K. H. Hoffmann

Karlsruhe Institute of Technology, Carlsruhe, Baden-Württemberg, Germany

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Publications (317)

  • Dataset · Jan 2016
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    [Show abstract] [Hide abstract] ABSTRACT: The degradation of signal in silicon sensors is studied under conditions expected at the CERN High-Luminosity LHC. 200 $\mu$m thick n-type silicon sensors are irradiated with protons of different energies to fluences of up to $3 \cdot 10^{15}$ neq/cm$^2$. Pulsed red laser light with a wavelength of 672 nm is used to generate electron-hole pairs in the sensors. The induced signals are used to determine the charge collection efficiencies separately for electrons and holes drifting through the sensor. The effective trapping rates are extracted by comparing the results to simulation. The electric field is simulated using Synopsys device simulation assuming two effective defects. The generation and drift of charge carriers are simulated in an independent simulation based on PixelAV. The effective trapping rates are determined from the measured charge collection efficiencies and the simulated and measured time-resolved current pulses are compared. The effective trapping rates determined for both electrons and holes are about 50% smaller than those obtained using standard extrapolations of studies at low fluences and suggests an improved tracker performance over initial expectations.
    Full-text Article · May 2015 · Journal of Instrumentation
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    [Show abstract] [Hide abstract] ABSTRACT: A search for new physics is performed based on events with jets and a pair of isolated, same-sign leptons. The results are obtained using a sample of proton-proton collision data collected by the CMS experiment at a centre-of-mass energy of 8 TeV at the LHC, corresponding to an integrated luminosity of 19.5 fb-1. In order to be sensitive to a wide variety of possible signals beyond the standard model, multiple search regions defined by the missing transverse energy, the hadronic energy, the number of jets and b-quark jets, and the transverse momenta of the leptons in the events are considered. No excess above the standard model background expectation is observed and constraints are set on a number of models for new physics, as well as on the same-sign top-quark pair and quadruple-top-quark production cross sections. Information on event selection efficiencies is also provided, so that the results can be used to confront an even broader class of new physics models. [Figure not available: see fulltext.]
    Full-text Article · Jan 2015 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: The razor approach to search for R–parity conserving supersymmetric particles is described in detail. Two analyses are considered: an inclusive search for new heavy particle pairs decaying to final states with at least two jets and missing transverse energy, and a dedicated search for final states with at least one jet originating from a bottom quark. For both the inclusive study and the study requiring a bottom-quark jet, the data are examined in exclusive final states corresponding to all-hadronic, single-lepton, and dilepton events. The study is based on the data set of proton-proton collisions at √s = 7 TeV collected with the CMS detector at the LHC in 2011, corresponding to an integrated luminosity of 4.7 fb[superscript −1]. The study consists of a shape analysis performed in the plane of two kinematic variables, denoted M[subscript R] and R[superscript 2], that correspond to the mass and transverse energy flow, respectively, of pair-produced, heavy, new-physics particles. The data are found to be compatible with the background model, defined by studying event simulations and data control samples. Exclusion limits for squark and gluino production are derived in the context of the constrained minimal supersymmetric standard model (CMSSM) and also for simplified-model spectra (SMS). Within the CMSSM parameter space considered, squark and gluino masses up to 1350 GeV are excluded at 95% confidence level, depending on the model parameters. For SMS scenarios, the direct production of pairs of top or bottom squarks is excluded for masses as high as 400 GeV.
    Full-text Article · Dec 2014
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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 (p_{T}) range of 80-250 GeV/c, and within pseudorapidity |η|<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 p_{T} 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 p_{T} range studied.
    Full-text Article · Sep 2014 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: Measurements of the muon charge asymmetry in inclusive pp to WX production at sqrt(s)=7 TeV are presented. The data sample corresponds to an integrated luminosity of 4.7 inverse femtobarns recorded with the CMS detector at the LHC. With a sample of more than twenty million W to mu mu events, the statistical precision is greatly improved in comparison to previous measurements. These new results provide additional constraints on the parton distribution functions of the proton in the range of the Bjorken scaling variable x from 10E-3 to 10E-1. These measurements and the recent CMS measurement of associated W + charm production are used together with the cross sections for inclusive deep inelastic ep scattering at HERA in a next-to-leading-order QCD analysis. The determination of the valence quark distributions is improved, and the strange-quark distribution is probed directly through the leading-order process g + c to W + c in proton-proton collisions at the LHC.
    Full-text Article · Aug 2014 · Physical Review D
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    [Show abstract] [Hide abstract] ABSTRACT: Dijet production has been measured in collisions at a nucleon-nucleon centre-of-mass energy of 5.02. A data sample corresponding to an integrated luminosity of 35 was collected using the Compact Muon Solenoid detector at the Large Hadron Collider. The dijet transverse momentum balance, azimuthal angle correlations, and pseudorapidity distributions are studied as a function of the transverse energy in the forward calorimeters (). For collisions, the dijet transverse momentum ratio and the width of the distribution of dijet azimuthal angle difference are comparable to the same quantities obtained from a simulated reference and insensitive to . In contrast, the mean value of the dijet pseudorapidity is found to change monotonically with increasing , indicating a correlation between the energy emitted at large pseudorapidity and the longitudinal motion of the dijet frame. The pseudorapidity distribution of the dijet system in minimum bias collisions is compared with next-to-leading-order perturbative QCD predictions obtained from both nucleon and nuclear parton distribution functions, and the data more closely match the latter.
    Full-text Article · Jul 2014 · European Physical Journal C
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    [Show abstract] [Hide abstract] ABSTRACT: Spectra of identified charged hadrons are measured in pPb collisions with the CMS detector at the LHC at √SNN = 5.02 TeV. Charged pions, kaons, and protons in the transverse-momentum range pT ≈ 0.1-1.7GeV/c and laboratory rapidity pipeypipe < 1 are identified via their energy loss in the silicon tracker. The average pT increases with particle mass and the charged multiplicity of the event. The increase of the average pT with charged multiplicity is greater for heavier hadrons. Comparisons to Monte Carlo event generators reveal that Epos Lhc, which incorporates additional hydrodynamic evolution of the created system, is able to reproduce most of the data features, unlike Hijing and Ampt. The pT spectra and integrated yields are also compared to those measured in pp and PbPb collisions at various energies. The average transverse momentum and particle ratio measurements indicate that particle production at LHC energies is strongly correlated with event particle multiplicity. © 2014 CERN for the benefit of the CMS collaboration.
    Full-text Article · Jun 2014 · European Physical Journal C
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    [Show abstract] [Hide abstract] ABSTRACT: A study of color coherence effects in pp collisions at a center-of-mass energy of 7TeV is presented. The data used in the analysis were collected in 2010 with the CMS detector at the LHC and correspond to an integrated luminosity of 36 pb-1. Events are selected that contain at least three jets and where the two jets with the largest transverse momentum exhibit a back-to-back topology. The measured angular correlation between the second- and third-leading jet is shown to be sensitive to color coherence effects, and is compared to the predictions of Monte Carlo models with various implementations of color coherence. None of the models describe the data satisfactorily. © 2014 The Author(s).
    Full-text Article · Jun 2014 · European Physical Journal C
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    [Show abstract] [Hide abstract] ABSTRACT: Measurements of Wγ and Zγ production in proton-proton collisions at s=7TeV are used to extract limits on anomalous triple gauge couplings. The results are based on data recorded by the CMS experiment at the LHC that correspond to an integrated luminosity of 5.0fb-1. The cross sections are measured for photon transverse momenta pTγ>15GeV, and for separations between photons and final-state charged leptons in the pseudorapidity-azimuthal plane of ΔR(,γ)>0.7 in νγ and γ final states, where refers either to an electron or a muon. A dilepton invariant mass requirement of m>50GeV is imposed for the Zγ process. No deviations are observed relative to predictions from the standard model, and limits are set on anomalous WWγ, ZZγ, and Zγγ triple gauge couplings. © 2014 CERN, for the CMS Collaboration.
    Full-text Article · May 2014 · Physical Review D
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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 19.5fb-1 and was collected at s=8TeV 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. © 2014 CERN, for the CMS Collaboration.
    Full-text Article · Apr 2014 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: Results are presented of a search for a "natural" supersymmetry scenario with gauge mediated symmetry breaking. It is assumed that only the supersymmetric partners of the top quark (the top squark) and the Higgs boson (Higgsino) are accessible. Events are examined in which there are two photons forming a Higgs boson candidate, and at least two b-quark jets. In 19.7fb-1 of proton-proton collision data at s=8TeV, recorded in the CMS experiment, no evidence of a signal is found and lower limits at the 95% confidence level are set, excluding the top squark mass below 360 to 410 GeV, depending on the Higgsino mass. © 2014 CERN, for the CMS Collaboration.
    Full-text Article · Apr 2014 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: Results are presented of a search for a "natural" supersymmetry scenario with gauge mediated symmetry breaking. It is assumed that only the supersymmetric partners of the top quark (the top squark) and the Higgs boson (Higgsino) are accessible. Events are examined in which there are two photons forming a Higgs boson candidate, and at least two b-quark jets. In 19.7 fb-1 of proton-proton collision data at s=8 TeV, recorded in the CMS experiment, no evidence of a signal is found and lower limits at the 95% confidence level are set, excluding the top squark mass below 360 to 410 GeV, depending on the Higgsino mass.
    Full-text Article · Apr 2014 · Physical Review Letters
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    [Show abstract] [Hide abstract] ABSTRACT: A search for WV gamma triple vector boson production is presented based on events containing a W boson decaying to a muon or an electron and a neutrino, a second V (W or Z) boson, and a photon. The data correspond to an integrated luminosity of 19.3 inverse femtobarns collected in 2012 with the CMS detector at the LHC in pp collisions at sqrt(s) = 8 TeV. An upper limit of 311 fb on the fiducial cross section for the WV gamma production process is obtained at 95% confidence level for photons with a transverse energy above 30 GeV and with an absolute value of pseudorapidity of less than 1.44. This limit is approximately a factor of 3.4 larger than the standard model predictions that are based on next-to-leading order QCD calculations. Since no evidence of anomalous WW gamma gamma or WWZ gamma quartic gauge boson couplings is found, this paper presents the first experimental limits on the dimension-8 parameter f[T,0] and the CP-conserving WWZ gamma parameters kappa[0,W] and kappa[C,W]. Limits are also obtained for the WW gamma gamma parameters a[0,W] and a[C,W].
    Full-text Article · Apr 2014 · Physical Review D
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    [Show abstract] [Hide abstract] ABSTRACT: The production of \( Y \)(1S), \( Y \)(2S), and \( Y \)(3S) is investigated in pPb and pp collisions at centre-of-mass energies per nucleon pair of 5.02 TeV and 2.76 TeV, respectively. The datasets correspond to integrated luminosities of about 31 nb−1 (pPb) and 5.4 pb−1 (pp), collected in 2013 by the CMS experiment at the LHC. Upsilons that decay into muons are reconstructed within the rapidity interval |y CM| < 1.93 in the nucleon-nucleon centre-of-mass frame. Their production is studied as a function of two measures of event activity, namely the charged-particle multiplicity measured in the pseudorapidity interval |η| < 2.4, and the sum of transverse energy deposited at forward pseudorapidity, 4.0 < |η| < 5.2. The \( Y \) cross sections normalized by their event activity integrated values, \( Y \)(nS)/〈\( Y \)(nS)〉, are found to rise with both measures of the event activity in pp and pPb. In both collision systems, the ratios of the excited to the ground state cross sections, \( Y \)(nS)/ \( Y \)(1S), are found to decrease with the charged-particle multiplicity, while as a function of the transverse energy the variation is less pronounced. The event activity integrated double ratios, [\( Y \)(nS)/ \( Y \)(1S)]pPb /[\( Y \)(nS)/ \( Y \)(1S)]pp, are also measured and found to be 0.83 ± 0.05 (stat.) ± 0.05 (syst.) and 0.71 ± 0.08 (stat.) ± 0.09 (syst.) for \( Y \)(2S) and \( Y \)(3S), respectively.
    Full-text Article · Apr 2014 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: A search for invisible decays of Higgs bosons is performed using the vector boson fusion and associated ZH production modes. In the ZH mode, the Z boson is required to decay to a pair of charged leptons or a b b-bar quark pair. The searches use the 8 TeV pp collision dataset collected by the CMS detector at the LHC, corresponding to an integrated luminosity of up to 19.7 inverse femtobarns. Certain channels include data from 7 TeV collisions corresponding to an integrated luminosity of 4.9 inverse femtobarns. The searches are sensitive to non-standard-model invisible decays of the recently observed Higgs boson, as well as additional Higgs bosons with similar production modes and large invisible branching fractions. In all channels, the observed data are consistent with the expected standard model backgrounds. Limits are set on the production cross section times invisible branching fraction, as a function of the Higgs boson mass, for the vector boson fusion and ZH production modes. By combining all channels, and assuming standard model Higgs boson cross sections and acceptances, the observed (expected) upper limit on the invisible branching fraction at m[H]=125 GeV is found to be 0.58 (0.44) at 95% confidence level. We interpret this limit in terms of a Higgs-portal model of dark matter interactions.
    Full-text Article · Apr 2014 · European Physical Journal C
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    [Show abstract] [Hide abstract] ABSTRACT: A search is presented for a massive particle, generically referred to as a Z', decaying into a tt̄ pair. The search focuses on Z' resonances that are sufficiently massive to produce highly Lorentz-boosted top quarks, which yield collimated decay products that are partially or fully merged into single jets. The analysis uses new methods to analyze jet substructure, providing suppression of the non-top multijet backgrounds. The analysis is based on a data sample of proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 5 fb -1. Upper limits in the range of 1 pb are set on the product of the production cross section and branching fraction for a topcolor Z' modeled for several widths, as well as for a Randall-Sundrum Kaluza-Klein gluon. In addition, the result contrain any enhancement in tt̄ production beyond expectations of the standard mode for tt invariant mass larger than 1TeV/c 2.
    Full-text Article · Mar 2014 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: Results are presented from a search for the production of a heavy gauge boson W' decaying into a top and a bottom quark, using a data set collected by the CMS experiment at sqrt(s)=8 TeV and corresponding to an integrated luminosity of 19.5 inverse femtobarns. Various models of W'-boson production are studied by allowing for an arbitrary combination of left- and right-handed couplings. The analysis is based on the detection of events with a lepton (e, mu), jets, and missing transverse energy in the final state. No evidence for W'-boson production is found and 95% confidence level upper limits on the production cross section times branching fraction are obtained. For W' bosons with purely right-handed couplings, and for those with left-handed couplings assuming no interference effects, the observed 95% confidence level limit is M(W')>2.05 TeV. For W' bosons with purely left-handed couplings, including interference effects, the observed 95% confidence level limit is M(W')>1.84 TeV. The results presented in this paper are the most stringent limits published to date.
    Full-text Article · Feb 2014 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: The production of a Z boson, decaying into two leptons and produced in association with one or more b jets, is studied using proton-proton collisions delivered by the LHC at a centre-of-mass energy of 7 TeV. The data were recorded in 2011 with the CMS detector and correspond to an integrated luminosity of 5 fb-1. The Z(ℓℓ) + b-jets cross sections (where ℓℓ = μμ or ee) are measured separately for a Z boson produced with exactly one b jet and with at least two b jets. In addition, a cross section ratio is extracted for a Z boson produced with at least one b jet, relative to a Z boson produced with at least one jet. The measured cross sections are compared to various theoretical predictions, and the data favour the predictions in the five-flavour scheme, where b quarks are assumed massless. The kinematic properties of the reconstructed particles are compared with the predictions from the MadGraph event generator using the pythia parton shower simulation. [Figure not available: see fulltext.] © 2014 The Author(s).
    Full-text Article · Feb 2014 · Journal of High Energy Physics
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    [Show abstract] [Hide abstract] ABSTRACT: A measurement of total and fiducial inclusive W and Z boson production cross sections in pp collisions at sqrt(s)=8 TeV is presented. Electron and muon final states are analyzed in a data sample collected with the CMS detector corresponding to an integrated luminosity of 18.2 +/- 0.5 inverse picobarns. The measured total inclusive cross sections times branching fractions are sigma(pp to W X) B(W to l nu ) = 12.21 +/- 0.03 (stat.) +/- 0.24 (syst.) +/- 0.32 (lum.) nb and sigma(pp to Z X) B(Z to l+ l-) = 1.15 +/- 0.01 (stat.) +/- 0.02 (syst.) +/- 0.03 (lum.) nb for the dilepton mass in the range of 60 to 120 GeV. The measured values agree with next-to-next-to-leading-order QCD cross section calculations. Ratios of cross sections are reported with a precision of 2%. This is the first measurement of inclusive W and Z boson production in proton-proton collisions at sqrt(s)=8 TeV.
    Full-text Article · Feb 2014 · Physical Review Letters

Publication Stats

11k Citations

Institutions

  • 2009-2014
    • Karlsruhe Institute of Technology
      • Institute of Experimental Nuclear Physics
      Carlsruhe, Baden-Württemberg, Germany
  • 2010-2013
    • Yerevan Physics Institute
      Ayrivan, Yerevan, Armenia
  • 2012
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2011
    • CERN
      Genève, Geneva, Switzerland