2,092 research outputs found

    Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV

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    The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8  TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ÏˆÎł, with the photons being measured through conversions to eâșe⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → ÎŒâșΌ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum

    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

    Evidence for X(3872) in Pb-Pb Collisions and Studies of its Prompt Production at sNN\sqrt{^{s}NN} =5.02 TeV

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    The first evidence for X(3872) production in relativistic heavy ion collisions is reported. The X(3872) production is studied in lead-lead (Pb-Pb) collisions at a center-of-mass energy of sNN\sqrt{^{s}NN}=5.02 TeV per nucleon pair, using the decay chain X(3872)→J/ψπ+^{+} π–^{–}→Ό+^{+}Ό–^{–}π+^{+} π–^{–}. The data were recorded with the CMS detector in 2018 and correspond to an integrated luminosity of 1.7 nb−1^{-1}. The measurement is performed in the rapidity and transverse momentum ranges |y|<1.6 and 15<pT<50 GeV/c. The significance of the inclusive X(3872) signal is 4.2 standard deviations. The prompt X(3872) to ψ2S yield ratio is found to be ρPb−Pb^{Pb-Pb}=1.08±0.49(stat)±0.52(syst), to be compared with typical values of 0.1 for pp collisions. This result provides a unique experimental input to theoretical models of the X(3872) production mechanism, and of the nature of this exotic state

    Measurement of b jet shapes in proton-proton collisions at s \sqrt{s} = 5.02 TeV

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    We present the first study of charged-hadron production associated with jets originating from b quarks in proton-proton collisions at a center-of-mass energy of 5.02 TeV. The data sample used in this study was collected with the CMS detector at the CERN LHC and corresponds to an integrated luminosity of 27.4 pb−1. To characterize the jet substructure, the differential jet shapes, defined as the normalized transverse momentum distribution of charged hadrons as a function of angular distance from the jet axis, are measured for b jets. In addition to the jet shapes, the per-jet yields of charged particles associated with b jets are also quantified, again as a function of the angular distance with respect to the jet axis. Extracted jet shape and particle yield distributions for b jets are compared with results for inclusive jets, as well as with the predictions from the pythia and herwig++ event generators

    Measurement of the Y(1S) pair production cross section and search for resonances decaying to Y(1S)ÎŒâșΌ⁻ in proton-proton collisions at √s = 13 TeV

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    The fiducial cross section for Y(1S) pair production in proton-proton collisions at a center-of-mass energy of 13 TeV in the region where both Y(1S) mesons have an absolute rapidity below 2.0 is measured to be 79±11(stat)±6(syst)±3(B) pb assuming the mesons are produced unpolarized. The last uncertainty corresponds to the uncertainty in the Y(1S) meson dimuon branching fraction. The measurement is performed in the final state with four muons using proton-proton collision data collected in 2016 by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9fb⁻Âč. This process serves as a standard model reference in a search for narrow resonances decaying to Y(1S)ÎŒâșΌ⁻ in the same final state. Such a resonance could indicate the existence of a tetraquark that is a bound state of two b quarks and two b antiquarks. The tetraquark search is performed for masses in the vicinity of four times the bottom quark mass, between 17.5 and 19.5 GeV, while a generic search for other resonances is performed for masses between 16.5 and 27 GeV. No significant excess of events compatible with a narrow resonance is observed in the data. Limits on the production cross section times branching fraction to four muons via an intermediate Y(1S) resonance are set as a function of the resonance mass

    Search for physics beyond the standard model in events with jets and two same-sign or at least three charged leptons in proton-proton collisions at s=13 TeV\sqrt{s}=13\,{\text {TeV}}

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    A data sample of events from proton-proton collisions with at least two jets, and two isolated same-sign or three or more charged leptons, is studied in a search for signatures of new physics phenomena. The data correspond to an integrated luminosity of 137fb−1^{-1} at a center-of-mass energy of 13TeV, collected in 2016–2018 by the CMS experiment at the LHC. The search is performed using a total of 168 signal regions defined using several kinematic variables. The properties of the events are found to be consistent with the expectations from standard model processes. Exclusion limits at 95% confidence level are set on cross sections for the pair production of gluinos or squarks for various decay scenarios in the context of supersymmetric models conserving or violating R parity. The observed lower mass limits are as large as 2.1TeV for gluinos and 0.9TeV for top and bottom squarks. To facilitate reinterpretations, model-independent limits are provided in a set of simplified signal regions
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