Measurement of the Inclusive $Z \to ee$ Production Cross Section in Proton-Proton Collisions at $\sqrt{s}$ = 7TeV and $Z \to ee$ Decays as Standard Candles for Luminosity at the Large Hadron Collider

Abstract

This thesis comprises a precision measurement of the inclusive \Zee production cross section in proton-proton collisions provided by the Large Hadron Collider (LHC) at a center-of-mass energy of $\sqrt{s}=7$~TeV and the absolute luminosity based on \Zee decays. The data was collected by the Compact Muon Solenoid (CMS) detector near Geneva, Switzerland during the year of 2010 and corresponds to an integrated luminosity of $\int\mathcal{L}dt = 35.9\pm 1.4$~pb$^{-1}$. Electronic decays of $Z$ bosons allow one of the first electroweak measurements at the LHC, making the cross section measurement a benchmark of physics performance after the first year of CMS detector and LHC machine operations. It is the first systematic uncertainty limited \Zee cross section measurement performed at $\sqrt{s}=7$~TeV. The measured cross section pertaining to the invariant mass window $M_{ee}\in (60,120)$~GeV is reported as: $\sigma(pp\to Z+X) \times \mathcal{B}( Z\to ee ) = 997 \pm 11 \mathrm{(sta t)} \pm 19 \mathrm{(syst)} \pm 40 \mathrm{(lumi)} \textrm{ pb}$, which agrees with the theoretical prediction calculated to NNLO in QCD. Leveraging \Zee decays as ``standard candles'' for measuring the absolute luminosity at the LHC is examined; they are produced copiously, are well understood, and have clean detector signatures. Thus the consistency of the inclusive \Zee production cross section measurement with the theoretical prediction motivates inverting the measurement to instead use the \Zee signal yield to measure the luminosity. The result, which agrees with the primary relative CMS luminosity measurement calibrated using Van der Meer separation scans, is not only the most precise absolute luminosity measurement performed to date at a hadron collider, but also the first one based on a physics signal at the LHC.