Browsing by Author "Michlin, B."
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Item Alignment of the CMS tracker with LHC and cosmic ray data(IOP Publishing, 2014) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The central component of the CMS detector is the largest silicon tracker ever built. The precise alignment of this complex device is a formidable challenge, and only achievable with a significant extension of the technologies routinely used for tracking detectors in the past. This article describes the full-scale alignment procedure as it is used during LHC operations. Among the specific features of the method are the simultaneous determination of up to 200 000 alignment parameters with tracks, the measurement of individual sensor curvature parameters, the control of systematic misalignment effects, and the implementation of the whole procedure in a multi-processor environment for high execution speed. Overall, the achieved statistical accuracy on the module alignment is found to be significantly better than 10μm.Item Combined Measurement of the Higgs Boson Mass in pp Collisions at √s=7 and 8 TeV with the ATLAS and CMS Experiments(American Physical Society, 2015) Adair, A.; Akgun, B.; Chen, Z.; Ecklund, K.M.; Geurts, F.J.M.; Guilbaud, M.; Li, W.; Michlin, B.; Northup, M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Rorie, J.; Tu, Z.; Zabel, J.A measurement of the Higgs boson mass is presented based on the combined data samples of the ATLAS and CMS experiments at the CERN LHC in the H→γγ and H→ZZ→4ℓ decay channels. The results are obtained from a simultaneous fit to the reconstructed invariant mass peaks in the two channels and for the two experiments. The measured masses from the individual channels and the two experiments are found to be consistent among themselves. The combined measured mass of the Higgs boson is mH=125.09±0.21 (stat)±0.11 (syst) GeV.Item Constraints on the Higgs boson width from off-shell production and decay to Z-boson pairs(Elsevier, 2014) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Constraints are presented on the total width of the recently discovered Higgs boson, ΓH, using its relative on-shell and off-shell production and decay rates to a pair of Z bosons, where one Z boson decays to an electron or muon pair, and the other to an electron, muon, or neutrino pair. The analysis is based on the data collected by the CMS experiment at the LHC in 2011 and 2012, corresponding to integrated luminosities of 5.1 fb−1 at a center-of-mass energy √s=7 TeV and 19.7 fb−1 at √s=8 TeV. A simultaneous maximum likelihood fit to the measured kinematic distributions near the resonance peak and above the Z-boson pair production threshold leads to an upper limit on the Higgs boson width of ΓH<22 MeV at a 95% confidence level, which is 5.4 times the expected value in the standard model at the measured mass of mH=125.6 GeV.Item Description and performance of track and primary-vertex reconstruction with the CMS tracker(IOP Publishing, 2014) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.A description is provided of the software algorithms developed for the CMS tracker both for reconstructing charged-particle trajectories in proton-proton interactions and for using the resulting tracks to estimate the positions of the LHC luminous region and individual primary-interaction vertices. Despite the very hostile environment at the LHC, the performance obtained with these algorithms is found to be excellent. For tbar t events under typical 2011 pileup conditions, the average track-reconstruction efficiency for promptly-produced charged particles with transverse momenta of pT > 0.9GeV is 94% for pseudorapidities of |η| < 0.9 and 85% for 0.9 < |η| < 2.5. The inefficiency is caused mainly by hadrons that undergo nuclear interactions in the tracker material. For isolated muons, the corresponding efficiencies are essentially 100%. For isolated muons of pT = 100GeV emitted at |η| < 1.4, the resolutions are approximately 2.8% in pT, and respectively, 10μm and 30μm in the transverse and longitudinal impact parameters. The position resolution achieved for reconstructed primary vertices that correspond to interesting pp collisions is 10–12μm in each of the three spatial dimensions. The tracking and vertexing software is fast and flexible, and easily adaptable to other functions, such as fast tracking for the trigger, or dedicated tracking for electrons that takes into account bremsstrahlung.Item Determination of the top-quark pole mass and strong coupling constant from the tt production cross section in pp collisions at √s = 7 TeV(Elsevier, 2014) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The inclusive cross section for top-quark pair production measured by the CMS experiment in proton–proton collisions at a center-of-mass energy of 7 TeV is compared to the QCD prediction at next-to-next-to-leading order with various parton distribution functions to determine the top-quark pole mass, View the MathML sourcemtpole, or the strong coupling constant, αSαS. With the parton distribution function set NNPDF2.3, a pole mass of 176.7 +3.8 −3.4 GeV is obtained when constraining αS at the scale of the Z boson mass, mZ, to the current world average. Alternatively, by constraining mtpole to the latest average from direct mass measurements, a value of αS(mZ)=0.1151−0.0032+0.0033 is extracted. This is the first determination of αS using events from top-quark production.Item Differential cross section measurements for the production of a W boson in association with jets in proton–proton collisions at √s=7 eV(Elsevier, 2015) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Measurements are reported of differential cross sections for the production of a W boson, which decays into a muon and a neutrino, in association with jets, as a function of several variables, including the transverse momenta (pT) and pseudorapidities of the four leading jets, the scalar sum of jet transverse momenta (HT), and the difference in azimuthal angle between the directions of each jet and the muon. The data sample of pp collisions at a centre-of-mass energy of 7 TeV was collected with the CMS detector at the LHC and corresponds to an integrated luminosity of 5.0 fb−1. The measured cross sections are compared to predictions from Monte Carlo generators, MADGRAPH + PYTHIA and SHERPA, and to next-to-leading-order calculations from BLACKHAT + SHERPA. The differential cross sections are found to be in agreement with the predictions, apart from the pT distributions of the leading jets at high pT values, the distributions of the HT at high-HT and low jet multiplicity, and the distribution of the difference in azimuthal angle between the leading jet and the muon at low valuesItem Erratum to: Search for new physics in events with same-sign dileptons and jets in pp collisions at √s = 8 TeV(Springer, 2015) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Item Evidence of b-Jet Quenching in PbPb Collisions at √sNN=2.76 TeV(American Physical Society, 2014) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The production of jets associated to bottom quarks is measured for the first time in PbPb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair. Jet spectra are reported in the transverse momentum (pT) range of 80–250 GeV/c, and within pseudorapidity |η|<2. The nuclear modification factor (RAA) calculated from these spectra shows a strong suppression in the b-jet yield in PbPb collisions relative to the yield observed in pp collisions at the same energy. The suppression persists to the largest values of pT studied, and is centrality dependent. The RAA is about 0.4 in the most central events, similar to previous observations for inclusive jets. This implies that jet quenching does not have a strong dependence on parton mass and flavor in the jet pT range studied.Item Measurement of differential cross sections for the production of a pair of isolated photons in pp collisions at √s=7TeV(Springer, 2014) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.A measurement of differential cross sections for the production of a pair of isolated photons in proton–proton collisions at √s=7TeV is presented. The data sample corresponds to an integrated luminosity of 5.0 fb−1 collected with the CMS detector. A data-driven isolation template method is used to extract the prompt diphoton yield. The measured cross section for two isolated photons, with transverse energy above 40 and 25 GeV respectively, in the pseudorapidity range |η|<2.5 , |η|∉[1.44,1.57] and with an angular separation ΔR>0.45 , is 17.2±0.2(stat)±1.9(syst)±0.4(lumi) \,pb . Differential cross sections are measured as a function of the diphoton invariant mass, the diphoton transverse momentum, the azimuthal angle difference between the two photons, and the cosine of the polar angle in the Collins–Soper reference frame of the diphoton system. The results are compared to theoretical predictions at leading, next-to-leading, and next-to-next-to-leading order in quantum chromodynamics.Item Measurement of J/ψ and ψ(2S) Prompt Double-Differential Cross Sections in pp Collisions at √s=7 TeV(American Physical Society, 2015) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The double-differential cross sections of promptly produced J/ψ and ψ(2S) mesons are measured in pp collisions at √s=7 TeV, as a function of transverse momentum pT and absolute rapidity |y|. The analysis uses J/ψ and ψ(2S) dimuon samples collected by the CMS experiment, corresponding to integrated luminosities of 4.55 and 4.90 fb−1, respectively. The results are based on a two-dimensional analysis of the dimuon invariant mass and decay length, and extend to pT=120 and 100 GeV for the J/ψ and ψ(2S), respectively, when integrated over the interval |y|<1.2. The ratio of the ψ(2S) to J/ψ cross sections is also reported for |y|<1.2, over the range 10Item Search for anomalous production of events with three or more leptons in pp collisions at √s=8 TeV(American Physical Society, 2014) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.A search for physics beyond the standard model in events with at least three leptons is presented. The data sample, corresponding to an integrated luminosity of 19.5 fb−1 of proton-proton collisions with center-of-mass energy √s=8 TeV, was collected by the CMS experiment at the LHC during 2012. The data are divided into exclusive categories based on the number of leptons and their flavor, the presence or absence of an opposite-sign, same-flavor lepton pair (OSSF), the invariant mass of the OSSF pair, the presence or absence of a tagged bottom-quark jet, the number of identified hadronically decaying τ leptons, and the magnitude of the missing transverse energy and of the scalar sum of jet transverse momenta. The numbers of observed events are found to be consistent with the expected numbers from standard model processes, and limits are placed on new-physics scenarios that yield multilepton final states. In particular, scenarios that predict Higgs boson production in the context of supersymmetric decay chains are examined. We also place a 95% confidence level upper limit of 1.3% on the branching fraction for the decay of a top quark to a charm quark and a Higgs boson (t→cH), which translates to a bound on the left- and right-handed top-charm flavor-violating Higgs Yukawa couplings, λHtc and λHct, respectively, of √|λHtc|2+|λHct|2−−−−−−−−−−−<0.21.Item Search for physics beyond the standard model in final states with a lepton and missing transverse energy in proton-proton collisions at √s=8 TeV(American Physical Society, 2015) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.A search for new physics in proton-proton collisions having final states with an electron or muon and missing transverse energy is presented. The analysis uses data collected in 2012 with the CMS detector, at an LHC center-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of 19.7 fb−1. No significant deviation of the transverse mass distribution of the charged lepton-neutrino system from the standard model prediction is found. Mass exclusion limits of up to 3.28 TeV at 95% confidence level for a W′-boson with the same couplings as that of the standard model W-boson are determined. Results are also derived in the framework of split universal extra dimensions, and exclusion limits on Kaluza–Klein W(2)KK states are found. The final state with large missing transverse energy also enables a search for dark matter production with a recoiling W-boson, with limits set on the mass and the production cross section of potential candidates. Finally, limits are established for a model including interference between a left-handed W′-boson and the standard model W-boson and for a compositeness model.Item Search for supersymmetry using razor variables in events with b-tagged jets in pp collisions at √s=8 TeV(American Physical Society, 2015) Adair, A.; Akgun, B.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Michlin, B.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.An inclusive search for supersymmetry in events with at least one b-tagged jet is performed using proton-proton collision data collected by the CMS experiment in 2012 at a center-of-mass energy of 8 TeV. The data set size corresponds to an integrated luminosity of 19.3 fb−1. The two-dimensional distribution of the razor variables R2 and MR is studied in events with and without leptons. The data are found to be consistent with the expected background, which is modeled with an empirical function. Exclusion limits on supersymmetric particle masses at a 95% confidence level are derived in several simplified supersymmetric scenarios for several choices of the branching fractions. By combining the likelihoods of a search in events without leptons and a search that requires a single lepton (electron or muon), an improved bound on the top-squark mass is obtained. Assuming the lightest supersymmetric particle to be stable and weakly interacting, and to have a mass of 100 GeV, the branching-fraction-dependent (-independent) production of gluinos is excluded for gluino masses up to 1310 (1175) GeV. The corresponding limit for top-squark pair production is 730 (645) GeV.