Recursos de colección

Caltech Authors (147.820 recursos)

Repository of works by Caltech published authors.

Group = CMS@Caltech

Mostrando recursos 1 - 20 de 524

  1. Radiation induced color centers and light monitoring for lead tungstate crystals

    Qu, Xiangdong; Zhang, Liyuan; Zhu, Ren-yuan
    This paper presents result of a study on undoped as well as various doped lead tungstate crystals. Radiation induced color center density was compared to radio luminescence. Light output degradation for crystals under irradiation was measured. Correlations between variations of crystal transmittance and light output were investigated. Monitoring wavelength was determined so that adequate sensitivity and good linearity may be achieved.

  2. Implementation of a Software Feedback Control for the CMS Monitoring Lasers

    Zhang, Liyuan; Bailleux, David; Bornheim, Adolf; Zhu, Kejun; Zhu, Ren-Yuan
    Light monitoring will play a crucial role in maintaining energy resolution for the CMS lead tungstate crystal calorimeter in situ at LHC. Since 2003, a laser based monitoring system in its final design has been installed and used in beam tests at CERN. While the stability of the laser pulse energy and FWHM width, measured in 24 hours, is at 3% level, a long term degradation and a drift of the laser pulse center timing at 2 ns/day were observed. The degradation and drift were caused by the aging of the DC Kr lamp used to pump the Nd:YLF laser,...

  3. The commissioning of the CMS electromagnetic calorimeter light monitoring system

    Orimoto, Toyoko J.
    The Compact Muon Solenoid (CMS) Experiment, located at the Large Hadron Collider at CERN, is a general purpose particle detector experiment with a high resolution electromagnetic calorimeter (ECAL). I report on the commissioning of the light monitoring system, which tracks changes in transparency of the lead tungstate crystals and is thus crucial for maintaining the energy resolution of the calorimeter.

  4. Search for high-mass Zy resonances in e^+e^- and µ+µ- y final states in proton-proton collisions at √s=8 and 13TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Lawhorn, J. M.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    This paper describes the search for a high-mass narrow-width scalar particle decaying into a Z boson and a photon. The analysis is performed using proton-proton collision data recorded with the CMS detector at the LHC at center-of-mass energies of 8 and 13 TeV, corresponding to integrated luminosities of 19.7 and 2.7 fb^(−1), respectively. The Z bosons are reconstructed from opposite-sign electron or muon pairs. No statistically significant deviation from the standard model predictions has been found in the 200-2000 GeV mass range. Upper limits at 95% confidence level have been derived on the product of the scalar particle production cross...

  5. Search for top quark decays via Higgs-boson-mediated flavor-changing neutral currents in pp collisions at √s = 8 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A search is performed for Higgs-boson-mediated flavor-changing neutral currents in the decays of top quarks. The search is based on proton-proton collision data corresponding to an integrated luminosity of 19.7 fb^(−1) at a center-of-mass energy of 8 TeV collected with the CMS detector at the LHC. Events in which a top quark pair is produced with one top quark decaying into a charm or up quark and a Higgs boson (H), and the other top quark decaying into a bottom quark and a W boson are selected. The Higgs boson in these events is assumed to subsequently decay into either...

  6. The CMS trigger system

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Pierini, M.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    This paper describes the CMS trigger system and its performance during Run 1 of the LHC. The trigger system consists of two levels designed to select events of potential physics interest from a GHz (MHz) interaction rate of proton-proton (heavy ion) collisions. The first level of the trigger is implemented in hardware, and selects events containing detector signals consistent with an electron, photon, muon, τ lepton, jet, or missing transverse energy. A programmable menu of up to 128 object-based algorithms is used to select events for subsequent processing. The trigger thresholds are adjusted to the LHC instantaneous luminosity during data...

  7. Search for anomalous Wtb couplings and flavour-changing neutral currents in t-channel single top quark production in pp collisions at √s = 7 and 8 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    Single top quark events produced in the t channel are used to set limits on anomalous Wtb couplings and to search for top quark flavour-changing neutral current (FCNC) interactions. The data taken with the CMS detector at the LHC in proton-proton collisions at s√ =7 and 8 TeV correspond to integrated luminosities of 5.0 and 19.7 fb^(−1), respectively. The analysis is performed using events with one muon and two or three jets. A Bayesian neural network technique is used to discriminate between the signal and backgrounds, which are observed to be consistent with the standard model prediction. The 95% confidence...

  8. Measurement of the production cross section of a W boson in association with two b jets in pp collisions at √s = 8 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Lawhorn, J. M.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    The production cross section of a W boson in association with two b jets is measured using a sample of proton–proton collisions at √s = 8TeV collected by the CMS experiment at the CERN LHC. The data sample corresponds to an integrated luminosity of 19.8 fb^(-1). The W bosons are reconstructed via their leptonic decays, W → ℓν, where ℓ = μ or ee . The fiducial region studied contains exactly one lepton with transverse momentum p^ℓ_T > 30 GeV and pseudorapidity |η^ℓ|<2.1, with exactly two b jets with p_T > 25 GeV and |η|< 2.4 and no other jets...

  9. Observation of the decay B^+ → ψ(2S)ϕ(1020)K^+ in pp collisions at √s = 8 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Lawhorn, J. M.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    The decay B^+ → ψ (2S) ϕ(1020)K^+B^+ → ψ(2S)ϕ(1020)K^+ is observed for the first time using data collected from pp collisions at √s = 8 TeV by the CMS experiment at the LHC, corresponding to an integrated luminosity of 19.6 fb^(-1). The branching fraction of this decay is measured, using the mode B+→ψ(2S)K+B+→ψ(2S)K+ as normalization, to be (4.0±0.4(stat) ± 0.6(syst)±0.2(B))×10^(−6), where the third uncertainty is from the measured branching fraction of the normalization channel.

  10. Measurement of the W^+ W^− cross section in pp collisions at √s = 8 TeV and limits on anomalous gauge couplings

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Pierini, M.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A measurement of the W boson pair production cross section in proton-proton collisions at √s = 8 TeV is presented. The data collected with the CMS detector at the LHC correspond to an integrated luminosity of 19.4 fb^(-1). The W^+W^− candidates are selected from events with two charged leptons, electrons or muons, and large missing transverse energy. The measured W^+W^− cross section is 60.1 ± 0.9(stat) ± 3.2 (exp) ± 3.1(theo)±1.6 (lumi)\,pb = 60.1 ± 4.8\,pb60.1 ± 0.9 (stat) ±3.2 (exp)±3.1 (theo) ± 1.6 (lumi)\,pb = 60.1 ± 4.8\, pb, consistent with the standard model prediction. The W^+W^− cross sections...

  11. Measurement of the double-differential inclusive jet cross section in proton–proton collisions at √s = 13 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A measurement of the double-differential inclusive jet cross section as a function of jet transverse momentum pT and absolute jet rapidity |y| is presented. The analysis is based on proton–proton collisions collected by the CMS experiment at the LHC at a centre-of-mass energy of 13TeV. The data samples correspond to integrated luminosities of 71 and 44 pb^(-1) for |y|<3 and 3.2<|y|<4.7, respectively. Jets are reconstructed with the anti-k_t clustering algorithm for two jet sizes, R, of 0.7 and 0.4, in a phase space region covering jet pT up to 2TeV and jet rapidity up to |y| = 4.7. Predictions of...

  12. Measurement of the W boson helicity fractions in the decays of top quark pairs to lepton+jets final states produced in pp collisions at √s=8 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    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 F_0 = 0.681 ± 0.012(stat) ± 0.023 (syst), f_L and F_R = -0.004 ± 0.005(stat)...

  13. Search for lepton flavour violating decays of the Higgs boson to eτ and eμ in proton–proton collisions at √s = 8 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    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....

  14. Measurement of the ZZ production cross section and Z → ℓ^+ℓ^-ℓ'+ℓ'^- branching fraction in pp collisions at √s = 13 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Lawhorn, J. M.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    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^(+1.9)_(-1.8) (stat)^(+0.5)_(-0.3) (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 604 GeV...

  15. Measurement of the tt production cross section in the eµ channel in proton-proton collisions at √s = 7 and 8 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    The inclusive cross section for top quark pair production is measured in proton-proton collisions at √s = 7 and 8 TeV, corresponding to 5.0 and 19.7 fb^(−1), respectively, with the CMS experiment at the LHC. The cross sections are measured in the electron-muon channel using a binned likelihood fit to multi-differential final state distributions related to identified b quark jets and other jets in the event. The measured cross section values are 173.6 ± 2.1(stat)_(−4.0)^(+4.5)(syst) ± 3.8(lumi)pb at √s = 7 TeV, and 244.9 ± 1.4(stat)_(−5.5)^(+6.3)(syst) ± 6.4(lumi)pb at √s = 8 TeV, in good agreement with QCD calculations at next-to-next-to-leading-order accuracy. The ratio of the cross...

  16. Measurements of the tt[bar] production cross section in lepton+jets final states in pp collisions at 8 TeV and ratio of 8 to 7 TeV cross sections

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Pierini, M.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A measurement of the top quark pair production (tt[bar]) cross section in proton–proton collisions at the centre-of-mass energy of 8TeV is presented using data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.6fb^(−1). This analysis is performed in the tt[bar] decay channels with one isolated, high transverse momentum electron or muon and at least four jets, at least one of which is required to be identified as originating from hadronization of a b quark. The calibration of the jet energy scale and the efficiency of b jet identification are determined from data. The measured...

  17. Search for heavy resonances decaying to two Higgs bosons in final states containing four b quarks

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A search is presented for narrow heavy resonances X decaying into pairs of Higgs bosons ( HH ) in proton-proton collisions collected by the CMS experiment at the LHC at √s = 8TeV. The data correspond to an integrated luminosity of 19.7 fb^(−1). The search considers HH resonances with masses between 1 and 3 TeV, having final states of two b quark pairs. Each Higgs boson is produced with large momentum, and the hadronization products of the pair of b quarks can usually be reconstructed as single large jets. The background from multijet and tt events is significantly reduced by...

  18. Search for dark matter particles in proton-proton collisions at √s = 8 TeV using the razor variables

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A search for dark matter particles directly produced in proton-proton collisions recorded by the CMS experiment at the LHC is presented. The data correspond to an integrated luminosity of 18.8 fb^(−1), at a center-of-mass energy of 8 TeV. The event selection requires at least two jets and no isolated leptons. The razor variables are used to quantify the transverse momentum balance in the jet momenta. The study is performed separately for events with and without jets originating from b quarks. The observed yields are consistent with the expected backgrounds and, depending on the nature of the production mechanism, dark matter...

  19. Azimuthal decorrelation of jets widely separated in rapidity in pp collisions at √s = 7 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Pierini, M.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.
    The decorrelation in the azimuthal angle between the most forward and the most backward jets (Mueller-Navelet jets) is measured in data collected in pp collisions with the CMS detector at the LHC at √s = 7 TeV. The measurement is presented in the form of distributions of azimuthal-angle differences, Δϕ, between the Mueller-Navelet jets, the average cosines of (π − Δϕ), 2(π − Δϕ), and 3(π − Δϕ), and ratios of these cosines. The jets are required to have transverse momenta, pT, in excess of 35 GeV and rapidities, |y|, of less than 4.7. The results are presented as a...

  20. Search for Dark Matter and Supersymmetry with a Compressed Mass Spectrum in the Vector Boson Fusion Topology in Proton-Proton Collisions at √s = 8  TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; Bornheim, A.; Bunn, J.; Chen, Y.; Duarte, J.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A first search for pair production of dark matter candidates through vector boson fusion in proton-proton collisions at √s = 8  TeV is performed with the CMS detector. The vector boson fusion topology enhances missing transverse momentum, providing a way to probe supersymmetry, even in the case of a compressed mass spectrum. The data sample corresponds to an integrated luminosity of 18.5  fb^(−1), recorded by the CMS experiment. The observed dijet mass spectrum is consistent with the standard model expectation. In an effective field theory, dark matter masses are explored as a function of contact interaction strength. The most stringent limit on...

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