[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
[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.
[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
[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
[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
Full-text Article · Apr 2014 · European Physical Journal C
[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
[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
[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
[Show abstract][Hide abstract]ABSTRACT: A search for the standard model Higgs boson decaying to a W-boson pair at the LHC is reported. The event sample corresponds to an integrated luminosity of 4.9 fb(-1) and 19.4 fb(-1) collected with the CMS detector in pp collisions at = 7 and 8 TeV, respectively. The Higgs boson candidates are selected in events with two or three charged leptons. An excess of events above background is observed, consistent with the expectation from the standard model Higgs boson with a mass of around 125 GeV. The probability to observe an excess equal or larger than the one seen, under the background-only hypothesis, corresponds to a significance of 4.3 standard deviations for m (H) = 125.6 GeV. The observed signal cross section times the branching fraction to WW for m (H) = 125.6 GeV is times the standard model expectation. The spin-parity J (P) = 0(+) hypothesis is favored against a narrow resonance with J (P) = 2(+) or J (P) = 0(-) that decays to a W-boson pair. This result provides strong evidence for a Higgs-like boson decaying to a W-boson pair.
Full-text Article · Jan 2014 · Journal of High Energy Physics
[Show abstract][Hide abstract]ABSTRACT: The first observation of the associated production of a single top quark and
a W boson is presented. The analysis is based on a data set corresponding to an
integrated luminosity of 12.2 inverse femtobarns of proton-proton collisions at
sqrt(s) = 8 TeV recorded by the CMS experiment at the LHC. Events with two
leptons and a jet originating from a b quark are selected. A multivariate
analysis based on kinematic and topological properties is used to separate the
signal from the dominant t t-bar background. An excess consistent with the
signal hypothesis is observed, with a significance which corresponds to 6.1
standard deviations above a background-only hypothesis. The measured production
cross section is 23.4 +/- 5.4 pb, in agreement with the standard model
Full-text Article · Jan 2014 · Physical Review Letters
[Show abstract][Hide abstract]ABSTRACT: The normalised differential top quark-antiquark production cross section is measured as a function of the jet multiplicity in proton-proton collisions at a centre-of-mass energy of 7 TeV at the LHC with the CMS detector. The measurement is performed in both the dilepton and lepton+jets decay channels using data corresponding to an integrated luminosity of 5.0 fb −1. Using a procedure to associate jets to decay products of the top quarks, the differential cross section of the tt production is determined as a function of the additional jet multiplicity in the lepton+jets channel. Furthermore , the fraction of events with no additional jets is measured in the dilepton channel, as a function of the threshold on the jet transverse momentum. The measurements are compared with predictions from perturbative quantum chro-modynamics and no significant deviations are observed.
Full-text Article · Jan 2014 · European Physical Journal C
[Show abstract][Hide abstract]ABSTRACT: Rapidity distributions are presented for events containing either a Z boson
or a photon with a single jet in proton-proton collisions produced at the CERN
LHC. The data, collected with the CMS detector at sqrt(s) = 7 TeV, correspond
to an integrated luminosity of 5.0 inverse femtobarns. The individual rapidity
distributions of the boson and the jet are consistent within 5% with
expectations from perturbative QCD. However, QCD predictions for the sum and
the difference in rapidities of the two final-state objects show discrepancies
with CMS data. In particular, next-to-leading-order QCD calculations, and two
common Monte Carlo event generators using different methods to match
matrix-element partons with parton showers, appear inconsistent with the data
as well as with each other.
[Show abstract][Hide abstract]ABSTRACT: A search for flavor-changing neutral currents in top-quark decays t to Zq is
performed in events produced from the decay chain t t-bar to Zq+Wb, where both
vector bosons decay leptonically, producing a final state with three leptons
(electrons or muons). A dataset collected with the CMS detector at the LHC is
used, corresponding to an integrated luminosity of 19.7 inverse femtobarns of
proton-proton collisions at a center-of-mass energy of 8 TeV. No excess is seen
in the observed number of events relative to the standard model prediction;
thus no evidence for flavor-changing neutral currents in top-quark decays is
found. A combination with a previous search at 7 TeV excludes a t to Zq
branching fraction greater than 0.05% at the 95% confidence level.
Full-text Article · Dec 2013 · Physical Review Letters