A combination of measurements of the inclusive top-quark pair production cross-section performed by ATLAS and CMS in proton-proton collisions at centre-of-mass energies of 7 and 8 TeV at the LHC is presented. The cross-sections are obtained using top-quark pair decays with an opposite-charge electron-muon pair in the final state and with data corresponding to an integrated luminosity of about 5 fb$^{-1}$ at $\sqrt{s}=7$ TeV and about 20 fb$^{-1}$ at $\sqrt{s}=8$ TeV for each experiment. The combined cross-sections are determined to be $178.5 \pm 4.7$ pb at $\sqrt{s}=7$ TeV and $243.3^{+6.0}_{-5.9}$ pb at $\sqrt{s}=8$ TeV with a correlation of 0.41, using a reference top-quark mass value of 172.5 GeV. The ratio of the combined cross-sections is determined to be $R_{8/7}= 1.363\pm 0.032$. The combined measured cross-sections and their ratio agree well with theory calculations using several parton distribution function (PDF) sets. The values of the top-quark pole mass (with the strong coupling fixed at 0.118) and the strong coupling (with the top-quark pole mass fixed at 172.5 GeV) are extracted from the combined results by fitting a next-to-next-to-leading-order plus next-to-next-to-leading-log QCD prediction to the measurements. Using a version of the NNPDF3.1 PDF set containing no top-quark measurements, the results obtained are $m_t^\text{pole} = 173.4^{+1.8}_{-2.0}$ GeV and $\alpha_\text{s}(m_Z)= 0.1170^{+ 0.0021}_{-0.0018}$.
Full covariance matrix including all systematic uncertainties expressed as nuisance parameters. With the exception of the cross section parameters, all parameters were normalised to 1 before the fit. Therefore, the diagonal elements represent the constraint in quadrature.
Full covariance matrix including all systematic uncertainties expressed as nuisance parameters. With the exception of the cross section parameters, all parameters were normalised to 1 before the fit. Therefore, the diagonal elements represent the constraint in quadrature.
A search for physics beyond the standard model in events with at least three leptons is presented. The data sample, corresponding to an integrated luminosity of 19.5 inverse femtobarns of proton-proton collisions with center-of-mass energy sqrt(s) = 8 TeV, was collected by the CMS experiment at the LHC during 2012. The data are divided into exclusive categories based on the number of leptons and their flavor, the presence or absence of an opposite-sign, same-flavor lepton pair (OSSF), the invariant mass of the OSSF pair, the presence or absence of a tagged bottom-quark jet, the number of identified hadronically decaying tau leptons, and the magnitude of the missing transverse energy and of the scalar sum of jet transverse momenta. The numbers of observed events are found to be consistent with the expected numbers from standard model processes, and limits are placed on new-physics scenarios that yield multilepton final states. In particular, scenarios that predict Higgs boson production in the context of supersymmetric decay chains are examined. We also place a 95% confidence level upper limit of 1.3% on the branching fraction for the decay of a top quark to a charm quark and a Higgs boson (t to c H), which translates to a bound on the left- and right-handed top-charm flavor-violating Higgs Yukawa couplings, lambda[H, tc] and lambda[H, ct], respectively, of sqrt(abs(lambda[H, tc])^2 + abs(lambda[H, ct])^2) < 0.21.
Observed and expected numbers of events with four or more leptons with the scalar sum of jet transverse momentum values HT > 200 GeV. "On-Z" refers to events with at least one E+ E- or MU+ MU- (OSSF) pair with dilepton mass between 75 and 105 GeV, while "Off-Z" refers to events with one or two OSSF pairs, none of which fall in this mass range. The OSSFN designation refers to the number of E+ E- and MU+ MU- pairs in the event. Search channels binned in ET have been combined into coarse ET bins for the purposes of presentation.
Observed and expected numbers of events with four or more leptons with the scalar sum of jet transverse momentum values HT < 200 GeV. "On-Z" refers to events with at least one E+ E- or MU+ MU- (OSSF) pair with dilepton mass between 75 and 105 GeV, while "Off-Z" refers to events with one or two OSSF pairs, none of which fall in this mass range. The OSSFN designation refers to the number of E+ E- and MU+ MU- pairs in the event. Search channels binned in ET have been combined into coarse ET bins for the purposes of presentation.
Observed and expected numbers of events with exactly three leptons with the scalar sum of jet transverse momentum values HT > 200 GeV. "On-Z" refers to events with at least one E+ E- or MU+ MU- (OSSF) pair with dilepton mass between 75 and 105 GeV, while "Off-Z" refers to events with one or two OSSF pairs, none of which fall in this mass range. The OSSFN designation refers to the number of E+ E- and MU+ MU- pairs in the event. Search channels binned in ET have been combined into coarse ET bins for the purposes of presentation.
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 $b\bar{b}$ 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 interactions.
Summary of 95% CL upper limits on SIG*BR(HIGGS --> INVISIBLE)/SIG(SM) obtained from the VBF search, the combined ZH searches, and the combination of all three searches.
Measurements are reported of the WZ and ZZ production cross sections in proton-proton collisions at $\sqrt{s}$ = 8 TeV in final states where one Z boson decays to b-tagged jets. The other gauge boson, either W or Z, is detected through its leptonic decay (either $W \to e\nu, \mu\nu$ or $Z \to e^+ e^-, \mu^+ \mu^-$, or $\nu\bar{\nu})$. The results are based on data corresponding to an integrated luminosity of 18.9 inverse-femtobarns collected with the CMS detector at the Large Hadron Collider. The measured cross sections, $\sigma(pp \to WZ)$ = 30.7 $\pm$ 9.3 (stat.) $\pm$ 7.1 (syst.) $\pm$ 4.1 (th.) $\pm$ 1.0 (lum.) pb and $\sigma(pp \to ZZ)$ = 6.5 $\pm$ 1.7 (stat.) $\pm$ 1.0 (syst.) $\pm$ 0.9 (th.) $\pm$ 0.2 (lum.) pb, are consistent with next-to-leading order quantum chromodynamics calculations.
The cross section for inclusive WZ production for the mass range 60 < M(Z) < 120 GeV.
The cross section for inclusive ZZ production for the mass range 60 < M(Z) < 120 GeV.
The cross section for inclusive WZ production in the region defined by 60 < M(Z) < 120 GeV and PT(W) > 100 GeV.
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 $\sigma(pp \to WX) \times B(W \to l\nu)$ = 12.21 +/- 0.03 (stat) +/- 0.24 (syst) +/- 0.32 (lum) nb, and $\sigma(pp \to ZX) \times B(Z \to l^{+}l^{-})$ = 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.
W+ total and fiducial production cross sections times branching fractions.
W- total and fiducial production cross sections times branching fractions.
(W+ + W-) total and fiducial production cross sections times branching fractions.
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 prediction.
Measured cross section for associated production of a single top quark and a W boson, where the uncertainty is mainly systematic.
The top-antitop quark (t t-bar) production cross section is measured in proton-proton collisions at sqrt(s) = 8 TeV with the CMS experiment at the LHC, using a data sample corresponding to an integrated luminosity of 5.3 inverse femtobarns. The measurement is performed by analysing events with a pair of electrons or muons, or one electron and one muon, and at least two jets, one of which is identified as originating from hadronisation of a bottom quark. The measured cross section is 239 +/- 2 (stat.) +/- 11 (syst.) +/- 6 (lum.) pb, for an assumed top-quark mass of 172.5 GeV, in agreement with the prediction of the standard model.
The total efficiencies etotal, i.e. the products of event acceptance, selection efficiency and branching fraction for the respective TOP TOPBAR final states, as estimated from simulation for a top-quark mass of 172.5 GeV, and the measured TOP TOPBAR production cross sections, where the uncertainties are from statistical, systematic and integrated luminosity components, respectively.
The TOP TOPBAR cross section obtained by combining all final states.
A search for the production of heavy partners of the top quark with charge 5/3 is performed in events with a pair of same-sign leptons. The data sample corresponds to an integrated luminosity of 19.5 inverse femtobarns and was collected at sqrt(s) = 8 TeV by the CMS experiment. No significant excess is observed in the data above the expected background and the existence of top-quark partners with masses below 800 GeV is excluded at a 95% confidence level, assuming they decay exclusively to tW. This is the first limit on these particles from the LHC, and it is significantly more restrictive than previous limits.
The distribution of HT for all channels combined after the full selection except for the HT requirement itself.
Expected and observed 95% CL limits on the $\mathrm{T}_{5/3}$ production cross section times the branching fraction for decay to same-sign dileptons.
The distribution of HT for all channels combined, after the requirement of same- sign dileptons, the Z-boson veto, and a requirement of at least two jets.
A search is performed for a massive new vector-like quark T, with charge 2/3, that is pair produced together with its antiparticle in proton-proton collisions. The data were collected by the CMS experiment at the Large Hadron Collider in 2012 at sqrt(s) = 8 TeV and correspond to an integrated luminosity of 19.5 inverse femtobarns. The T quark is assumed to decay into three different final states, bW, tZ, and tH. The search is carried out using events with at least one isolated lepton. No deviations from standard model expectations are observed, and lower limits are set on the T quark mass at 95% confidence level. The lower limit lies between 687 and 782 GeV for all possible values of the branching fractions into the three different final states assuming strong production. These limits are the most stringent constraints to date on the existence of such a quark.
Number of events predicted for background processes and observed in the single-lepton sample. The uncertainty in the total background expectation is computed including the correlations between the systematic uncertainties of the individual contributions.
Number of events predicted for background processes and observed in the opposite-sign dilepton samples with two or three jets (OS1) and with at least 5 jets (OS2), the same-sign dilepton sample (SS), and the trilepton sample. An entry "-" means that the background source is not applicable to the channel.
Lower limits for the T quark mass, at 95% CL, for different combinations of T quark branching fractions.
A search is performed for pair-produced spin-3/2 excited top quarks ($t^*\bar{t}^*$), each decaying to a top quark and a gluon. The search uses data collected with the CMS detector from pp collisions at a center-of-mass energy of $\sqrt{s}$=8 TeV, selecting events that have a single isolated muon or electron, an imbalance in transverse momentum, and at least six jets, of which one must be compatible with originating from the fragmentation of a b quark. The data, corresponding to an integrated luminosity of 19.5 inverse femtobarns, show no significant excess over standard model predictions, and provide a lower limit of 803 GeV at 95% confidence on the mass of the spin-3/2 t* quark in an extension of the Randall-Sundrum model, assuming a 100% branching fraction of its decay into a top quark and a gluon. This is the first search for a spin-3/2 excited top quark performed at the LHC.
Kinematic distributions of single l + > 5-jet events in data (points), compared to MC simulation normalized to the number of events observed in data. Shown are pT spectra for muons (a) and electrons (b), and jet spectra for the channels $\mu$+jets (c) and e+jets (d). The reconstructed mtg distribution is shown for the $\mu$+jets channel in (e) and for e+jets in (f).
Kinematic distributions of single l + > 5-jet events in data (points), compared to MC simulation normalized to the number of events observed in data. Shown are pT spectra for muons (a) and electrons (b), and jet spectra for the channels $\mu$+jets (c) and e+jets (d). The reconstructed mtg distribution is shown for the $\mu$+jets channel in (e) and for e+jets in (f).
Kinematic distributions of single l + > 5-jet events in data (points), compared to MC simulation normalized to the number of events observed in data. Shown are pT spectra for muons (a) and electrons (b), and jet spectra for the channels $\mu$+jets (c) and e+jets (d). The reconstructed mtg distribution is shown for the $\mu$+jets channel in (e) and for e+jets in (f).
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