[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: The standard model of particle physics describes the fundamental particles and their interactions via the strong, electromagnetic and weak forces. It provides precise predictions for measurable quantities that can be tested experimentally. The probabilities, or branching fractions, of the strange B meson (B-s(0)) and the B-0 meson decaying into two oppositely charged muons (mu(+) and mu(-)) are especially interesting because of their sensitivity to theories that extend the standard model. The standard model predicts that the B-s(0)->mu(+)mu(-) and B-0 ->mu(+)mu(-) decays are very rare, with about four of the former occurring for every billion B-s(0) mesons produced, and one of the latter occurring for every ten billion B-0 mesons(1). A difference in the observed branching fractions with respect to the predictions of the standard model would provide a direction in which the standard model should be extended. Before the Large Hadron Collider (LHC) at CERN2 started operating, no evidence for either decay mode had been found. Upper limits on the branching fractions were an order of magnitude above the standard model predictions. The CMS (Compact Muon Solenoid) and LHCb(Large Hadron Collider beauty) collaborations have performed a joint analysis of the data from proton-proton collisions that they collected in 2011 at a centre-of-mass energy of seven teraelectronvolts and in 2012 at eight teraelectronvolts. Here we report the first observation of the B-s(0)->mu(+)mu(-) decay, with a statistical significance exceeding six standard deviations, and the best measurement so far of its branching fraction. Furthermore, we obtained evidence for the B-0 ->mu(+)mu(-) decay with a statistical significance of three standard deviations. Both measurements are statistically compatible with standard model predictions and allow stringent constraints to be placed on theories beyond the standard model. The LHC experiments will resume taking data in 2015, recording proton-proton collisions at a centre-of-mass energy of 13 teraelectronvolts, which will approximately double the production rates of B-s(0) and B-0 mesons and lead to further improvements in the precision of these crucial tests of the standard model.
[Show abstract][Hide abstract] ABSTRACT: Epitaxial GaAs material shows a great potential in X-ray spectroscopy and radiography applications due to its high absorption efficiency and low defect density. Fabrication of pixel radiation detectors from high-purtity epitaxial GaAs has been developed further. The process is based on mesa etching for pixellisation and sputtering for metallization. The leakage currents of processed pad detectors are below 10 nA/cm(2) at a reverse bias of 100V and decrease exponentially with the temperature. Measurement with transient current technique (TCT) shows that electrons have a trapping time of 8 ns. Good spectroscopic result were obtained from both a pad detector and a hybridized Medipix GaAs detector.
No preview · Article · Dec 2014 · Journal of Instrumentation
[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.
Full-text · Article · Aug 2014 · Physical Review D
[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: A search is reported for massive resonances decaying into a quark and a
vector boson (W or Z), or two vector bosons (WW, WZ, or ZZ). The analysis is
performed on an inclusive sample of multijet events corresponding to an
integrated luminosity of 19.7 inverse femtobarns, collected in proton-proton
collisions at a centre-of-mass energy of 8 TeV with the CMS detector at the
LHC. The search uses novel jet-substructure identification techniques that
provide sensitivity to the presence of highly boosted vector bosons decaying
into a pair of quarks. Exclusion limits are set at a confidence level of 95% on
the production of: (i) excited quark resonances q* decaying to qW and qZ for
masses less than 3.2 TeV and 2.9 TeV, respectively, (ii) a Randall-Sundrum
graviton G[RS] decaying into WW for masses below 1.2 TeV, and (iii) a heavy
partner of the W boson W' decaying into WZ for masses less than 1.7 TeV. For
the first time mass limits are set on W' to WZ and G[RS] to WW in the all-jets
final state. The mass limits on q* to qW, q* to qZ, W' to WZ, G[RS] to WW are
the most stringent to date. A model with a "bulk" graviton G[Bulk] that decays
into WW or ZZ bosons is also studied.
Full-text · Article · May 2014 · Journal of High Energy Physics
[Show abstract][Hide abstract] ABSTRACT: A search for WV gamma triple vector boson production is presented based on
events containing a W boson decaying to a muon or an electron and a neutrino, a
second V (W or Z) boson, and a photon. The data correspond to an integrated
luminosity of 19.3 inverse femtobarns collected in 2012 with the CMS detector
at the LHC in pp collisions at sqrt(s) = 8 TeV. An upper limit of 311 fb on the
fiducial cross section for the WV gamma production process is obtained at 95%
confidence level for photons with a transverse energy above 30 GeV and with an
absolute value of pseudorapidity of less than 1.44. This limit is approximately
a factor of 3.4 larger than the standard model predictions that are based on
next-to-leading order QCD calculations. Since no evidence of anomalous WW gamma
gamma or WWZ gamma quartic gauge boson couplings is found, this paper presents
the first experimental limits on the dimension-8 parameter f[T,0] and the
CP-conserving WWZ gamma parameters kappa[0,W] and kappa[C,W]. Limits are also
obtained for the WW gamma gamma parameters a[0,W] and a[C,W].
Full-text · Article · Apr 2014 · Physical Review D
[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: 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: 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.
Full-text · Article · Dec 2013 · Physical Review D
[Show abstract][Hide abstract] ABSTRACT: A study of proton-proton collisions in which two b hadrons are produced in association with a Z boson is reported. The collisions were recorded at a centre-of-mass energy of 7 TeVwith the CMS detector at the LHC, for an integrated luminosity of 5.2 fb(-1). The b hadrons are identified by means of displaced secondary vertices, without the use of reconstructed jets, permitting the study of b-hadron pair production at small angular separation. Differential cross sections are presented as a function of the angular separation of the b hadrons and the Z boson. In addition, inclusive measurements are presented. For both the inclusive and differential studies, different ranges of Z boson momentum are considered, and each measurement is compared to the predictions from different event generators at leading-order and next-to-leading-order accuracy.
Full-text · Article · Dec 2013 · Journal of High Energy Physics
[Show abstract][Hide abstract] ABSTRACT: Measurements of the differential and double-differential Drell-Yan cross
sections are presented using an integrated luminosity of 4.5(4.8) inverse
femtobarns in the dimuon (dielectron) channel of proton-proton collision data
recorded with the CMS detector at the LHC at sqrt(s) = 7 TeV. The measured
inclusive cross section in the Z-peak region (60-120 GeV) is sigma(ell ell) =
986.4 +/- 0.6 (stat.) +/- 5.9 (exp. syst.) +/- 21.7 (th. syst.) +/- 21.7 (lum.)
pb for the combination of the dimuon and dielectron channels. Differential
cross sections d[sigma]/d[m] for the dimuon, dielectron, and combined channels
are measured in the mass range 15 to 1500 GeV and corrected to the full phase
space. Results are also presented for the measurement of the
double-differential cross section d2[sigma]/d[m]d[abs(y)] in the dimuon channel
over the mass range 20 to 1500 GeV and absolute dimuon rapidity from 0 to 2.4.
These measurements are compared to the predictions of perturbative QCD
calculations at next-to-leading and next-to-next-to-leading orders using
various sets of parton distribution functions.
Full-text · Article · Oct 2013 · Journal of High Energy Physics
[Show abstract][Hide abstract] ABSTRACT: A measurement of W+W- production in pp collisions at is presented. The data were collected with the CMS detector at the LHC, and correspond to an integrated luminosity of 4.92 +/- 0.11 fb(-1). The W+W- candidates consist of two oppositely charged leptons, electrons or muons, accompanied by large missing transverse energy. The W+W- production cross section is measured to be 52.4 +/- 2.0 (stat.)+/- 4.5 (syst.)+/- 1.2 (lum.) pb. This measurement is consistent with the standard model prediction of 47.0 +/- 2.0 pb at next-to-leading order. Stringent limits on the WW gamma and WWZ anomalous triple gauge-boson couplings are set.
Full-text · Article · Oct 2013 · European Physical Journal C
[Show abstract][Hide abstract] ABSTRACT: An inclusive search for supersymmetric processes that produce final states with jets and missing transverse energy is performed in pp collisions at a centre-of-mass energy of 8 TeV. The data sample corresponds to an integrated luminosity of 11.7 fb−1 collected by the CMS experiment at the LHC. In this search, a dimensionless kinematic variable, α
T, is used to discriminate between events with genuine and misreconstructed missing transverse energy. The search is based on an examination of the number of reconstructed jets per event, the scalar sum of transverse energies of these jets, and the number of these jets identified as originating from bottom quarks. No significant excess of events over the standard model expectation is found. Exclusion limits are set in the parameter space of simplified models, with a special emphasis on both compressed-spectrum scenarios and direct or gluino-induced production of third-generation squarks. For the case of gluino-mediated squark production, gluino masses up to 950–1125 GeV are excluded depending on the assumed model. For the direct pair-production of squarks, masses up to 450 GeV are excluded for a single light first- or second-generation squark, increasing to 600 GeV for bottom squarks.
Full-text · Article · Sep 2013 · European Physical Journal C