expectations from Monte Carlo simulation show a very good agreement. pTmiss could also be used as an extra tool for background rejection in the presence of pile-up.The measurement of the Wγ/Zγproduction cross section in proton-proton collisions provides an important test of the Standard Model theory. The pro-duction of Wγand Zγprocesses are directly sensitive to the triple gauge boson couplings, which has been predicted by the non-Abelian SU(2)L×U(1)Y gauge group of the electroweak sector. Physics beyond the SM (composite structure of W and Z bosons, new vector bosons, etc.) will enhance the Wγand Zγcross sections and alter the production kinematics. Data taken with the ATLAS detector provide a new opportunity to study Wγand Zγproduction using the high energy pp collisions provided by the Large Hadron Collider. Previous mea- surements have been made at the Fermilab TeVatron collider by the CDF [1] and DO [2] experiments using pp collisions at (?)=1.8 TeV and (?)= 1.96 TeV.The cross sections of p+p→e+v+γ+X and p+p→e+e+γ+ X is measured, where the photon has ET>15 GeV andΔR (e,γ)>0.74 in a pp collisions at center of mass (?)=7 TeV. The sources of Wγand Z-γinclude photon radiation from the final state leptons, the initial state quarks and from hard fragmentation of a quark or gluon. In the case of W bosons, there is an additional contribution, from the W boson via the direct WWγcoupling. The results are based on 95 Wγand 25 Zγcandidate events collected in a data set corresponding to an integrated luminosity of approximately 35 pb-1, based on the full 2010 pp data collected by the ATLAS detectors with fully operational detector and stable beam conditions.The efficiency of the electron trigger that is used to collect the events for the Wγand Zγanalysis, is measured from data using the "W Tag and Probe" method. The electron identification efficiency is measured using the signal Monte Carlo simulation sample with correction factors obtained from "Tag and Probe" measurements using W→eνand Z→ee events, based on data-driven methods. The photon selection efficiency is factorized into several components and mea-sured separately. The photon identification efficiency is measured using the signal Monte Carlo simulation sample with fudge factor corrections, which accounts for the small discrepancy between the data and simulation in the distributions of the photon identification variables. The photon isolation efficiency (εiso) is estimated from Wγand Zγsignal Monte Carlo and cross checked with data using probe electrons extrapolation from the Z→ee sample. The efficiency of the photon component in the fragmentation is calculated using W+1jet Monte Carlo sim-ulation sample by selecting a high transverse energy photon coming from the jet The ATLAS Coordinate System is a right-handed system. The nominal interaction point is defined as the origin of the coordinate system, while the z-axis is defined as the beam direction and the x-y plane is transverse to the beam direction. The positive x-axis is defined as pointing from the interaction point to the center of the LHC ring and the positive y-axis is defined as pointing upwards. The azimuthal angleφis measured around the beam axis and the polar angleθis the angle from the beam axis. The pseudorapidity is defined asη=-lntan(θ/2). The polar angleθis measured from the positive z axis. The distanceΔR inηφspace is defined asΔR=(?). fragmentation.The dominant source of background to the Wγ(Zγ) measurement is the W(Z)+jets process, when photons, present in the decay products cascade of mesons produced in the jet hadronization, pass our selection criteria ("Jet-Fake"). Since the jet faking photon processes is not well modeled by the W+jets Monte Carlo simulation, leading to very large systematic uncertainties, we use a data-driven method to estimate the amount of W+jets events present in our Wγselected events. The small contributions from other background processes are estimated using Monte Carlo simulation. The same data driven technique can in principle be adopted to estimate the Z+jets background in Zγevents. However the limited data sample size (total 25 Zy candidates in real data) currently available docs not allow a reliable estimation of such a background. Therefore, for the Zγanalysis, the Z+jets background is also estimated from Monte Carlo with a large associated uncertainty (100%). Furthermore, the photon purity in the selected events is measured to be 80%. The background subtracted data are compared to Standard Model predictions of the signal.In this thesis, both fiducial cross sections and total cross sections are mea-sured. The systematic uncertainties for the fiducial cross sections measurement are mostly from the photon identification and electromagnetic energy scale. For the total cross sections measurement, the additional significant uncertainties are the uncertainties from parton distribution functions and higher order QCD cor-rections. The fiducial cross sections areσWγ(?)fiducial=5.4±0.7(stat)±0.9(syst) 0.2(lumi) pb andσZγfiducial=2.2±0.6(stat)±0.5(syst)±0.1(lumi)pb. The to-tal inclusive Wγand Zγproduction cross sections times the respective lcpton-ic branching ratios for the electron channels arc measured to beσWγ(?)total=48.9±6.8(stat)±8.2(syst)±1.7(lumi) pb andσZγtotal=9.0±2.5(stat)±2.1(syst)±0.3(lumi) pb. The measured production cross sections, together with the kine-matic distributions of the electrons and photons in data signal candidate events, are found to agree with leading order Standard Model predictions after NLO k-factor corrections.