Recursos de colección

Caltech Authors (143.226 recursos)

Repository of works by Caltech published authors.

Group = CMS@Caltech

Mostrando recursos 1 - 20 de 511

  1. 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....

  2. 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...

  3. 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...

  4. 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...

  5. 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...

  6. 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...

  7. 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...

  8. 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...

  9. Search for supersymmetry in pp collisions at √s = 13 TeV in the single-lepton final state using the sum of masses of large-radius jets

    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.
    Results are reported from a search for supersymmetric particles in proton-proton collisions in the final state with a single, high transverse momentum lepton; multiple jets, including at least one b-tagged jet; and large missing transverse momentum. The data sample corresponds to an integrated luminosity of 2.3 fb^(−1) at √s = 13 TeV, recorded by the CMS experiment at the LHC. The search focuses on processes leading to high jet multiplicities, such as gluino pair production with g → ttχ^0_1. The quantity MJ, defined as the sum of the masses of the large-radius jets in the event, is used in conjunction...

  10. Search for new physics in final states with two opposite-sign, same-flavor leptons, jets, and missing transverse momentum in pp collisions at √s = 13 TeV

    Khachatryan, V.; Anderson, D.; Apresyan, A.; Bendavid, J.; 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 physics beyond the standard model in final states with two opposite-sign, same-flavor leptons, jets, and missing transverse momentum. The data sample corresponds to an integrated luminosity of 2.3 fb^(−1) of proton-proton collisions at √s=13 TeV collected with the CMS detector at the LHC in 2015. The analysis uses the invariant mass of the lepton pair, searching for a kinematic edge or a resonant-like excess compatible with the Z boson mass. Both search modes use several event categories in order to increase the sensitivity to new physics. These categories are based on the rapidity of the...

  11. Measurement of dijet azimuthal decorrelation in pp 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 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 TeV. The analysis is based on the proton-proton collision data collected with the CMS experiment at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.7fb^(−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...

  12. Searches for R-parity-violating supersymmetry in pp collisions at √s = 8 TeV in final states with 0–4 leptons

    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.
    Results are presented from searches for R-parity-violating supersymmetry in events produced in pp collisions at √s = 8 TeV at the LHC. Final states with 0, 1, 2, or multiple leptons are considered independently. The analysis is performed on data collected by the CMS experiment corresponding to an integrated luminosity of 19.5 fb^(−1). No excesses of events above the standard model expectations are observed, and 95% confidence level limits are set on supersymmetric particle masses and production cross sections. The results are interpreted in models featuring R-parity-violating decays of the lightest supersymmetric particle, which in the studied scenarios can be...

  13. ϒ(nS) polarizations versus particle multiplicity 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.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.
    The polarizations of the ϒ(1S), ϒ(2S), and ϒ(3S) mesons are measured as a function of the charged particle multiplicity in proton–proton collisions at √s = 7 TeV. 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 ϒ(nS) transverse momentum (10–15 and 15–35 GeV), and in the rapidity interval |y|<1.2. The results do not show significant changes from low- to high-multiplicity pp collisions, although large uncertainties preclude definite statements in the ϒ(2S)...

  14. Search for direct pair production of supersymmetric top quarks decaying to all-hadronic final states in pp 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.; Pierini, M.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.
    Results are reported from a search for the pair production of top squarks, the supersymmetric partners of top quarks, in final states with jets and missing transverse momentum. The data sample used in this search was collected by the CMS detector and corresponds to an integrated luminosity of 18.9fb^(-1) of proton-proton collisions at a centre-of-mass energy of 8 TeV produced by the LHC. The search features novel background suppression and prediction methods, including a dedicated top quark pair reconstruction algorithm. The data are found to be in agreement with the predicted backgrounds. Exclusion limits are set in simplified supersymmetry models...

  15. Inclusive Searches for Supersymmetry with the CMS detector at √s = 8 TeV

    Duarte, Javier Mauricio
    We discuss two complementary searches for supersymmetry and their combination carried out on approximately 19 fb^(-1) of data collected by CMS during the √s = 8 TeV LHC run. Results for a variety of final state signatures, including multi-bottom, multi-top, and mixed top-plus-bottom quark topologies, are presented. The combination of results yields an improved bound on the top-squark mass. For a neutralino mass of 100 GeV, the branching fraction dependent (independent) pair production of gluinos and top squarks is excluded for gluino masses up to 1310 (1175) GeV and for top squark masses up to 730 (645) GeV.

  16. Measurement of the differential cross section and charge asymmetry for inclusive pp → W^(±) + X production 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 differential cross section and charge asymmetry for inclusive pp → W^(±) + X → μ^(±)ν + X production at √s = 8 TeV are measured as a function of muon pseudorapidity. The data sample corresponds to an integrated luminosity of 18.8 fb^(−1) recorded with the CMS detector at the LHC. These results provide important constraints on the parton distribution functions of the proton in the range of the Bjorken scaling variable x from 10^(−3) to 10^(−1).

  17. Measurement of the integrated and differential tt production cross sections for high-pT top quarks 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 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...

  18. Measurement of the inelastic cross section in proton–lead collisions at √SNN = 5.02 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 inelastic hadronic cross section in proton–lead collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV is measured with the CMS detector at the LHC. The data sample, corresponding to an integrated luminosity of L=12.6±0.4 nb^(−1), has been collected with an unbiased trigger for inclusive particle production. The cross section is obtained from the measured number of proton–lead collisions with hadronic activity produced in the pseudorapidity ranges 3<η<5 and/or −5<η<−3, corrected for photon-induced contributions, experimental acceptance, and other instrumental effects. The inelastic cross section is measured to be σ_(inel)(pPb)=2061±3(stat)±34(syst)±72(lumi) mb. Various Monte Carlo generators, commonly used in heavy...

  19. Search for neutral resonances decaying into a Z boson and a pair of b jets or τ leptons

    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 search is performed for a new resonance decaying into a lighter resonance and a Z boson. Two channels are studied, targeting the decay of the lighter resonance into either a pair of oppositely charged τ leptons or a View the MathML sourcebb‾ pair. The Z boson is identified via its decays to electrons or muons. The search exploits data collected by the CMS experiment at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.8 fb^(−1). No significant deviations are observed from the standard model expectation and limits are set on production cross sections and parameters...

  20. Study of Z boson production in pPb collisions at √SNN = 5.02 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.
    The production of Z bosons in pPb collisions at √SNN = 5.02 TeV is studied by the CMS experiment via the electron and muon decay channels. The inclusive cross section is compared to pp collision predictions, and found to scale with the number of elementary nucleon–nucleon collisions. The differential cross sections as a function of the Z boson rapidity and transverse momentum are measured. Though they are found to be consistent within uncertainty with theoretical predictions both with and without nuclear effects, the forward–backward asymmetry suggests the presence of nuclear effects at large rapidities. These results provide new data for...

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