H. C. Kaestli

Paul Scherrer Institut, Филлиген, Aargau, Switzerland

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Publications (454)

  • Article · Dec 2016
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    [Show abstract] [Hide abstract] ABSTRACT: The decay is observed for the first time using data collected from pp collisions at by the CMS experiment at the LHC, corresponding to an integrated luminosity of 19.6 . The branching fraction of this decay is measured, using the mode as normalization, to be , where the third uncertainty is from the measured branching fraction of the normalization channel.
    Full-text available · Article · Nov 2016 · Physics Letters B
  • The CMS collaboration · V. Khachatryan · A. M. Sirunyan · [...] · N. Woods
    [Show abstract] [Hide abstract] ABSTRACT: Interactions between jets and the quark-gluon plasma produced in heavy ion collisions are studied via the angular distributions of summed charged-particle transverse momenta (pT) with respect to both the leading and subleading jet axes in high-pT dijet events. The contributions of charged particles in different momentum ranges to the overall event pT balance are decomposed into short-range jet peaks and a long-range azimuthal asymmetry in charged-particle pT. The results for PbPb collisions are compared to those in pp collisions using data collected in 2011 and 2013, at collision energy \( \sqrt{s_{\mathrm{N}\;\mathrm{N}}}=2.76 \) TeV with integrated luminosities of 166 μb−1 and 5.3 pb−1, respectively, by the CMS experiment at the LHC. Measurements are presented as functions of PbPb collision centrality, charged-particle pT, relative azimuth, and radial distance from the jet axis for balanced and unbalanced dijets.
    Article · Nov 2016 · Journal of High Energy Physics
  • Article · Nov 2016 · Journal of High Energy Physics
  • V. Khachatryan · A. M. Sirunyan · A. Tumasyan · [...] · N. Woods
    [Show abstract] [Hide abstract] ABSTRACT: A measurement is presented of the cross section for the electroweak production of a W boson in association with two jets in proton-proton collisions at a center-of-mass energy of 8 TeV. The data set was collected with the CMS detector and corresponds to an integrated luminosity of 19.3 fb−1. The measured fiducial cross section for W bosons decaying to electrons or muons and for pTj1> 60 GeV, pTj2> 50 GeV, |ηj| < 4.7, and mjj> 1000 GeV is 0.42 ± 0.04 (stat) ± 0.09 (syst) ± 0.01 (lumi) pb. This result is consistent with the standard model leading-order prediction of 0.50 ± 0.02 (scale) ± 0.02 (PDF) pb obtained with MadGraph5_amc@nlo 2.1 interfaced to pythia 6.4. This is the first cross section measurement for this process.
    Article · Nov 2016 · Journal of High Energy Physics
  • [Show abstract] [Hide abstract] ABSTRACT: Four-lepton production in proton-proton collisions, pp→(Z/γ⁎)(Z/γ⁎)→ℓ+ℓ−ℓ′+ℓ′−, where ℓ,ℓ′=e or μ, is studied at a center-of-mass energy of 13 TeV with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 2.6 fb⁻¹. The ZZ production cross section, σ(pp→ZZ)=14.6−1.8+1.9(stat)−0.3+0.5(syst)±0.2(theo)±0.4(lumi)pb, is measured for events with two opposite-sign, same-flavor lepton pairs produced in the mass region 60<mℓ+ℓ−,mℓ′+ℓ′−<120 GeV. The Z boson branching fraction to four leptons is measured to be B(Z→ℓ+ℓ−ℓ′+ℓ′−)=4.9−0.7+0.8 (stat) −0.2+0.3 (syst) −0.1+0.2(theo)±0.1(lumi)×10−6 for the four-lepton invariant mass in the range 80<mℓ+ℓ−ℓ′+ℓ′−<100 GeV and dilepton mass mℓ+ℓ−>4 GeV for all opposite-sign, same-flavor lepton pairs. The results are in agreement with standard model predictions.
    Article · Oct 2016 · Physics Letters B
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    [Show abstract] [Hide abstract] ABSTRACT: The cross section for pair production of top quarks (tt) with high transverse momenta is measured in pp collisions, collected with the CMS detector at the LHC with s=8 TeV in data corresponding to an integrated luminosity of 19.7 fb-1. The measurement is performed using lepton+jets events, where one top quark decays semileptonically, while the second top quark decays to a hadronic final state. The hadronic decay is reconstructed as a single, large-radius jet, and identified as a top quark candidate using jet substructure techniques. The integrated cross section and the differential cross sections as a function of top quark pT and rapidity are measured at particle level within a fiducial region related to the detector-level requirements and at parton level. The particle-level integrated cross section is found to be σtt=0.499±0.035(stat+syst)±0.095(theo)±0.013(lumi) pb for top quark pT>400 GeV. The parton-level measurement is σtt=1.44±0.10(stat+syst)±0.29(theo)±0.04(lumi) pb. The integrated and differential cross section results are compared to predictions from several event generators.
    Full-text available · Article · Oct 2016
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    [Show abstract] [Hide abstract] ABSTRACT: The W boson helicity fractions from top quark decays in tt‾ events are measured using data from proton–proton collisions at a centre-of-mass energy of 8TeV. The data were collected in 2012 with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.8fb−1. Events are reconstructed with either one muon or one electron, along with four jets in the final state, with two of the jets being identified as originating from b quarks. The measured helicity fractions from both channels are combined, yielding F0=0.681±0.012(stat)±0.023(syst), FL=0.323±0.008(stat)±0.014(syst), and FR=−0.004±0.005(stat)±0.014(syst) for the longitudinal, left-, and right-handed components of the helicity, respectively. These measurements of the W boson helicity fractions are the most accurate to date and they agree with the predictions from the standard model.
    Full-text available · Article · Oct 2016 · Physics Letters B
  • [Show abstract] [Hide abstract] ABSTRACT: A direct search for lepton flavour violating decays of the Higgs boson (H) in the H→eτ and H→eμ channels is described. The data sample used in the search was collected in proton–proton collisions at s=8 TeV with the CMS detector at the LHC and corresponds to an integrated luminosity of 19.7 fb−1. No evidence is found for lepton flavour violating decays in either final state. Upper limits on the branching fractions, B(H→eτ)<0.69% and B(H→eμ)<0.035%, are set at the 95% confidence level. The constraint set on B(H→eτ) is an order of magnitude more stringent than the existing indirect limits. The limits are used to constrain the corresponding flavour violating Yukawa couplings, absent in the standard model.
    Article · Oct 2016 · Physics Letters B
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    [Show abstract] [Hide abstract] ABSTRACT: A measurement of the decorrelation of azimuthal angles between the two jets with the largest transverse momenta is presented for seven regions of leading jet transverse momentum up to 2.2\(\,\mathrm{TeV}\). The analysis is based on the proton-proton collision data collected with the CMS experiment at a centre-of-mass energy of 8\(\,\mathrm{TeV}\) corresponding to an integrated luminosity of 19.7\(\,\text {fb}^{-1}\). The dijet azimuthal decorrelation is caused by the radiation of additional jets and probes the dynamics of multijet production. The results are compared to fixed-order predictions of perturbative quantum chromodynamics (QCD), and to simulations using Monte Carlo event generators that include parton showers, hadronization, and multiparton interactions. Event generators with only two outgoing high transverse momentum partons fail to describe the measurement, even when supplemented with next-to-leading-order QCD corrections and parton showers. Much better agreement is achieved when at least three outgoing partons are complemented through either next-to-leading-order predictions or parton showers. This observation emphasizes the need to improve predictions for multijet production.
    Full-text available · Article · Oct 2016 · European Physical Journal C
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    [Show abstract] [Hide abstract] ABSTRACT: Searches for new physics by the CMS collaboration are interpreted in the framework of the phenomenological minimal supersymmetric standard model (pMSSM). The data samples used in this study were collected at \( \sqrt{s}=7 \) and 8 TeV and have integrated luminosities of 5.0 fb−1 and 19.5 fb−1, respectively. A global Bayesian analysis is performed, incorporating results from a broad range of CMS supersymmetry searches, as well as constraints from other experiments. Because the pMSSM incorporates several well-motivated assumptions that reduce the 120 parameters of the MSSM to just 19 parameters defined at the electroweak scale, it is possible to assess the results of the study in a relatively straightforward way. Approximately half of the model points in a potentially accessible subspace of the pMSSM are excluded, including all pMSSM model points with a gluino mass below 500 GeV, as well as models with a squark mass less than 300 GeV. Models with chargino and neutralino masses below 200 GeV are disfavored, but no mass range of model points can be ruled out based on the analyses considered. The nonexcluded regions in the pMSSM parameter space are characterized in terms of physical processes and key observables, and implications for future searches are discussed.
    Full-text available · Article · Oct 2016 · Journal of High Energy Physics
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    The CMS collaboration · V. Khachatryan · A. M. Sirunyan · [...] · N. Woods
    [Show abstract] [Hide abstract] ABSTRACT: A search for new physics is performed using events that contain one or more jets, no isolated leptons, and a large transverse momentum imbalance, as measured through the MT2 variable, which is an extension of the transverse mass in events with two invisible particles. The results are based on a sample of proton-proton collisions collected at a center-of-mass energy of 13 TeV with the CMS detector at the LHC, and that corresponds to an integrated luminosity of 2.3 fb−1. The observed event yields in the data are consistent with predictions for the standard model backgrounds. The results are interpreted using simplified models of supersymmetry and are expressed in terms of limits on the masses of potential new colored particles. Assuming that the lightest neutralino is stable and has a mass less than about 500 GeV, gluino masses up to 1550-1750 GeV are excluded at 95% confidence level, depending on the gluino decay mechanism. For the scenario of direct production of squark-antisquark pairs, top squarks with masses up to 800 GeV are excluded, assuming a 100% branching fraction for the decay to a top quark and neutralino. Similarly, bottom squark masses are excluded up to 880 GeV, and masses of light-flavor squarks are excluded up to 600-1260 GeV, depending on the degree of degeneracy of the squark masses.
    Full-text available · Article · Oct 2016 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: A search is presented for the production of two Higgs bosons in final states containing two photons and two bottom quarks. Both resonant and nonresonant hypotheses are investigated. The analyzed data correspond to an integrated luminosity of 19.7 fb-1 of proton-proton collisions at s=8 TeV collected with the CMS detector. Good agreement is observed between data and predictions of the standard model (SM). Upper limits are set at 95% confidence level on the production cross section of new particles and compared to the prediction for the existence of a warped extra dimension. When the decay to two Higgs bosons is kinematically allowed, assuming a mass scale ΛR=1 TeV for the model, the data exclude a radion scalar at masses below 980 GeV. The first Kaluza-Klein excitation mode of the graviton in the RS1 Randall-Sundrum model is excluded for masses between 325 and 450 GeV. An upper limit of 0.71 pb is set on the nonresonant two-Higgs-boson cross section in the SM-like hypothesis. Limits are also derived on nonresonant production assuming anomalous Higgs-boson couplings.
    Full-text available · Article · Sep 2016
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    [Show abstract] [Hide abstract] ABSTRACT: Measurements are reported of the normalized differential cross sections for top quark pair production with respect to four kinematic event variables: the missing transverse energy; the scalar sum of the jet transverse momentum (pT); the scalar sum of the pT of all objects in the event; and the pT of leptonically decaying W bosons from top quark decays. The data sample, collected using the CMS detector at the LHC, consists of 5.0 fb-1 of proton-proton collisions at s=7 TeV and 19.7 fb-1 at s=8 TeV. Top quark pair events containing one electron or muon are selected. The results are presented after correcting for detector effects to allow direct comparison with theoretical predictions. No significant deviations from the predictions of several standard model event simulation generators are observed. © 2016 CERN, for the CMS Collaboration. Published by the American Physical Society under the terms of the »http://creativecommons.org/licenses/by/3.0/» Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    Full-text available · Article · Sep 2016
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    [Show abstract] [Hide abstract] ABSTRACT: A search is presented for single top quark production in the s channel in proton-proton collisions with the CMS detector at the CERN LHC in decay modes of the top quark containing a muon or an electron in the final state. The signal is extracted through a maximum-likelihood fit to the distribution of a multivariate discriminant defined using boosted decision trees to separate the expected signal contribution from background processes. The analysis uses data collected at centre-of-mass energies of 7 and 8 TeV and corresponding to integrated luminosities of 5.1 and 19.7 fb−1, respectively. The measured cross sections of 7.1 ± 8.1 pb (at 7 TeV) and 13.4 ± 7.3 pb (at 8 TeV) result in a best fit value of 2.0 ± 0.9 for the combined ratio of the measured and expected values. The signal significance is 2.5 standard deviations, and the upper limit on the rate relative to the standard model expectation is 4.7 at 95% confidence level.
    Full-text available · Article · Sep 2016 · Journal of High Energy Physics
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    Full-text available · Article · Sep 2016 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: A search is presented for the Higgs boson off-shell production in gluon fusion and vector boson fusion processes with the Higgs boson decaying into a W+W− pair and the W bosons decaying leptonically. The data observed in this analysis are used to constrain the Higgs boson total decay width. The analysis is based on the data collected by the CMS experiment at the LHC, corresponding to integrated luminosities of 4.9 fb−1 at a centre-of-mass energy of 7 TeV and 19.4 fb−1 at 8 TeV, respectively. An observed (expected) upper limit on the off-shell Higgs boson event yield normalised to the standard model prediction of 2.4 (6.2) is obtained at the 95% CL for the gluon fusion process and of 19.3 (34.4) for the vector boson fusion process. Observed and expected limits on the total width of 26 and 66 MeV are found, respectively, at the 95% confidence level (CL). These limits are combined with the previous result in the ZZ channel leading to observed and expected 95% CL upper limits on the width of 13 and 26 MeV, respectively.
    Full-text available · Article · Sep 2016 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: The polarizations of the , , and mesons are measured as a function of the charged particle multiplicity in proton-proton collisions at . The measurements are performed with a dimuon data sample collected in 2011 by the CMS experiment, corresponding to an integrated luminosity of 4.9 fb−1. The results are extracted from the dimuon decay angular distributions, in two ranges of transverse momentum (10–15 and 15–35 GeV), and in the rapidity interval . The results do not show significant changes from low- to high-multiplicity pp collisions, although large uncertainties preclude definite statements in the and cases.
    Full-text available · Article · Aug 2016 · Physics Letters B
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    [Show abstract] [Hide abstract] ABSTRACT: A search for new physics is performed using events with two isolated same-sign leptons, two or more jets, and missing transverse momentum. The results are based on a sample of proton–proton collisions at a center-of-mass energy of 13\(\,\text {TeV}\) recorded with the CMS detector at the LHC, corresponding to an integrated luminosity of 2.3 \(\mathrm{fb}^{1}\). Multiple search regions are defined by classifying events in terms of missing transverse momentum, the scalar sum of jet transverse momenta, the transverse mass associated with a \(\mathrm {W}\) boson candidate, the number of jets, the number of \(\mathrm{b} \) quark jets, and the transverse momenta of the leptons in the event. The analysis is sensitive to a wide variety of possible signals beyond the standard model. No excess above the standard model background expectation is observed. Constraints are set on various supersymmetric models, with gluinos and bottom squarks excluded for masses up to 1300 and 680\(\,\text {GeV}\), respectively, at the 95 % confidence level. Upper limits on the cross sections for the production of two top quark-antiquark pairs (119\(\,\text {fb}\)) and two same-sign top quarks (1.7\(\,\text {pb}\)) are also obtained. Selection efficiencies and model independent limits are provided to allow further interpretations of the results.
    Full-text available · Article · Aug 2016 · European Physical Journal C
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    G. Aad · B. Abbott · J. Abdallah · [...] · N. Woods
    [Show abstract] [Hide abstract] ABSTRACT: Combined ATLAS and CMS measurements of the Higgs boson production and decay rates, as well as constraints on its couplings to vector bosons and fermions, are presented. The combination is based on the analysis of five production processes, namely gluon fusion, vector boson fusion, and associated production with a W or a Z boson or a pair of top quarks, and of the six decay modes H → ZZ, W W , γγ, ττ, bb, and μμ. All results are reported assuming a value of 125.09 GeV for the Higgs boson mass, the result of the combined measurement by the ATLAS and CMS experiments. The analysis uses the CERN LHC proton-proton collision data recorded by the ATLAS and CMS experiments in 2011 and 2012, corresponding to integrated luminosities per experiment of approximately 5 fb−1 at \( \sqrt{s}=7 \) TeV and 20 fb−1 at \( \sqrt{s}=8 \) TeV. The Higgs boson production and decay rates measured by the two experiments are combined within the context of three generic parameterisations: two based on cross sections and branching fractions, and one on ratios of coupling modifiers. Several interpretations of the measurements with more model-dependent parameterisations are also given. The combined signal yield relative to the Standard Model prediction is measured to be 1.09 ± 0.11. The combined measurements lead to observed significances for the vector boson fusion production process and for the H → ττ decay of 5.4 and 5.5 standard deviations, respectively. The data are consistent with the Standard Model predictions for all parameterisations considered.
    Full-text available · Article · Aug 2016 · Journal of High Energy Physics

Publication Stats

13k Citations

Institutions

  • 2004-2016
    • Paul Scherrer Institut
      Филлиген, Aargau, Switzerland
  • 2014
    • Bhabha Atomic Research Centre
      Mumbai, Maharashtra, India
  • 2013
    • Sharif University of Technology
      Teheran, Tehrān, Iran
  • 2010-2013
    • Yerevan Physics Institute
      Ayrivan, Yerevan, Armenia
  • 2012
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States