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Open Access Publications from the University of California

Scholarly Works (7 results)

  • Article
  • Peer Reviewed
UC San Diego Previously Published Works (2021)
A search is presented for supersymmetric partners of the top quark (top squarks) in final states with two oppositely charged leptons (electrons or muons), jets identified as originating from b quarks, and missing transverse momentum. The search uses data from proton-proton collisions at s=13TeV collected with the CMS detector, corresponding to an integrated luminosity of 137 fb-1 . Hypothetical signal events are efficiently separated from the dominant top quark pair production background with requirements on the significance of the missing transverse momentum and on transverse mass variables. No significant deviation is observed from the expected background. Exclusion limits are set in the context of simplified supersymmetric models with pair-produced lightest top squarks. For top squarks decaying exclusively to a top quark and a lightest neutralino, lower limits are placed at 95% confidence level on the masses of the top squark and the neutralino up to 925 and 450 GeV , respectively. If the decay proceeds via an intermediate chargino, the corresponding lower limits on the mass of the lightest top squark are set up to 850 GeV for neutralino masses below 420 GeV . For top squarks undergoing a cascade decay through charginos and sleptons, the mass limits reach up to 1.4 TeV and 900 GeV respectively for the top squark and the lightest neutralino.
    Cover page: Search for top squark pair production using dilepton final states in pp collision data collected at s=13TeV.
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2021)
    A search for dark matter particles is performed using events with a Z boson candidate and large missing transverse momentum. The analysis is based on proton-proton collision data at a center-of-mass energy of 13 Te , collected by the CMS experiment at the LHC in 2016-2018, corresponding to an integrated luminosity of 137 fb-1 . The search uses the decay channels Z→ee and Z→μμ . No significant excess of events is observed over the background expected from the standard model. Limits are set on dark matter particle production in the context of simplified models with vector, axial-vector, scalar, and pseudoscalar mediators, as well as on a two-Higgs-doublet model with an additional pseudoscalar mediator. In addition, limits are provided for spin-dependent and spin-independent scattering cross sections and are compared to those from direct-detection experiments. The results are also interpreted in the context of models of invisible Higgs boson decays, unparticles, and large extra dimensions.
      Cover page: Search for dark matter produced in association with a leptonically decaying Z boson in proton-proton collisions at s=13Te.
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2020)
      We describe a method to obtain point and dispersion estimates for the energies of jets arising from b quarks produced in proton-proton collisions at an energy of s=13TeV at the CERN LHC. The algorithm is trained on a large sample of simulated b jets and validated on data recorded by the CMS detector in 2017 corresponding to an integrated luminosity of 41 fb-1 . A multivariate regression algorithm based on a deep feed-forward neural network employs jet composition and shape information, and the properties of reconstructed secondary vertices associated with the jet. The results of the algorithm are used to improve the sensitivity of analyses that make use of b jets in the final state, such as the observation of Higgs boson decay to bb¯ .
        Cover page: A Deep Neural Network for Simultaneous Estimation of b Jet Energy and Resolution
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2022)
        Measurements of the associated production of a W boson and a charm ( c ) quark in proton-proton collisions at a centre-of-mass energy of 8 TeV are reported. The analysis uses a data sample corresponding to a total integrated luminosity of 19.7 fb-1 collected by the CMS detector at the LHC. The W bosons are identified through their leptonic decays to an electron or a muon, and a neutrino. Charm quark jets are selected using distinctive signatures of charm hadron decays. The product of the cross section and branching fraction σ(pp→W+c+X)B(W→ℓν) , where ℓ=e or μ , and the cross section ratio σ(pp→W++c¯+X)/σ(pp→W-+c+X) are measured in a fiducial volume and differentially as functions of the pseudorapidity and of the transverse momentum of the lepton from the W boson decay. The results are compared with theoretical predictions. The impact of these measurements on the determination of the strange quark distribution is assessed.
          Cover page: Measurements of the associated production of a W boson and a charm quark in proton-proton collisions at s=8TeV.
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2021)
          The rate for Higgs ( H ) bosons production in association with either one ( tH ) or two ( tt¯H ) top quarks is measured in final states containing multiple electrons, muons, or tau leptons decaying to hadrons and a neutrino, using proton-proton collisions recorded at a center-of-mass energy of 13TeV by the CMS experiment. The analyzed data correspond to an integrated luminosity of 137 fb-1 . The analysis is aimed at events that contain H→WW , H→ττ , or H→ZZ decays and each of the top quark(s) decays either to lepton+jets or all-jet channels. Sensitivity to signal is maximized by including ten signatures in the analysis, depending on the lepton multiplicity. The separation among tH , tt¯H , and the backgrounds is enhanced through machine-learning techniques and matrix-element methods. The measured production rates for the tt¯H and tH signals correspond to 0.92±0.19(stat)-0.13+0.17(syst) and 5.7±2.7(stat)±3.0(syst) of their respective standard model (SM) expectations. The corresponding observed (expected) significance amounts to 4.7 (5.2) standard deviations for tt¯H , and to 1.4 (0.3) for tH production. Assuming that the Higgs boson coupling to the tau lepton is equal in strength to its expectation in the SM, the coupling yt of the Higgs boson to the top quark divided by its SM expectation, κt=yt/ytSM , is constrained to be within -0.9<κt<-0.7 or 0.7<κt<1.1 , at 95% confidence level. This result is the most sensitive measurement of the tt¯H production rate to date.
            Cover page: Measurement of the Higgs boson production rate in association with top quarks in final states with electrons, muons, and hadronically decaying tau leptons at s=13TeV.
            • Article
            • Peer Reviewed
            UC San Diego Previously Published Works (2021)
            Results of the Model Unspecific Search in CMS (MUSiC), using proton-proton collision data recorded at the LHC at a centre-of-mass energy of 13 TeV , corresponding to an integrated luminosity of 35.9 fb-1 , are presented. The MUSiC analysis searches for anomalies that could be signatures of physics beyond the standard model. The analysis is based on the comparison of observed data with the standard model prediction, as determined from simulation, in several hundred final states and multiple kinematic distributions. Events containing at least one electron or muon are classified based on their final state topology, and an automated search algorithm surveys the observed data for deviations from the prediction. The sensitivity of the search is validated using multiple methods. No significant deviations from the predictions have been observed. For a wide range of final state topologies, agreement is found between the data and the standard model simulation. This analysis complements dedicated search analyses by significantly expanding the range of final states covered using a model independent approach with the largest data set to date to probe phase space regions beyond the reach of previous general searches.
              Cover page: MUSiC: a model-unspecific search for new physics in proton-proton collisions at s=13TeV.
              • Article
              • Peer Reviewed
              UC Riverside Previously Published Works (2020)
              Measurements are presented of the single-diffractive dijet cross section and the diffractive cross section as a function of the proton fractional momentum loss ξ and the four-momentum transfer squared t. Both processes pp→pX and pp→Xp , i.e. with the proton scattering to either side of the interaction point, are measured, where X includes at least two jets; the results of the two processes are averaged. The analyses are based on data collected simultaneously with the CMS and TOTEM detectors at the LHC in proton-proton collisions at s=8Te during a dedicated run with β∗=90m at low instantaneous luminosity and correspond to an integrated luminosity of 37.5nb-1 . The single-diffractive dijet cross section σjjpX , in the kinematic region ξ<0.1 , 0.03<|t|<1Ge2 , with at least two jets with transverse momentum pT>40Ge , and pseudorapidity |η|<4.4 , is 21.7±0.9(stat)-3.3+3.0(syst)±0.9(lumi)nb . The ratio of the single-diffractive to inclusive dijet yields, normalised per unit of ξ , is presented as a function of x, the longitudinal momentum fraction of the proton carried by the struck parton. The ratio in the kinematic region defined above, for x values in the range -2.9≤log10x≤-1.6 , is R=(σjjpX/Δξ)/σjj=0.025±0.001(stat)±0.003(syst) , where σjjpX and σjj are the single-diffractive and inclusive dijet cross sections, respectively. The results are compared with predictions from models of diffractive and nondiffractive interactions. Monte Carlo predictions based on the HERA diffractive parton distribution functions agree well with the data when corrected for the effect of soft rescattering between the spectator partons.
                Cover page: Measurement of single-diffractive dijet production in proton–proton collisions at s=8Te with the CMS and TOTEM experiments
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