Measurement of the low mass Drell-Yan cross section in the di-muon channel in proton-proton collisions at √s = 7 TeV with the ATLAS detector.
Abstract
The low mass Drell-Yan di-muon process is investigated with the ATLAS detector at
the LHC, in order to provide information that advances our knowledge of the Parton
Density Functions in a region of phase space unaccessible at previous experiments.
A cut-based selection of di-muon events is performed, using 2010 data with a
centre of mass energy of the proton-proton collisions of 7TeV, and an integrated
luminosity of 36 pb−1. The analysis probes the region of low muon transverse momentum
(pT > 6GeV), and low di-muon mass region 12 < M < 66GeV.
The main challenges of the analysis are the understanding of the muon isolation
and the trigger efficiency. In order to reject the large QCD background the analysis
relies on stringent isolation criteria. The efficiency of the chosen selection is presented
in detail. The second main part of the analysis is the measurement of the
trigger efficiency for low pT threshold muon triggers. This is an important aspect of
the cross section measurement, since the pT spectrum of leptons from the low mass
Drell-Yan process are soft and populate the trigger threshold region.
In order to measure the differential cross section in mass d
dM in the fiducial
region of |η| < 2.4 and pT,μ1 > 9GeV and pT,μ2 > 6GeV (asymmetric analysis) or
pT,μ1 > 6GeV and pT,μ2 > 6GeV (symmetric analysis), a one dimensional bin-by-bin
unfolding is adopted to account for detector reconstruction and resolution effects;
all the associated uncertainties are also presented. The fiducial and extrapolated
differential cross sections are measured at the Born level. Dressed level corrections
are also given. The measured cross sections are shown to agree with theoretical
predictions within the margin of error.
A precision of 9.7% is achieved in the asymmetric analysis in the lowest invariant
mass bin between 12 and 17GeV; the statistical and systematic uncertainties in the
same bin are 4.2% and 8.7% respectively. In the remaining mass region the total
uncertainty is smaller. The luminosity error during the 2010 data taking period is
estimated to be 3.4%.
In addition to the Drell-Yan cross section measurement, the thesis describes the
study performed in order to extract the Lorentz angle value in the ATLAS Semiconductor
Tracker. The Lorentz angle is computed through the study of the SCT
cluster width from both cosmic and collision data and comparison with simulation
is shown. The track selection on collision data is defined and the fitting range is
optimised to give robust results.
Throughout this thesis the convention c = 1 is adopted.
Authors
Piccaro, ElisaCollections
- Theses [4114]