Search

A search for narrow resonances in dielectron and dimuon invariant mass spectra has been performed using data obtained from proton–proton collisions at s=13 TeV collected with the CMS detector. The integrated luminosity for the dielectron sample is 2.7 fb−1 and for the dimuon sample 2.9 fb−1. The sensitivity of the search is increased by combining these data with a previously analyzed set of data obtained at s=8 TeV and corresponding to a luminosity of 20 fb−1. No evidence for non-standard-model physics is found, either in the 13 TeV data set alone, or in the combined data set. Upper limits on the product of production cross section and branching fraction have also been calculated in a model-independent manner to enable interpretation in models predicting a narrow dielectron or dimuon resonance structure. Limits are set on the masses of hypothetical particles that could appear in new-physics scenarios. For the ZSSM′ particle, which arises in the sequential standard model, and for the superstring inspired Zψ′ particle, 95% confidence level lower mass limits for the combined data sets and combined channels are found to be 3.37 and 2.82 TeV, respectively. The corresponding limits for the lightest Kaluza–Klein graviton arising in the Randall–Sundrum model of extra dimensions with coupling parameters 0.01 and 0.10 are 1.46 and 3.11 TeV, respectively. These results significantly exceed the limits based on the 8 TeV LHC data.

Abstract: A search for narrow, high-mass resonances decaying to electron or muon pairs has been performed using pp collision data collected at by the CMS experiment in 2011. The data sample corresponds to an integrated luminosity of approximately 5 fb−1. The event yields observed in the signal regions are consistent with predictions of the standard model backgrounds, and upper limits on the cross section times branching fraction for a resonance decaying to dileptons are extracted from a shape analysis of the dilepton invariant mass distribution. The resulting mass limits at 95% confidence level are 2330 GeV for the in the Sequential Standard Model, 2000 GeV for the superstring-inspired resonance, 890 (540) GeV for the Stueckelberg extension with the mass parameter (0.04), and 2140 (1810) GeV for Kaluza–Klein gravitons with the coupling parameter of 0.10 (0.05). These limits are the most stringent to date. [Copyright &y& Elsevier]

Abstract: We report measurements of electron pair production in elementary and reactions at with the HADES spectrometer. For the first time, the electron pairs were reconstructed for reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass is about an order of magnitude larger in reactions as compared to . A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in reactions are compatible with a superposition of elementary and collisions, leaving little room for additional electron pair sources in such light collision systems. [Copyright &y& Elsevier]

Abstract: We derive the forward photoproduction amplitude for the diffractive reaction in the momentum space, within the formalism of -factorization. Predictions for the reaction are given using unintegrated gluon distribution from the literature. We calculate the total cross section as a function of photon–proton center of mass energy and the invariant mass distribution of the lepton pair. We also discuss whether the production of timelike virtual photons can be approximated by continuing to the spacelike domain . The present calculation provides an input for future predictions for exclusive hadroproduction in the reaction. [Copyright &y& Elsevier]

Abstract: We consider a hard jet production tagged by a muon pair in ultrarelativistic heavy ion collisions. The process cross section is calculated by the CompHEP Monte Carlo generator taking into account full interference pattern at LHC energies. We have found that reasonable statistics, ∼1000 events per 1 month of LHC run with lead beams, can be expected for realistic geometrical acceptance and kinematic cuts. The transverse momentum imbalance due to interactions of jet partons in the medium is evaluated for pair + jet correlation, as well as for the correlation between pair and a leading particle in a jet. Theoretical and experimental uncertainties of these observables are discussed. [Copyright &y& Elsevier]