A search for resonances and quantum black holes is performed using the dijet mass spectra measured in proton-proton collisions at sqrt(s) = 8 TeV with the CMS detector at the LHC. The data set corresponds to an integrated luminosity of 19.7 inverse femtobarns. In a search for narrow resonances that couple to quark-quark, quark-gluon, or gluon-gluon pairs, model-independent upper limits, at 95% confidence level, are obtained on the production cross section of resonances, with masses above 1.2 TeV. When interpreted in the context of specific models the limits exclude: string resonances with masses below 5.0 TeV; excited quarks below 3.5 TeV; scalar diquarks below 4.7 TeV; W' bosons below 1.9 TeV or between 2.0 and 2.2 TeV; Z' bosons below 1.7 TeV; and Randall-Sundrum gravitons below 1.6 TeV. A separate search is conducted for narrow resonances that decay to final states including b quarks. The first exclusion limit is set for excited b quarks, with a lower mass limit between 1.2 and 1.6 TeV depending on their decay properties. Searches are also carried out for wide resonances, assuming for the first time width-to-mass ratios up to 30%, and for quantum black holes with a range of model parameters. The wide resonance search excludes axigluons and colorons with mass below 3.6 TeV, and color-octet scalars with mass below 2.5 TeV. Lower bounds between 5.0 and 6.3 TeV are set on the masses of quantum black holes.
The differential cross-section as a function of rapidity has been measured for the exclusive production of $J/\psi$ and $\psi(2S)$ mesons in proton-proton collisions at $\sqrt{s}=7$ TeV, using data collected by the LHCb experiment, corresponding to an integrated luminosity of 930 pb$^{-1}$. The cross-sections times branching fractions to two muons having pseudorapidities between 2.0 and 4.5 are measured to be $$\begin{array}{rl} \sigma_{pp\rightarrow J/\psi\rightarrow{\mu^+}{\mu^-}}(2.0<\eta_{\mu^\pm }<4.5)=&291\pm 7\pm19 {\rm \ pb},\\ \sigma_{pp\rightarrow\psi(2S)\rightarrow{\mu^+}{\mu^-}}(2.0<\eta_{\mu^\pm}<4.5)=&6.5\pm 0.9\pm 0.4 {\rm \ pb},\end{array}$$ where the first uncertainty is statistical and the second is systematic. The measurements agree with next-to-leading order QCD predictions as well as with models that include saturation effects.
This article reports on a search for dark matter pair production in association with bottom or top quarks in 20.3 fb$^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 8$ TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing $b$-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter--nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter.
We present the first measurement at the LHC of exclusive J/$\psi$ photoproduction off protons, in ultra-peripheral proton-lead collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. Events are selected with a dimuon pair produced either in the rapidity interval, in the laboratory frame, $2.5<y<4$ (p-Pb) or $-3.6<y<-2.6$ (Pb--p), and no other particles observed in the ALICE acceptance. The measured cross sections $\sigma (\gamma + {\rm p} \rightarrow J/\psi + {\rm p})$ are 33.2 $\pm$ 2.2 (stat) $\pm$ 3.1 (syst) $\pm$ 0.7 (theo) nb in p-Pb and 284 $\pm$ 36 (stat) $^{+27}_{-32}$ (syst) $\pm$ 26 (theo) nb in Pb-p collisions. We measure this process up to about 700 GeV in the $\gamma {\rm p}$ centre-of-mass, which is a factor of two larger than the highest energy studied at HERA. The data are consistent with a power law dependence of the $J/\psi$ photoproduction cross section in $\gamma {\rm p}$ energies from about 20 to 700 GeV, or equivalently, from Bjorken-$x$ between $\sim 2\times 10^{-2}$ to $\sim 2\times 10^{-5}$, thus indicating no significant change in the gluon density behaviour of the proton between HERA and LHC energies.
A search for neutral Higgs bosons predicted in the minimal supersymmetric standard model (MSSM) for mu+ mu- decay channels is presented. The analysis uses data collected by the CMS experiment at the LHC in proton-proton collisions at centre-of-mass energies of 7 and 8 TeV, corresponding to integrated luminosities of 5.1 and 19.3 inverse femtobarns, respectively. The search is sensitive to Higgs bosons produced through the gluon fusion process or in association with a bb quark pair. No statistically significant excess is observed in the mu+ mu- mass spectrum. Results are interpreted in the framework of several benchmark scenarios, and the data are used to set an upper limit on the MSSM parameter tan(beta) as a function of the mass of the pseudoscalar A boson in the range from 115 to 300 GeV. Model independent upper limits are given for the product of the cross section and branching fraction for gluon fusion and b quark associated production. They are the most stringent limits obtained to date in this channel.
Measurements of the cross sections for top quark pairs produced in association with a W or Z boson are presented, using 8 TeV pp collision data corresponding to an integrated luminosity of 19.5 inverse femtobarns, collected by the CMS experiment at the LHC. Final states are selected in which the associated W boson decays to a charged lepton and a neutrino or the Z boson decays to two charged leptons. Signal events are identified by matching reconstructed objects in the detector to specific final state particles from ttW or ttZ decays. The ttW cross section is measured to be 382 +117 -102 fb with a significance of 4.8 standard deviations from the background-only hypothesis. The ttZ cross section is measured to be 242 +65 -55 fb with a significance of 6.4 standard deviations from the background-only hypothesis. These measurements are used to set bounds on five anomalous dimension-six operators that would affect the ttW and ttZ cross sections.
As part of a study of large p T phenomena in photon-proton collisions at the CERN ISR, a search for direct single photon production has been performed. A statistical division of the data sample into the fraction consistent with single photon and the fraction due to multiphoton decays of neutral hadrons is accomplished by measuring the average conversion probability for the sample in a one radiation length thick converter. The fraction of the sample attributable to direct single photon production is 〈 γ /all〉 = 0.074 ± 0.012 for 6 GeV/ c < p T 10 GeV/ c , and 〈 γ /all〉 = 0.26 ± 0.04 for p T > 10 GeV/ c , with an additional systematic uncertainty of ±0.05 for both values.
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We measure the ttbar production cross section in ppbar collisions at sqrt{s}=1.96 TeV in the lepton+jets channel. Two complementary methods discriminate between signal and background, b-tagging and a kinematic likelihood discriminant. Based on 0.9 fb-1 of data collected by the D0 detector at the Fermilab Tevatron Collider, we measure sigma_ttbar=7.62+/-0.85 pb, assuming the current world average m_t=172.6 GeV. We compare our cross section measurement with theory predictions to determine a value for the top quark mass of 170+/-7 GeV.
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