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"The first measurement of the differential cross section of the associated production of a photon and a bottom quark in proton-proton collisions is presented. Protons are accelerated and brought into collision at a center-of-mass energy of 8 TeV by the Large Hadron Collider. The collisions are recorded with the ATLAS detector and correspond to an integrated luminosity of up to 20.2 fb-1. The measurement is performed in two regions of the absolute value of the photon pseudorapidity: from 0 to 1.37 and from 1.56 to 2.37. The measurement is performed as a function of the photon transverse energy, from 25 to 400 GeV in the former pseudorapidity region and from 25 to 350 GeV in the latter region. The relative uncertainty in the measurement varies between 13% and 54% and is dominated by the uncertainty in the efficiency of identifying jets containing bottom quarks. The ratio of the cross section in the two pseudorapidity regions is also measured to reduce the positively correlated systematic uncertainties in the measurement between the two regions. The measurement is compared to predictions of perturbative quantum chromodynamics at the leading order and at the next-to-leading order. At low transverse energy, a good agreement is observed between the measured and the predicted values. At high transverse energy, however, the measured values are observed to be up to a factor of two larger than the most precise predicted values. This measurement can help improve the modelling of bottom quarks in perturbative quantum chromodynamics." --
The associated production of a W boson and a single charm quark (W+c) is the only process in proton-proton collisions that directly probes the strange quark content of the proton. In this thesis, W+charm production is measured in proton-proton collisions at the LHC at 13 TeV, as recorded by the Compact Muon Solenoid (CMS) experiment. The analysis focuses on the identification of W bosons in their leptonic decay to a muon and a neutrino and charm quarks are tagged via the full reconstruction of D*-Mesons. The measured cross sections of W+c production are used, in combination with other relevant CMS results and the most precise HERA DIS data, in a QCD analysis to determine the strange quark content of the proton. The resulting strange quark distribution and suppression, with respect to the other light sea quarks, are in good agreement with those obtained in neutrino scattering experiments and extend their kinematic reach.
The first measurement of the cross section for top-quark pair production in proton-proton collisions at the Large Hadron Collider at center-of-mass energy of 7 TeV has been performed using a data sample corresponding to an integrated luminosity of 3 pb-1 recorded by the CMS detector. This result utilizes the final state with two isolated, highly energetic charged leptons, large missing transverse energy, and two or more jets. Backgrounds from Drell-Yan and non-W/Z boson production are estimated from data. Eleven events are observed in the data with 2.1+/-1.0 events expected from background. The measured cross section is 194+/-72 (statistical) +/-24 (systematic) +/-21 (luminosity) pb. The result is consistent with next-to-leading order theoretical predictions.
The proceedings report results on all aspects of high energy photon interactions on photon, proton and Pomeron targets. There are significant contributions from the LEP experiments, from ZEUS and H1, from CLEO II and from the TRISTAN experiments in Japan, accompanied by extensive theoretical discussion and predictions for future gamma-gamma colliders.
The top quark pair production cross section is measured for the first time in proton-proton collisions at √s= 13 TeV by theCMS experiment at the CERN LHC, using data corresponding to an integrated luminosity of 42 pb-1. The measurement is performed by analyzing events with at least one electron and one muon of opposite charge, and at least two jets. We then measured the cross section and found that was 769 ± 60 (stat) ± 55 (syst) ± 92 (lumi) pb, in agreement with the expectation from the standard model.
Scientific knowledge experienced an age of explosion during the XX century, fruitful in discoveries that radically changed the perspective we had of Nature. Our current quest is, however, to continue questioning the fundamental aspects of everything that surrounds us, to test until the very end how far the great theories can go, and to pursue with the undeniable task of human restlessness to answer the question “and why is that?”. In the specific topic of this work, we will make use of the largest machine ever built to obtain information about some of the smallest elements of matter, measuring with the best precision up to date the cross section of a process that could shed some light on the mystery of fundamental asymmetry between matter and antimatter in the Universe. Our mind demands symmetry...