Measurement of ttbar production differential cross section at \sqrt{s}=8 TeV at ATLAS

Top quark studies play an important role in the physics program of the Large Hadron Collider (LHC). The energy and luminosity reached allow the acquisition of a large amount of data especially in kinematic regions never studied before. In this thesis is presented the measurement of the ttbar production differential cross section on data collected by ATLAS in 2012 in proton proton collisions at \sqrt{s} = 8 TeV, corresponding to an integrated luminosity of 20.3 fb^{−1}. The measurement is performed for ttbar events in the semileptonic channel where the hadronically decaying top quark has a transverse momentum above 300 GeV. The hadronic top quark decay is reconstructed as a single large radius jet and identified using jet substructure properties. The final differential cross section result has been compared with several theoretical distributions obtaining a discrepancy of about the 25% between data and predictions, depending on the MC generator. Furthermore the kinematic distributions of the ttbar production process are very sensitive to the choice of the parton distribution function (PDF) set used in the simulations and could provide constraints on gluons PDF. In particular in this thesis is performed a systematic study on the PDF of the protons, varying several PDF sets and checking which one better describes the experimental distributions. The boosted techniques applied in this measurement will be fundamental in the next data taking at \sqrt{s}=13 TeV when will be produced a large amount of heavy particles with high momentum.

Measurement of the top quark mass in multijet events with the CMS experiment at LHC

La massa del quark top è qui misurata per mezzo dei dati raccolti dall’esperimento CMS in collisioni protone-protone ad LHC, con energia nel centro di massa pari ad 8 TeV. Il campione di dati raccolto corrisponde ad una luminosità integrata pari a 18.2 /fb. La misura è effettuata su eventi con un numero di jet almeno pari a 6, di cui almeno due b-taggati (ovvero identificati come prodotto dell’adronizzazione di due quark bottom). Il valore di massa trovato è di (173.95 +- 0.43 (stat)) GeV/c2, in accordo con la media mondiale. The top quark mass is here measured by using the data that have been collected with the CMS experiment in proton-proton collisions at the LHC, at a center-of-mass energy of 8 TeV. The dataset which was used, corresponds to an integrated luminosiy of 18.2 /fb. The mass measurement is carried out by using events characterized by six or more jets, two of which identified as being originated by the hadronization of bottom quarks. The result of the measurement of the top quark mass performed here is: (173.95 +- 0.43 (stat)) GeV/c2, in accordance with the recently published world average.

Measurement of the inclusive and differential t/t cross sections at 13 TeV in the all-jets boosted regime with CMS

The t/t production cross section is measured with the CMS detector in the all-jets channel in $pp$ collisions at the centre-of-mass energy of 13 TeV. The analysis is based on the study of t/t events in the boosted topology, namely events in which decay products of the quark top have a high Lorentz boost and are thus reconstructed in the detector as a single, wide jet. The data sample used in this analysis corresponds to an integrated luminosity of 2.53 fb-1. The inclusive cross section is found to be sigma(t/t) = 727 +- 46 (stat.) +115-112 (sys.) +- 20~(lumi.) pb, a value which is consistent with the theoretical predictions. The differential, detector-level cross section is measured as a function of the transverse momentum of the leading jet and compared to the QCD theoretical predictions. Finally, the differential, parton-level cross section is reported, measured as a function of the transverse momentum of the leading parton, extrapolated to the full phase space and compared to the QCD predictions.