Displaying publications 1 - 20 of 119 in total

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  1. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, Brandstetter J, et al.
    Phys. Rev. Lett., 2020 May 22;124(20):202001.
    PMID: 32501048 DOI: 10.1103/PhysRevLett.124.202001
    A measurement is reported of the jet mass distribution in hadronic decays of boosted top quarks produced in pp collisions at sqrt[s]=13  TeV. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of 35.9  fb^{-1}. The measurement is performed in the lepton+jets channel of tt[over ¯] events, where the lepton is an electron or muon. The products of the hadronic top quark decay t→bW→bqq[over ¯]^{'} are reconstructed as a single jet with transverse momentum larger than 400 GeV. The tt[over ¯] cross section as a function of the jet mass is unfolded at the particle level and used to extract a value of the top quark mass of 172.6±2.5  GeV. A novel jet reconstruction technique is used for the first time at the LHC, which improves the precision by a factor of 3 relative to an earlier measurement. This highlights the potential of measurements using boosted top quarks, where the new technique will enable future precision measurements.
  2. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, Dragicevic M, et al.
    Phys. Rev. Lett., 2020 Apr 24;124(16):162002.
    PMID: 32383915 DOI: 10.1103/PhysRevLett.124.162002
    The polarizations of promptly produced χ_{c1} and χ_{c2} mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at sqrt[s]=8  TeV. The χ_{c} states are reconstructed via their radiative decays χ_{c}→J/ψγ, with the photons being measured through conversions to e^{+}e^{-}, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_{c2} to χ_{c1} yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ→μ^{+}μ^{-} decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum.
  3. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, Dragicevic M, et al.
    Phys. Rev. Lett., 2020 Apr 03;124(13):131802.
    PMID: 32302170 DOI: 10.1103/PhysRevLett.124.131802
    A search is presented for a narrow resonance decaying to a pair of oppositely charged muons using sqrt[s]=13  TeV proton-proton collision data recorded at the LHC. In the 45-75 and 110-200 GeV resonance mass ranges, the search is based on conventional triggering and event reconstruction techniques. In the 11.5-45 GeV mass range, the search uses data collected with dimuon triggers with low transverse momentum thresholds, recorded at high rate by storing a reduced amount of trigger-level information. The data correspond to integrated luminosities of 137 and 96.6  fb^{-1} for conventional and high-rate triggering, respectively. No significant resonant peaks are observed in the probed mass ranges. The search sets the most stringent constraints to date on a dark photon in the ∼30-75 and 110-200 GeV mass ranges.
  4. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, Brandstetter J, et al.
    Phys. Rev. Lett., 2020 Jan 31;124(4):041803.
    PMID: 32058742 DOI: 10.1103/PhysRevLett.124.041803
    The first search for supersymmetry in events with an experimental signature of one soft, hadronically decaying τ lepton, one energetic jet from initial-state radiation, and large transverse momentum imbalance is presented. These event signatures are consistent with direct or indirect production of scalar τ leptons (τ[over ˜]) in supersymmetric models that exhibit coannihilation between the τ[over ˜] and the lightest neutralino (χ[over ˜]_{1}^{0}), and that could generate the observed relic density of dark matter. The data correspond to an integrated luminosity of 77.2  fb^{-1} of proton-proton collisions at sqrt[s]=13  TeV collected with the CMS detector at the LHC in 2016 and 2017. The results are interpreted in a supersymmetric scenario with a small mass difference (Δm) between the chargino (χ[over ˜]_{1}^{±}) or next-to-lightest neutralino (χ[over ˜]_{2}^{0}), and the χ[over ˜]_{1}^{0}. The mass of the τ[over ˜] is assumed to be the average of the χ[over ˜]_{1}^{±} and χ[over ˜]_{1}^{0} masses. The data are consistent with standard model background predictions. Upper limits at 95% confidence level are set on the sum of the χ[over ˜]_{1}^{±}, χ[over ˜]_{2}^{0}, and τ[over ˜] production cross sections for Δm(χ[over ˜]_{1}^{±},χ[over ˜]_{1}^{0})=50  GeV, resulting in a lower limit of 290 GeV on the mass of the χ[over ˜]_{1}^{±}, which is the most stringent to date and surpasses the bounds from the LEP experiments.
  5. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Asilar E, Bergauer T, et al.
    Eur Phys J C Part Fields, 2020;80(1):43.
    PMID: 32026888 DOI: 10.1140/epjc/s10052-019-7585-7
    A measurement is presented of electroweak (EW) production of a
    W
    boson in association with two jets in proton-proton collisions at


    s

    =
    13


    Te



    . The data sample was recorded by the CMS Collaboration at the LHC and corresponds to an integrated luminosity of 35.9



    fb

    -
    1



    . The measurement is performed for the


    ν

    jj final state (with


    ν

    indicating a lepton-neutrino pair, and j representing the quarks produced in the hard interaction) in a kinematic region defined by invariant mass


    m
    jj

    >
    120


    Ge



    and transverse momenta


    p

    T
    j


    >
    25


    Ge



    . The cross section of the process is measured in the electron and muon channels yielding


    σ
    EW


    (
    W
    jj
    )

    =
    6.23
    ±
    0.12

    (stat)
    ±
    0.61

    (syst)

    pb

    per channel, in agreement with leading-order standard model predictions. The additional hadronic activity of events in a signal-enriched region is studied, and the measurements are compared with predictions. The final state is also used to perform a search for anomalous trilinear gauge couplings. Limits on anomalous trilinear gauge couplings associated with dimension-six operators are given in the framework of an effective field theory. The corresponding 95% confidence level intervals are

    -
    2.3
    <

    c

    W
    W
    W


    /

    Λ
    2

    <
    2.5



    Te



    -
    2



    ,

    -
    8.8
    <

    c
    W

    /

    Λ
    2

    <
    16



    Te



    -
    2



    , and

    -
    45
    <

    c
    B

    /

    Λ
    2

    <
    46



    Te



    -
    2



    . These results are combined with the CMS EW
    Zjj
    analysis, yielding the constraint on the

    c

    W
    W
    W


    coupling:

    -
    1.8
    <

    c

    W
    W
    W


    /

    Λ
    2

    <
    2.0



    Te



    -
    2



    .
  6. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, Brandstetter J, et al.
    Eur Phys J C Part Fields, 2020;80(2):75.
    PMID: 32108833 DOI: 10.1140/epjc/s10052-019-7593-7
    The standard model (SM) production of four top quarks (

    t


    t
    ¯

    t


    t
    ¯


    ) in proton-proton collisions is studied by the CMS Collaboration. The data sample, collected during the 2016-2018 data taking of the LHC, corresponds to an integrated luminosity of 137



    fb

    -
    1



    at a center-of-mass energy of 13


    TeV

    . The events are required to contain two same-sign charged leptons (electrons or muons) or at least three leptons, and jets. The observed and expected significances for the

    t


    t
    ¯

    t


    t
    ¯


    signal are respectively 2.6 and 2.7 standard deviations, and the

    t


    t
    ¯

    t


    t
    ¯


    cross section is measured to be

    12
    .

    6

    -
    5.2


    +
    5.8



    fb

    . The results are used to constrain the Yukawa coupling of the top quark to the Higgs boson,

    y
    t

    , yielding a limit of


    |


    y
    t

    /

    y

    t

    SM


    |
    <
    1.7


    at

    95
    %

    confidence level, where

    y

    t

    SM

    is the SM value of

    y
    t

    . They are also used to constrain the oblique parameter of the Higgs boson in an effective field theory framework,


    H
    ^

    <
    0.12

    . Limits are set on the production of a heavy scalar or pseudoscalar boson in Type-II two-Higgs-doublet and simplified dark matter models, with exclusion limits reaching 350-470


    GeV

    and 350-550


    GeV

    for scalar and pseudoscalar bosons, respectively. Upper bounds are also set on couplings of the top quark to new light particles.
  7. CMS Collaboration, Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, et al.
    Eur Phys J C Part Fields, 2020;80(3):189.
    PMID: 32226948 DOI: 10.1140/epjc/s10052-020-7739-7
    A search is presented for

    τ



    slepton pairs produced in proton-proton collisions at a center-of-mass energy of 13


    TeV

    . The search is carried out in events containing two

    τ



    leptons in the final state, on the assumption that each

    τ



    slepton decays primarily to a

    τ



    lepton and a neutralino. Events are considered in which each

    τ



    lepton decays to one or more hadrons and a neutrino, or in which one of the

    τ



    leptons decays instead to an electron or a muon and two neutrinos. The data, collected with the CMS detector in 2016 and 2017, correspond to an integrated luminosity of 77.2



    fb

    -
    1



    . The observed data are consistent with the standard model background expectation. The results are used to set 95% confidence level upper limits on the cross section for

    τ



    slepton pair production in various models for

    τ



    slepton masses between 90 and 200


    GeV

    and neutralino masses of 1, 10, and 20


    GeV

    . In the case of purely left-handed

    τ



    slepton production and decay to a

    τ



    lepton and a neutralino with a mass of 1


    GeV

    , the strongest limit is obtained for a

    τ



    slepton mass of 125


    GeV

    at a factor of 1.14 larger than the theoretical cross section.
  8. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, Brandstetter J, et al.
    Eur Phys J C Part Fields, 2020;80(5):370.
    PMID: 32633732 DOI: 10.1140/epjc/s10052-020-7858-1
    A measurement is presented of differential cross sections for t-channel single top quark and antiquark production in proton-proton collisions at a centre-of-mass energy of 13



    Te



    by the CMS experiment at the LHC. From a data set corresponding to an integrated luminosity of 35.9



    fb

    -
    1



    , events containing one muon or electron and two or three jets are analysed. The cross section is measured as a function of the top quark transverse momentum (

    p
    T

    ), rapidity, and polarisation angle, the charged lepton

    p
    T

    and rapidity, and the

    p
    T

    of the

    W



     boson from the top quark decay. In addition, the charge ratio is measured differentially as a function of the top quark, charged lepton, and

    W



     boson kinematic observables. The results are found to be in agreement with standard model predictions using various next-to-leading-order event generators and sets of parton distribution functions. Additionally, the spin asymmetry, sensitive to the top quark polarisation, is determined from the differential distribution of the polarisation angle at parton level to be

    0.440
    ±
    0.070

    , in agreement with the standard model prediction.
  9. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, Brandstetter J, et al.
    Phys. Rev. Lett., 2019 Dec 13;123(24):241801.
    PMID: 31922872 DOI: 10.1103/PhysRevLett.123.241801
    Results are reported from a search for new particles that decay into a photon and two gluons, in events with jets. Novel jet substructure techniques are developed that allow photons to be identified in an environment densely populated with hadrons. The analyzed proton-proton collision data were collected by the CMS experiment at the LHC, in 2016 at sqrt[s]=13  TeV, and correspond to an integrated luminosity of 35.9  fb^{-1}. The spectra of total transverse hadronic energy of candidate events are examined for deviations from the standard model predictions. No statistically significant excess is observed over the expected background. The first cross section limits on new physics processes resulting in such events are set. The results are interpreted as upper limits on the rate of gluino pair production, utilizing a simplified stealth supersymmetry model. The excluded gluino masses extend up to 1.7 TeV, for a neutralino mass of 200 GeV and exceed previous mass constraints set by analyses targeting events with isolated photons.
  10. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, Brandstetter J, et al.
    Phys. Rev. Lett., 2019 Dec 06;123(23):231803.
    PMID: 31868480 DOI: 10.1103/PhysRevLett.123.231803
    A search for narrow low-mass resonances decaying to quark-antiquark pairs is presented. The search is based on proton-proton collision events collected at 13 TeV by the CMS detector at the CERN LHC. The data sample corresponds to an integrated luminosity of 35.9  fb^{-1}, recorded in 2016. The search considers the case where the resonance has high transverse momentum due to initial-state radiation of a hard photon. To study this process, the decay products of the resonance are reconstructed as a single large-radius jet with two-pronged substructure. The signal would be identified as a localized excess in the jet invariant mass spectrum. No evidence for such a resonance is observed in the mass range 10 to 125 GeV. Upper limits at the 95% confidence level are set on the coupling strength of resonances decaying to quark pairs. The results obtained with this photon trigger strategy provide the first direct constraints on quark-antiquark resonance masses below 50 GeV obtained at a hadron collider.
  11. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, Brandstetter J, et al.
    Phys. Rev. Lett., 2019 Sep 27;123(13):131802.
    PMID: 31697516 DOI: 10.1103/PhysRevLett.123.131802
    A search for a light charged Higgs boson (H^{+}) decaying to a W boson and a CP-odd Higgs boson (A) in final states with eμμ or μμμ is performed using data from pp collisions at sqrt[s]=13  TeV, recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of 35.9  fb^{-1}. In this search, it is assumed that the H^{+} boson is produced in decays of top quarks, and the A boson decays to two oppositely charged muons. The presence of signals for H^{+} boson masses between 100 and 160 GeV and A boson masses between 15 and 75 GeV is investigated. No evidence for the production of the H^{+} boson is found. Upper limits at 95% confidence level are obtained on the combined branching fraction for the decay chain, t→bH^{+}→bW^{+}A→bW^{+}μ^{+}μ^{-}, of 1.9×10^{-6} to 8.6×10^{-6}, depending on the masses of the H^{+} and A bosons. These are the first limits for these decay modes of the H^{+} and A bosons.
  12. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Asilar E, Bergauer T, et al.
    Phys. Rev. Lett., 2019 Jul 12;123(2):022001.
    PMID: 31386524 DOI: 10.1103/PhysRevLett.123.022001
    The transverse momentum spectra of D^{0} mesons from b hadron decays are measured at midrapidity (|y|<1) in pp and Pb-Pb collisions at a nucleon-nucleon center of mass energy of 5.02 TeV with the CMS detector at the LHC. The D^{0} mesons from b hadron decays are distinguished from prompt D^{0} mesons by their decay topologies. In Pb-Pb collisions, the B→D^{0} yield is found to be suppressed in the measured p_{T} range from 2 to 100  GeV/c as compared to pp collisions. The suppression is weaker than that of prompt D^{0} mesons and charged hadrons for p_{T} around 10  GeV/c. While theoretical calculations incorporating partonic energy loss in the quark-gluon plasma can successfully describe the measured B→D^{0} suppression at higher p_{T}, the data show an indication of larger suppression than the model predictions in the range of 2
  13. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Asilar E, Bergauer T, et al.
    Phys. Rev. Lett., 2019 Apr 19;122(15):152001.
    PMID: 31050516 DOI: 10.1103/PhysRevLett.122.152001
    The modification of jet shapes in Pb-Pb collisions, relative to those in pp collisions, is studied for jets associated with an isolated photon. The data were collected with the CMS detector at the LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Jet shapes are constructed from charged particles with track transverse momenta (p_{T}) above 1  GeV/c in annuli around the axes of jets with p_{T}^{jet}>30  GeV/c associated with an isolated photon with p_{T}^{γ}>60  GeV/c. The jet shape distributions are consistent between peripheral Pb-Pb and pp collisions, but are modified for more central Pb-Pb collisions. In these central Pb-Pb events, a larger fraction of the jet momentum is observed at larger distances from the jet axis compared to pp, reflecting the interaction between the partonic medium created in heavy ion collisions and the traversing partons.
  14. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Asilar E, Bergauer T, et al.
    Phys. Rev. Lett., 2019 Apr 19;122(15):151802.
    PMID: 31050519 DOI: 10.1103/PhysRevLett.122.151802
    For the first time, a search for the rare decay of the W boson to three charged pions has been performed. Proton-proton collision data recorded by the CMS experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 77.3  fb^{-1}, have been analyzed. No significant excess is observed above the background expectation. An upper limit of 1.01×10^{-6} is set at 95% confidence level on the branching fraction of the W boson to three charged pions. This provides a strong motivation for theoretical calculations of this branching fraction.
  15. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Bergauer T, Brandstetter J, et al.
    Phys. Rev. Lett., 2019 Apr 05;122(13):132001.
    PMID: 31012626 DOI: 10.1103/PhysRevLett.122.132001
    Signals consistent with the B_{c}^{+}(2S) and B_{c}^{*+}(2S) states are observed in proton-proton collisions at sqrt[s]=13  TeV, in an event sample corresponding to an integrated luminosity of 143  fb^{-1}, collected by the CMS experiment during the 2015-2018 LHC running periods. These excited b[over ¯]c states are observed in the B_{c}^{+}π^{+}π^{-} invariant mass spectrum, with the ground state B_{c}^{+} reconstructed through its decay to J/ψπ^{+}. The two states are reconstructed as two well-resolved peaks, separated in mass by 29.1±1.5(stat)±0.7(syst)  MeV. The observation of two peaks, rather than one, is established with a significance exceeding five standard deviations. The mass of the B_{c}^{+}(2S) meson is measured to be 6871.0±1.2(stat)±0.8(syst)±0.8(B_{c}^{+})  MeV, where the last term corresponds to the uncertainty in the world-average B_{c}^{+} mass.
  16. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Asilar E, Bergauer T, et al.
    Phys. Rev. Lett., 2019 Apr 05;122(13):132003.
    PMID: 31012605 DOI: 10.1103/PhysRevLett.122.132003
    The observation of single top quark production in association with a Z boson and a quark (tZq) is reported. Events from proton-proton collisions at a center-of-mass energy of 13 TeV containing three charged leptons (either electrons or muons) and at least two jets are analyzed. The data were collected with the CMS detector in 2016 and 2017 and correspond to an integrated luminosity of 77.4fb^{-1}. The increased integrated luminosity, a multivariate lepton identification, and a redesigned analysis strategy improve significantly the sensitivity of the analysis compared to previous searches for tZq production. The tZq signal is observed with a significance well over 5 standard deviations. The measured tZq production cross section is σ(pp→tZq→tℓ^{+}ℓ^{-}q)=111±13(stat)_{-9}^{+11}(syst)  fb, for dilepton invariant masses above 30 GeV, in agreement with the standard model expectation.
  17. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Asilar E, Bergauer T, et al.
    Phys. Rev. Lett., 2019 Mar 29;122(12):121803.
    PMID: 30978057 DOI: 10.1103/PhysRevLett.122.121803
    This Letter describes a search for Higgs boson pair production using the combined results from four final states: bbγγ, bbττ, bbbb, and bbVV, where V represents a W or Z boson. The search is performed using data collected in 2016 by the CMS experiment from LHC proton-proton collisions at sqrt[s]=13  TeV, corresponding to an integrated luminosity of 35.9  fb^{-1}. Limits are set on the Higgs boson pair production cross section. A 95% confidence level observed (expected) upper limit on the nonresonant production cross section is set at 22.2 (12.8) times the standard model value. A search for narrow resonances decaying to Higgs boson pairs is also performed in the mass range 250-3000 GeV. No evidence for a signal is observed, and upper limits are set on the resonance production cross section.
  18. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Asilar E, Bergauer T, et al.
    Phys. Rev. Lett., 2019 Mar 01;122(8):081804.
    PMID: 30932612 DOI: 10.1103/PhysRevLett.122.081804
    A search for heavy, narrow resonances decaying to a Higgs boson and a photon (Hγ) has been performed in proton-proton collision data at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9  fb^{-1} collected with the CMS detector at the LHC in 2016. Events containing a photon and a Lorentz-boosted hadronically decaying Higgs boson reconstructed as a single, large-radius jet are considered, and the γ+jet invariant mass spectrum is analyzed for the presence of narrow resonances. To increase the sensitivity of the search, events are categorized depending on whether or not the large-radius jet can be identified as a result of the merging of two jets originating from b quarks. Results in both categories are found to agree with the predictions of the standard model. Upper limits on the production rate of Hγ resonances are set as a function of their mass in the range of 720-3250 GeV, representing the most stringent constraints to date.
  19. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Asilar E, Bergauer T, et al.
    Phys. Rev. Lett., 2019 Jan 18;122(2):021801.
    PMID: 30720313 DOI: 10.1103/PhysRevLett.122.021801
    A search for the Higgs boson decaying to two oppositely charged muons is presented using data recorded by the CMS experiment at the CERN LHC in 2016 at a center-of-mass energy sqrt[s]=13  TeV, corresponding to an integrated luminosity of 35.9  fb^{-1}. Data are found to be compatible with the predicted background. For a Higgs boson with a mass of 125.09 GeV, the 95% confidence level observed (background-only expected) upper limit on the production cross section times the branching fraction to a pair of muons is found to be 3.0 (2.5) times the standard model expectation. In combination with data recorded at center-of-mass energies sqrt[s]=7 and 8 TeV, the background-only expected upper limit improves to 2.2 times the standard model value with a standard model expected significance of 1.0 standard deviation. The corresponding observed upper limit is 2.9 with an observed significance of 0.9 standard deviation. This corresponds to an observed upper limit on the standard model Higgs boson branching fraction to muons of 6.4×10^{-4} and to an observed signal strength of 1.0±1.0(stat)±0.1(syst).
  20. Sirunyan AM, Tumasyan A, Adam W, Ambrogi F, Asilar E, Bergauer T, et al.
    Phys. Rev. Lett., 2019 Jan 11;122(1):011803.
    PMID: 31012697 DOI: 10.1103/PhysRevLett.122.011803
    A search is performed for dark matter particles produced in association with a top quark pair in proton-proton collisions at sqrt[s]=13  TeV. The data correspond to an integrated luminosity of 35.9  fb^{-1} recorded by the CMS detector at the LHC. No significant excess over the standard model expectation is observed. The results are interpreted using simplified models of dark matter production via spin-0 mediators that couple to dark matter particles and to standard model quarks, providing constraints on the coupling strength between the mediator and the quarks. These are the most stringent collider limits to date for scalar mediators, and the most stringent for pseudoscalar mediators at low masses.
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