Measurement of Associated Z$^0$-Boson and b-Jet Production in Proton-Proton Collisions with the CMS Experiment

In this thesis, data of proton-proton collisions at a centre-of-mass energyof s = 7 TeV, delivered by LHC and recorded with the CMS experimentin 2010 and 2011, is used to perform one technical and two physics analyses. The technical one develops and uses a method to estimate the precisionof the knowledge of the true geometry of the CMS inner tracking system in2011 data. It is used to quantify the remaining deviation between assumedand true geometry, and this is employed in the reconstruction of trajectoriesof charged particles, and thus optimises the performance of CMS.The first physics analysis calculates the ratio of cross sections of top-pairproduction (tt) and inclusive Z0 -boson production (i.e. Z0/γ ∗ restricted tomasses around the Z0 peak) in their muonic decay modes – leading to twospatially isolated high-momentum muons – on the full 2010 dataset spanning35.9 pb−1 . The advantage over absolute cross-section measurements is thatcommon experimental or theoretical uncertainties cancel, like it is definitelythe case for the measured luminosity. Furthermore, the tt cross section can benormalised using the ratio and the theoretical Z0 cross section, instead of theluminosity measurement. The processes are separated by the invariant dimuonmass, which shows a peak around the Z0 mass for the Z0 events, and a smoothwide curve for tt events. The final combination of the tt cross section measurements normalised to luminosity, respectively to the Z0 as standard candle,results in improved precision.The second physics analysis is the cross-section measurement of associatedZ -boson and b-jet production (bZ0 ) in 2.2 fb−1 of 2011 data, again in thedecay channel Z0 → µ+ µ− , but now requiring in addition the presence of atleast one b-tagged jet. This process is very similar to tt, since both containtwo muons and two b jets in the final state. Since their cross sections are alsosimilar, both processes are the main backgrounds for each other—however theycan be separated by their difference in missing transverse energy ET , arising//in tt from the undetected neutrinos, while there is no natural ET in generalin bZ0 . The focus lies on the estimation of the tt background, measuring thiscontribution in the dimuon invariant mass sideband region above the Z0 peakwhere it dominates, and extrapolating it into the signal region around the Z0bZ0peak. The inclusive cross section σinc , defined for 60 GeV < MZ0 < 120 GeV,Z0 → µ+ µ− , and at least one b jet on hadron-level in the kinematic rangebZ0pb > 25 GeV, |η b | < 2.1 and ∆R(jet,muons) > 0.5 yields σinc = 5.97 ±T0.10 (stat.)+0..78 (syst.) pb. The result is slightly above the prediction—this can−0 93be an indication that potential associated Higgs-boson and b-jet productioncross sections might also be above the prediction.0idea of matter consisting of different elements and of atomism – fundamentalbuilding blocks of matter – can be traced back to the ancient Greeks. A longtime later, after the concepts of atoms and of particles such as electrons andalpha particles were established by scientific research in physics and chemistry,Rutherford’s scattering experiment discovering the atomic nucleus founded avital technique to analyse the composition of matter in sub-atomic dimensions.Research on the experimental side – mainly via scattering experiments – andon the theoretical side, extended the picture and the new discipline particlephysics was born. The knowledge obtained throughout the last hundred yearsis described nowadays by the Standard Model of particle physics, which reduces all matter and its behaviour to a set of fundamental particles—matterconstituents which interact by exchange of force mediating particles. All butthe so-called Higgs boson are meanwhile established. However, there are indications that this theory is an approximation in the energy range investigated,by reason of divergencies when extrapolating to higher energies—several theoretical approaches, e.g. introducing additional heavy particles or extra spacedimensions, try to describe physics beyond the Standard Model. Since November 2009 the Large Hadron Collider (LHC) [1] at the European Organizationfor Nuclear Research CERN (see Figure 1.1) accelerates protons and providesproton-proton collisions, embracing an extensive physics programme. This establishes a new era of particle physics, providing measurements at a previouslyunknown energy scale.√ In this thesis, collisions provided by LHC at a centre-of-mass energy ofs = 7 TeV and recorded with the Compact Muon Solenoid Experiment(CMS) [2] are analysed; three different studies, a technical and two physicsanalyses, are presented. CMS is a multi-purpose detector with the typicalarrangement of different detector systems. The inner tracking system (calledtracker) is the biggest all-silicon tracker ever built. It is the entity of interest in the technical analysis presented in this work. The tracker is intended1