615 research outputs found

    Effect of hadronic cascade time on freeze-out properties of Identified Hadrons in Au+Au Collisions at sNN\sqrt{s_{NN}} = 7.7-39 GeV from AMPT Model

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    We report the transverse momentum pTp_T spectra of identified hadrons (π±\pi^\pm, K±K^\pm and p(pˉ)p(\bar p)) in Au+Au collisions at sNN\sqrt{s_{NN}} = 7.7 - 39 GeV from A Multi Phase Transport Model with string melting effect (AMPT-SM). During this study, a new set of parameters are explored to study the effect of hadronic cascade by varying hadronic cascade time tmaxt_{max} = 30 ffm/cc and 0.4 ffm/cc. No significant effect of this change is observed in the pTp_T spectra of light hadrons and the AMPT-SM model reasonably reproduces the experimental data. To investigate the kinetic freeze-out properties the blast wave fit is performed to the pTp_T spectra and it is found that the blast wave model describes the AMPT-SM simulations well. We additionally observe that the kinetic freeze-out temperature (TkinT_{kin}) increases from central to peripheral collisions, which is consistent with the argument of short-lived fireball in peripheral collisions. Whereas the transverse flow velocity, shows a decreasing trend from central to peripheral collisions indicating a more rapid expansion in the central collisions. Both, $T_{kin}$ and show a weak dependence on the collision energy at most energies. We also observe a strong anti-correlation between TkinT_{kin} and . The extracted freeze-out parameters from the AMPT-SM simulations agree with the experimental data as opposed to earlier studies that reported some discrepancies. Whereas, no significant effect is found on the freeze-out parameters by varying the tmaxt_{max}. We also report the pTp_T spectra of light hadrons and their freeze-out parameters by AMPT-SM simulations at sNN\sqrt{s_{NN}} = 14.5 GeV, where no experimental data is available for comparison. Overall, the set of parameters used in this study well describes the experimental data at BES energies.Comment: 12 pages, 7 figures, 2 table

    Search for New Physics with a Monojet and Missing Transverse Energy in pp Collisions at √s= 7 TeV

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    A study of events with missing transverse energy and an energetic jet is performed using pp collision data at a center-of-mass energy of 7 TeV. The data were collected by the CMS detector at the LHC, and correspond to an integrated luminosity of 36 pb-1. An excess of these events over standard model contributions is a signature of new physics such as large extra dimensions and unparticles. The number of observed events is in good agreement with the prediction of the standard model, and significant extension of the current limits on parameters of new physics benchmark models is achieved

    Search for New Physics with a Monojet and Missing Transverse Energy in pp Collisions at √s= 7 TeV

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    A study of events with missing transverse energy and an energetic jet is performed using pp collision data at a center-of-mass energy of 7 TeV. The data were collected by the CMS detector at the LHC, and correspond to an integrated luminosity of 36 pb-1. An excess of these events over standard model contributions is a signature of new physics such as large extra dimensions and unparticles. The number of observed events is in good agreement with the prediction of the standard model, and significant extension of the current limits on parameters of new physics benchmark models is achieved

    Measurement of the t-Channel Single Top Quark Production Cross Section in pp̅ Collisions at √s= 7 TeV

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    Electroweak production of the top quark is measured for the first time in pp collisions at √s= 7 TeV, using a data set collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 36 pb-1. With an event selection optimized for t-channel production, two complementary analyses are performed. The first one exploits the special angular properties of the signal, together with background estimates from the data. The second approach uses a multivariate analysis technique to probe the compatibility with signal topology expected from electroweak top-quark production. The combined measurement of the cross section is 83.6±29.8 (stat + syst) ± 3.3 (lumi) pb, consistent with the standard model expectation

    Measurement of the B0s Production Cross Section with B0s → J/ Decays in pp Collisions at √s = 7 TeV

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    The B0s differential production cross section is measured as functions of the transverse momentum and rapidity in pp collisions at √s= 7 TeV, using the B0s → J/UO decay, and compared with predictions based on perturbative QCD calculations at next-to-leading order. The data sample, collected by the CMS experiment at the LHC, corresponds to an integrated luminosity of 40 pb-1. The B0s is reconstructed from the decays J/U → µ+µ- and O→ K+K- The integrated B0s cross section times B0s→ J/UO branching fraction in the range 8BT\u3c 50 GeV/c and │yB \u3c 2.4 is measured to be 6.9 ± 0.6 ± 0.6 nb, where the first uncertainty is statistical and the second is systematic

    Study of W boson production in PbPb and pp collisions at √sNN = 2.76 TeV

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    A measurement is presented of W-boson production in PbPb collisions carried out at a nucleon–nucleon (NN) centre-of-mass energy √sNN of 2.76 TeV at the LHC using the CMS detector. In data corresponding to an integrated luminosity of 7.3 μb−1, the number of W → μνμ decays is extracted in the region of muon pseudorapidity |ημ| \u3c 2.1 and transverse momentum pμT \u3e 25 GeV/c. Yields of muons found per unit of pseudorapidity correspond to (159 ± 10(stat.) ± 12(syst.)) × 10−8 W+ and (154 ± 10(stat.) ± 12(syst.)) × 10−8 W− bosons per minimum-bias PbPb collision. The dependence of W production on the centrality of PbPb collisions is consistent with a scaling of the yield by the number of incoherent NN collisions. The yield of W bosons is also studied in a sample of pp interactions at √s = 2.76 TeV corresponding to an integrated luminosity of 231 nb−1. The individual W+ and W− yields in PbPb and pp collisions are found to agree, once the neutron and proton content in Pb nuclei is taken into account. Likewise, the difference observed in the dependence of the positive and negative muon production on pseudorapidity is consistent with next-to-leading-order perturbative QCD calculations

    Search for physics beyond the standard model in events with a Z boson, jets, and missing transverse energy in pp collisions at √s = 7 TeV

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    A search is presented for physics beyond the standard model (BSM) in events with a Z boson, jets, and missing transverse energy (EmissT ). This signature is motivated by BSM physics scenarios, including supersymmetry. The study is performed using a sample of proton–proton collision data collected at √s = 7 TeV with the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.98 fb−1. The contributions from the dominant standard model backgrounds are estimated from data using two complementary strategies, the jet-Z balance technique and a method based on modeling EmissT with data control samples. In the absence of evidence for BSM physics, we set limits on the non-standard-model contributions to event yields in the signal regions and interpret the results in the context of simplified model spectra. Additional information is provided to facilitate tests of other BSM physics models

    Search for Three-Jet Resonances in pp Collisions at √s= 7 TeV

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    A search for three-jet hadronic resonance production in pp collisions at a center-of-mass energy of 7 TeV has been conducted by the CMS Collaboration at the LHC, using a data sample corresponding to an integrated luminosity of 35 pb-1. Events with high jet multiplicity and a large scalar sum of jet transverse momenta are analyzed using a signature-based approach. The number of expected standard model background events is found to be in good agreement with the observed events. Limits on the cross section times branching ratio are set in a model of gluino pair production with an R-parity-violating decay to three quarks, and the data rule out such particles within the mass range of 200 to 280 GeV/c2

    Search for narrow resonances in dilepton mass spectra in pp collisions at √s = 7 TeV

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    A search for narrow, high-mass resonances decaying to electron or muon pairs has been performed using pp collision data collected at √s = 7 TeV by the CMS experiment in 2011. The data sample corresponds to an integrated luminosity of approximately 5 fb−1. The event yields observed in the signal regions are consistent with predictions of the standard model backgrounds, and upper limits on the cross section times branching fraction for a resonance decaying to dileptons are extracted from a shape analysis of the dilepton invariant mass distribution. The resulting mass limits at 95% confidence level are 2330 GeV for the Z\u27 in the Sequential Standard Model, 2000 GeV for the superstring-inspired Z\u27ψ resonance, 890 (540) GeV for the Stueckelberg extension Z\u27St with the mass parameter є = 0.06 (0.04), and 2140 (1810) GeV for Kaluza–Klein gravitons with the coupling parameter k/M̅Pl of 0.10 (0.05). These limits are the most stringent to date

    Search for aVectorlike Quarkwith Charge 2/3 in t + Z Events from pp Collisions at √s = 7 TeV

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    A search for pair-produced heavy vectorlike charge-2/3 quarks, T, in pp collisions at a center-of-mass energy of 7 TeV, is performed with the CMS detector at the LHC. Events consistent with the flavorchanging- neutral-current decay of a T quark to a top quark and a Z boson are selected by requiring two leptons from the Z-boson decay, as well as an additional isolated charged lepton. In a data sample corresponding to an integrated luminosity of 1.14 fb-1, the number of observed events is found to be consistent with the standard model background prediction. Assuming a branching fraction of 100% for the decay T → tZ, a T quark with a mass less than 475 GeV/c2 is excluded at the 95% confidence level
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