Search

A search for quark compositeness in the form of quark contact interactions, based on hadronic jet pairs (dijets) produced in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$, is described. The data sample of the study corresponds to an integrated luminosity of $2.9{\mathrm{pb}}^{-1}$ collected with the CMS detector at the LHC. The dijet centrality ratio, which quantifies the angular distribution of the dijets, is measured as a function of the invariant mass of the dijet system and is found to agree with the predictions of the standard model. A statistical analysis of the data provides a lower limit on the energy scale of quark contact interactions. The sensitivity of the analysis is such that the expected limit is 2.9 TeV; because the observed value of the centrality ratio at high invariant mass is below the expectation, the observed limit is 4.0 TeV at the 95% confidence level.

Charged-hadron transverse-momentum and pseudorapidity distributions in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$ are measured with the inner tracking system of the CMS detector at the LHC. The charged-hadron yield is obtained by counting the number of reconstructed hits, hit pairs, and fully reconstructed charged-particle tracks. The combination of the three methods gives a charged-particle multiplicity per unit of pseudorapidity $d{N}_{\mathrm{ch}}/d\eta {|}_{|\eta |<0.5}=5.78\pm 0.01(\mathrm{stat})\pm 0.23(\mathrm{syst})$ for non-single-diffractive events, higher than predicted by commonly used models. The relative increase in charged-particle multiplicity from $\sqrt{s}=0.9$ to 7 TeV is $[66.1\pm 1.0(\mathrm{stat})\pm 4.2(\mathrm{syst})]\%$. The mean transverse momentum is measured to be $0.545\pm 0.005(\mathrm{stat})\pm 0.015(\mathrm{syst})\mathrm{GeV}/c$. The results are compared with similar measurements at lower energies.

A precision measurement by the Alpha Magnetic Spectrometer on the International Space Station of the positron fraction in primary cosmic rays in the energy range from 0.5 to 350 GeV based on $6.8\times {10}^6$ positron and electron events is presented. The very accurate data show that the positron fraction is steadily increasing from 10 to $\sim 250\mathrm{GeV}$, but, from 20 to 250 GeV, the slope decreases by an order of magnitude. The positron fraction spectrum shows no fine structure, and the positron to electron ratio shows no observable anisotropy. Together, these features show the existence of new physical phenomena.

Bose-Einstein correlations have been measured using samples of proton-proton collisions at 0.9 and 2.36 TeV center-of-mass energies, recorded by the CMS experiment at the CERN Large Hadron Collider. The signal is observed in the form of an enhancement of pairs of same-sign charged particles with small relative four-momentum. The size of the correlated particle emission region is seen to increase significantly with the particle multiplicity of the event.

A search for narrow resonances in the dijet mass spectrum is performed using data corresponding to an integrated luminosity of $2.9{\mathrm{pb}}^{-1}$ collected by the CMS experiment at the Large Hadron Collider. Upper limits at the 95% confidence level are presented on the product of the resonance cross section, branching fraction into dijets, and acceptance, separately for decays into quark-quark, quark-gluon, or gluon-gluon pairs. The data exclude new particles predicted in the following models at the 95% confidence level: string resonances, with mass less than 2.50 TeV, excited quarks, with mass less than 1.58 TeV, and axigluons, colorons, and $E_6$ diquarks, in specific mass intervals. This extends previously published limits on these models.