35 research outputs found

    Event activity dependence of Y (nS) production in √sNN = 5.02 TeV pPb and √s = 2.76 TeV pp collisions

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    The production of isolated photons in PbPb and pp collisions at √s_(NN) = 5.02 TeV

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    The transverse energy (E^Îł_T) spectra of photons isolated from other particles are measured using proton-proton (pp) and lead-lead (PbPb) collisions at the LHC at √s_(NN) = 5.02 TeV with integrated luminosities of 27.4 pb⁻Âč and 404 ÎŒb⁻Âč for pp and PbPb data, respectively. The results are presented for photons with 25 < E^Îł_T < 200 GeV in the pseudorapidity range |η| < 1.44, and for different centrality intervals for PbPb collisions. Photon production in PbPb collisions is consistent with that in pp collisions scaled by the number of binary nucleon-nucleon collisions, demonstrating that photons do not interact with the quark-gluon plasma. Therefore, isolated photons can provide information about the initial energy of the associated parton in photon+jet measurements. The results are compared with predictions from the next-to-leading-order jetphox generator for different parton distribution functions (PDFs) and nuclear PDFs (nPDFs). The comparisons can help to constrain the nPDFs global fits

    The production of isolated photons in PbPb and pp collisions at sNN \sqrt{s_{\mathrm{NN}}} = 5.02 TeV

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    The transverse energy (ETÎł_{T}^{Îł}) spectra of photons isolated from other particles are measured using proton-proton (pp) and lead-lead (PbPb) collisions at the LHC at sNN \sqrt{s_{\mathrm{NN}}} = 5.02 TeV TeV with integrated luminosities of 27.4 pb−1^{-1} and 404 ÎŒb−1^{-1} for pp and PbPb data, respectively. The results are presented for photons with 25 <ETÎł_{T}^{Îł}< 200 GeV in the pseudorapidity range |η| < 1.44, and for different centrality intervals for PbPb collisions. Photon production in PbPb collisions is consistent with that in pp collisions scaled by the number of binary nucleon-nucleon collisions, demonstrating that photons do not interact with the quark-gluon plasma. Therefore, isolated photons can provide information about the initial energy of the associated parton in photon+jet measurements. The results are compared with predictions from the next-to-leading-order jetphox generator for different parton distribution functions (PDFs) and nuclear PDFs (nPDFs). The comparisons can help to constrain the nPDFs global fits

    Measurement of top quark pair production in association with a Z boson in proton-proton collisions at s \sqrt{\mathrm{s}} = 13 TeV

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    A measurement of the inclusive cross section of top quark pair production in association with a Z boson using proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC is performed. The data sample corresponds to an integrated luminosity of 77.5 fb−1, collected by the CMS experiment during 2016 and 2017. The measurement is performed using final states containing three or four charged leptons (electrons or muons), and the Z boson is detected through its decay to an oppositely charged lepton pair. The production cross section is measured to be σ(tt ÂŻ Z ttÂŻZ ) = 0.95 ± 0.05 (stat) ± 0.06 (syst) pb. For the first time, differential cross sections are measured as functions of the transverse momentum of the Z boson and the angular distribution of the negatively charged lepton from the Z boson decay. The most stringent direct limits to date on the anomalous couplings of the top quark to the Z boson are presented, including constraints on the Wilson coefficients in the framework of the standard model effective field theory

    Observation of Single Top Quark Production in Association with a Z Boson in Proton-Proton Collisions at root s=13 TeV

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    The observation of single top quark production in association with a Z boson and a quark (tZq) is reported. Events from proton-proton collisions at a center-of-mass energy of 13 TeV containing three charged leptons (either electrons or muons) and at least two jets are analyzed. The data were collected with the CMS detector in 2016 and 2017 and correspond to an integrated luminosity of 77.4fb(-1). The increased integrated luminosity, a multivariate lepton identification, and a redesigned analysis strategy improve significantly the sensitivity of the analysis compared to previous searches for tZq production. The tZq signal is observed with a significance well over 5 standard deviations. The measured tZq production cross section is sigma(pp -> tZq -> tl(+) l(-) q) = 111 +/- 13 (stat)(-9)(+11) (syst) fb, for dilepton invariant masses above 30 GeV, in agreement with the standard model expectation

    Measurement of the cross section for electroweak production of a Z boson, a photon and two jets in proton-proton collisions at √s = 13 TeV and constraints on anomalous quartic couplings

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    A measurement is presented of the cross section for electroweak production of a Z boson and a photon in association with two jets (ZÎłjj) in proton-proton collisions. The Z boson candidates are selected through their decay into a pair of electrons or muons. The process of interest, electroweak ZÎłjj production, is isolated by selecting events with a large dijet mass and a large pseudorapidity gap between the two jets. The measurement is based on data collected at the CMS experiment at √s = 13 TeV, corresponding to an integrated luminosity of 35.9 fb⁻Âč. The observed significance of the signal is 3.9 standard deviations, where a significance of 5.2 standard deviations is expected in the standard model. These results are combined with published results by CMS at √s = 8 TeV, which leads to observed and expected respective significances of 4.7 and 5.5 standard deviations. From the 13 TeV data, a value is obtained for the signal strength of electroweak ZÎłjj production and bounds are given on quartic vector boson interactions in the framework of dimension-eight effective field theory operators
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