Search

SPIRIT checklist. (DOC 121Â kb)

The large number of top quarks produced in proton-proton (pp) collisions at the LargeHadron Collider facilitates precision measurements of top-quark properties that are crucialfor testing the consistency of the Standard Model (SM) of particle physics. In particular,differential measurements of the top-quark pair (tt) production cross section are essentialfor comparison with state-of-the-art predictions of Quantum Chromodynamics (QCD)within the SM and for searches for new physics phenomena beyond it. In this thesis,normalised differential tt cross sections are measured at a pp centre-of-mass energy of√s = 8 TeV. The analysed dataset is recorded by the CMS experiment and correspondsto an integrated luminosity of 19.7 fb−1 . The tt production cross section is measureddifferentially as a function of eighteen kinematic quantities of the top quarks and theirdecay products. The achieved precision of typically 3 − 7 % allows to distinguish betweendifferent QCD predictions for several of the analysed quantities. In particular, the measuredtt production cross section as a function of the top-quark transverse momentum and itsimpact on different top-quark related measurements, also outside the scope of this work,is discussed in detail. In addition, the top-quark mass is determined from the normaliseddifferential cross section as a function of the invariant mass of the tt system and oneadditional jet. The obtained result is compatible with other measurements, including theworld-average top-quark mass, within the achieved precision of about 1.5 %.

A measurement is presented of the normalized differential cross sections of top-quark pair production in pp collisions at a center of mass energy of 7 TeV, using 1.1 fb$^{-1}$ of data collected in 2011 by the CMS experiment. The measurement is performed using five different final states with one or two muons or electrons in the final state. The event selections yield high purity ${\rm t}\bar{\rm t}$ samples. Kinematic reconstructions are performed to obtain full information of the top quarks, the ${\rm t}\bar{\rm t}$ system and the final state leptons. The results are compared to several QCD predictions up to next-to-leading order.