R Ciesielski

Yerevan Physics Institute, Ayrivan, Yerevan, Armenia

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Publications (600)

  • [Show abstract] [Hide abstract] ABSTRACT: 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 5.1fb -1 at a center-of-mass energy s=7TeV and 19.7fb -1 at s=8TeV. 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 < 22MeV at a 95% confidence level, which is 5.4 times the expected value in the standard model at the measured mass of mH = 125.6GeV. © 2014 Elsevier B.V.
    Article · Jan 2014 · Physics Letters B
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    Full-text Article · Jan 2014 · Physics Letters B
  • [Show abstract] [Hide abstract] ABSTRACT: The production cross section for a W boson and two b jets is measured using proton-proton collisions at √s=7 TeV in a data sample collected with the CMS experiment at the LHC corresponding to an integrated luminosity of 5.0 fb-1. The W+bb- events are selected in the W→μν decay mode by requiring a muon with transverse momentum pT > 25 GeV and pseudorapidity |η| < 2.1, and exactly two b-tagged jets with pT > 25 GeV and |η| < 2.4. The measured W+bb- production cross section in the fiducial region, calculated at the level of final-state particles, is σ(pp→W+bb-)×B(W→μν)=0.50.05(stat.) 0.09(syst.)±0.06(theo.)±0.01(lum.) pb, in agreement with the standard model prediction. In addition, kinematic distributions of the W+bb- system are in agreement with the predictions of a simulation using MadGraph and pythia. © 2014 Elsevier B.V.
    Article · Jan 2014 · Physics Letters B
  • [Show abstract] [Hide abstract] ABSTRACT: A search is presented for standard model (SM) production of four top quarks (Formula presented.) in pp collisions in the lepton + jets channel. The data correspond to an integrated luminosity of 19.6 fb−1 recorded at a centre-of-mass energy of 8 TeV with the CMS detector at the CERN LHC. The expected cross section for SM (Formula presented.) production is (Formula presented.). A combination of kinematic reconstruction and multivariate techniques is used to distinguish between the small signal and large background. The data are consistent with expectations of the SM, and an upper limit of 32 fb is set at a 95% confidence level on the cross section for producing four top quarks in the SM, where a limit of 32 ± 17 fb is expected.[Figure not available: see fulltext.].
    Article · Jan 2014 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: A search for three-jet hadronic resonance production in pp collisions at a centre-of-mass energy of 8 TeV has been conducted by the CMS Collaboration at the LHC with a data sample corresponding to an integrated luminosity of 19.4fb. The search method is model independent, and events are selected that have high jet multiplicity and large values of jet transverse momenta. The signal models explored assume R-parity-violating supersymmetric gluino pair production and have final states with either only light-flavour jets or both light- and heavy-flavour jets. No significant deviation is found between the selected events and the expected standard model multijet and tt- background. For a gluino decaying into light-flavour jets, a lower limit of 650 GeV on the gluino mass is set at a 95% confidence level, and for a gluino decaying into one heavy- and two light-flavour jets, gluino masses between 200 and 835 GeV are, for the first time, likewise excluded. © 2014 The Authors.
    Full-text Article · Jan 2014 · Physics Letters B
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    [Show abstract] [Hide abstract] ABSTRACT: Results are reported from a search for supersymmetry in pp collisions at a center-of-mass energy of 8 TeV, based on events with a single isolated lepton (electron or muon) and multiple jets, at least two of which are identified as b jets. The data sample corresponds to an integrated luminosity of 19.3 fbrecorded by the CMS experiment at the LHC in 2012. The search is motivated by supersymmetric models that involve strong-production processes and cascade decays of new particles. The resulting final states contain multiple jets as well as missing transverse momentum from weakly interacting particles. The event yields, observed across several kinematic regions, are consistent with the expectations from standard model processes. The results are interpreted in the context of simplified supersymmetric scenarios with pair production of gluinos, where each gluino decays to a top quark-antiquark pair and the lightest neutralino. For the case of decays via virtual top squarks, gluinos with a mass smaller than 1.26 TeV are excluded for low neutralino masses. © 2014 The Authors.
    Full-text Article · Jan 2014 · Physics Letters B
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    Full-text Article · Jan 2014 · European Physical Journal C
  • [Show abstract] [Hide abstract] ABSTRACT: A peaking structure in the J/ψφ mass spectrum near threshold is observed in B →J/ψφK decays, produced in pp collisions at √s=7 TeV collected with the CMS detector at the LHC. The data sample, selected on the basis of the dimuon decay mode of the J/ψ, corresponds to an integrated luminosity of 5.2 fb. Fitting the structure to an S-wave relativistic Breit-Wigner lineshape above a three-body phase-space nonresonant component gives a signal statistical significance exceeding five standard deviations. The fitted mass and width values are m = 4148.0 ± 2.4 (stat.) ± 6.3 (syst.)MeV and Γ=28 (stat.)±19(syst.)MeV, respectively. Evidence for an additional peaking structure at higher J/ψφ mass is also reported. © 2014 The Authors.
    Article · Jan 2014 · Physics Letters B
  • [Show abstract] [Hide abstract] ABSTRACT: Measurements of the top quark-antiquark (tt¯) spin correlations and the top quark polarization are presented for tt¯ pairs produced in pp collisions at s=8 TeV. The data correspond to an integrated luminosity of 19.5 fb-1 collected with the CMS detector at the LHC. The measurements are performed using events with two oppositely charged leptons (electrons or muons) and two or more jets, where at least one of the jets is identified as originating from a bottom quark. The spin correlations and polarization are measured from the angular distributions of the two selected leptons, both inclusively and differentially, with respect to the invariant mass, rapidity, and transverse momentum of the tt¯ system. The measurements are unfolded to the parton level and found to be in agreement with predictions of the standard model. A search for new physics in the form of anomalous top quark chromo moments is performed. No evidence of new physics is observed, and exclusion limits on the real part of the chromo-magnetic dipole moment and the imaginary part of the chromo-electric dipole moment are evaluated.
    Article · Jan 2014 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: Rapidity distributions are presented for events containing either a Z boson or a photon with a single jet in proton-proton collisions produced at the CERN LHC. The data, collected with the CMS detector at sqrt(s) = 7 TeV, correspond to an integrated luminosity of 5.0 inverse femtobarns. The individual rapidity distributions of the boson and the jet are consistent within 5% with expectations from perturbative QCD. However, QCD predictions for the sum and the difference in rapidities of the two final-state objects show discrepancies with CMS data. In particular, next-to-leading-order QCD calculations, and two common Monte Carlo event generators using different methods to match matrix-element partons with parton showers, appear inconsistent with the data as well as with each other.
    Full-text Article · Dec 2013 · Physical Review D
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    [Show abstract] [Hide abstract] ABSTRACT: A search for flavor-changing neutral currents in top-quark decays t to Zq is performed in events produced from the decay chain t t-bar to Zq+Wb, where both vector bosons decay leptonically, producing a final state with three leptons (electrons or muons). A dataset collected with the CMS detector at the LHC is used, corresponding to an integrated luminosity of 19.7 inverse femtobarns of proton-proton collisions at a center-of-mass energy of 8 TeV. No excess is seen in the observed number of events relative to the standard model prediction; thus no evidence for flavor-changing neutral currents in top-quark decays is found. A combination with a previous search at 7 TeV excludes a t to Zq branching fraction greater than 0.05% at the 95% confidence level.
    Full-text Article · Dec 2013 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: 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, and within pseudorapidity abs(eta < 2). The nuclear modification factor (R[AA]) 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 R[AA] 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.
    Full-text Article · Dec 2013 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: A search for the production of heavy partners of the top quark with charge 5/3 is performed in events with a pair of same-sign leptons. The data sample corresponds to an integrated luminosity of 19.5 inverse femtobarns and was collected at sqrt(s) = 8 TeV by the CMS experiment. No significant excess is observed in the data above the expected background and the existence of top-quark partners with masses below 800 GeV is excluded at a 95% confidence level, assuming they decay exclusively to tW. This is the first limit on these particles from the LHC, and it is significantly more restrictive than previous limits.
    Full-text Article · Dec 2013 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at sqrt(s[NN]) = 2.76 TeV by the CMS collaboration, using data from the 2011 LHC heavy-ion run. The data set includes a sample of ultra-central (0-0.2% centrality) PbPb events collected using a trigger based on total transverse energy in the hadron forward calorimeters and the total multiplicity of pixel clusters in the silicon pixel tracker. A total of about 1.8 million ultra-central events were recorded, corresponding to an integrated luminosity of 120 inverse microbarns. The observed correlations in ultra-central PbPb events are expected to be particularly sensitive to initial-state fluctuations. The single-particle anisotropy Fourier harmonics, from v[2] to v[6], are extracted as a function of particle transverse momentum. At higher transverse momentum, the v[2] harmonic becomes significantly smaller than the higher-order v[n] (n greater than or equal to 3). The pt-averaged v[2] and v[3] are found to be equal within 2%, while higher-order v[n] decrease as n increases. The breakdown of factorization of dihadron correlations into single-particle azimuthal anisotropies is observed. This effect is found to be most prominent in the ultra-central PbPb collisions, where the initial-state fluctuations play a dominant role. A comparison of the factorization data to hydrodynamic predictions with event-by-event fluctuating initial conditions is also presented.
    Full-text Article · Dec 2013 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: A study of proton-proton collisions in which two b hadrons are produced in association with a Z boson is reported. The collisions were recorded at a centre-of-mass energy of 7 TeVwith the CMS detector at the LHC, for an integrated luminosity of 5.2 fb(-1). The b hadrons are identified by means of displaced secondary vertices, without the use of reconstructed jets, permitting the study of b-hadron pair production at small angular separation. Differential cross sections are presented as a function of the angular separation of the b hadrons and the Z boson. In addition, inclusive measurements are presented. For both the inclusive and differential studies, different ranges of Z boson momentum are considered, and each measurement is compared to the predictions from different event generators at leading-order and next-to-leading-order accuracy.
    Full-text Article · Dec 2013 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: A search for anomalous production of events with three or more isolated leptons and bottom-quark jets produced in pp collisions at √s=8 TeV is presented. The analysis is based on a data sample corresponding to an integrated luminosity of 19.5 fb(-1) collected by the CMS experiment at the LHC in 2012. No excess above the standard model expectations is observed. The results are interpreted in the context of supersymmetric models with signatures that have low missing transverse energy arising from light top-squark pair production with R-parity-violating decays of the lightest supersymmetric particle. In two models with different R-parity-violating couplings, top squarks are excluded below masses of 1020 GeV and 820 GeV when the lightest supersymmetric particle has a mass of 200 GeV.
    Full-text Article · Nov 2013 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: Spin correlations in the t-tbar quark system and the polarization of the top quark are measured using dilepton final states produced in pp collisions at the LHC at sqrt(s)=7 TeV. The data correspond to an integrated luminosity of 5.0 inverse femtobarns collected with the CMS detector. The measurements are performed using events with two oppositely charged leptons (electrons or muons), a significant imbalance in transverse momentum, and two or more jets, where at least one of the jets is identified as likely originating from a b quark. The spin correlations and polarization are measured through asymmetries in angular distributions of the two selected leptons, unfolded to the parton level. All measurements are found to be in agreement with predictions of the standard model.
    Full-text Article · Nov 2013 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: Searches for anomalous top quark-antiquark production are presented, based on pp collisions at √s=8 TeV. The data, corresponding to an integrated luminosity of 19.7 fb−1, were collected with the CMS detector at the LHC. The observed ttbar invariant mass spectrum is found to be compatible with the standard model prediction. Limits on the production cross section times branching fraction probe, for the first time, a region of parameter space for certain models of new physics not yet constrained by precision measurements.
    Full-text Article · Nov 2013 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: Measurements of the differential and double-differential Drell-Yan cross sections are presented using an integrated luminosity of 4.5(4.8) inverse femtobarns in the dimuon (dielectron) channel of proton-proton collision data recorded with the CMS detector at the LHC at sqrt(s) = 7 TeV. The measured inclusive cross section in the Z-peak region (60-120 GeV) is sigma(ell ell) = 986.4 +/- 0.6 (stat.) +/- 5.9 (exp. syst.) +/- 21.7 (th. syst.) +/- 21.7 (lum.) pb for the combination of the dimuon and dielectron channels. Differential cross sections d[sigma]/d[m] for the dimuon, dielectron, and combined channels are measured in the mass range 15 to 1500 GeV and corrected to the full phase space. Results are also presented for the measurement of the double-differential cross section d2[sigma]/d[m]d[abs(y)] in the dimuon channel over the mass range 20 to 1500 GeV and absolute dimuon rapidity from 0 to 2.4. These measurements are compared to the predictions of perturbative QCD calculations at next-to-leading and next-to-next-to-leading orders using various sets of parton distribution functions.
    Full-text Article · Oct 2013 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: A measurement of W+W- production in pp collisions at is presented. The data were collected with the CMS detector at the LHC, and correspond to an integrated luminosity of 4.92 +/- 0.11 fb(-1). The W+W- candidates consist of two oppositely charged leptons, electrons or muons, accompanied by large missing transverse energy. The W+W- production cross section is measured to be 52.4 +/- 2.0 (stat.)+/- 4.5 (syst.)+/- 1.2 (lum.) pb. This measurement is consistent with the standard model prediction of 47.0 +/- 2.0 pb at next-to-leading order. Stringent limits on the WW gamma and WWZ anomalous triple gauge-boson couplings are set.
    Full-text Article · Oct 2013 · European Physical Journal C

Publication Stats

12k Citations

Institutions

  • 2011-2013
    • Yerevan Physics Institute
      Ayrivan, Yerevan, Armenia
  • 2007-2013
    • Deutsches Elektronen-Synchrotron
      • DESY Photon Science
      Hamburg, Hamburg, Germany
  • 2012
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
    • Vrije Universiteit Brussel
      • Accountancy, Auditing and Business Financing (ACCO)
      Bruxelles, Brussels Capital Region, Belgium
  • 2008
    • Argonne National Laboratory
      • Division of Physics
      Lemont, Illinois, United States
    • Università degli Studi di Torino
      Torino, Piedmont, Italy
  • 2006-2007
    • INFN - Istituto Nazionale di Fisica Nucleare
      Frascati, Latium, Italy
  • 2004-2005
    • Humboldt-Universität zu Berlin
      Berlín, Berlin, Germany
  • 2000-2004
    • University of Warsaw
      • Institute of Experimental Physics
      Warszawa, Masovian Voivodeship, Poland
  • 1999-2004
    • University of Freiburg
      Freiburg, Baden-Württemberg, Germany