Search for anomalous t t-bar production in the highly-boosted all-hadronic final state

A search is presented for a massive particle, generically referred to as a Z', decaying into a t t-bar 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 inverse femtobarns. 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 results constrain any enhancement in t t-bar production beyond expectations of the standard model for t t-bar invariant masses larger than 1 TeV.Comment: Submitted to the Journal of High Energy Physics; this version includes a minor typo correction that will be submitted as an erratu

The dependence of dijet production on photon virtuality in ep collisions at HERA

The dependence of dijet production on the virtuality of the exchanged photon, Q^2, has been studied by measuring dijet cross sections in the range 0 < Q^2 < 2000 GeV^2 with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb^-1. Dijet cross sections were measured for jets with transverse energy E_T^jet > 7.5 and 6.5 GeV and pseudorapidities in the photon-proton centre-of-mass frame in the range -3 < eta^jet <0. The variable xg^obs, a measure of the photon momentum entering the hard process, was used to enhance the sensitivity of the measurement to the photon structure. The Q^2 dependence of the ratio of low- to high-xg^obs events was measured. Next-to-leading-order QCD predictions were found to generally underestimate the low-xg^obs contribution relative to that at high xg^obs. Monte Carlo models based on leading-logarithmic parton-showers, using a partonic structure for the photon which falls smoothly with increasing Q^2, provide a qualitative description of the data.Comment: 35 pages, 6 eps figures, submitted to Eur.Phys.J.

Search for same-sign top-quark pair production at √s = 7 TeV and limits on flavour changing neutral currents in the top sector

An inclusive search for same-sign top-quark pair production in pp collisions at √s = 7 TeV is performed using a data sample recorded with the CMS detector in 2010, corresponding to an integrated luminosity of 35 pb^(−1). This analysis is motivated by recent studies of pp → tt reporting mass-dependent forward-backward asymmetries larger than expected from the standard model. These asymmetries could be due to Flavor Changing Neutral Currents (FCNC) in the top sector induced by t-channel exchange of a massive neutral vector boson (Z′). Models with such a Z′ also predict enhancement of same-sign top-pair production in pp or pp collisions. Limits are set as a function of the Z′ mass and its couplings to u and t quarks. These limits disfavour the FCNC interpretation of the Tevatron results

Measurement of the ratio of the 3-jet to 2-jet cross sections in pp collisions at √s = 7 TeV

A measurement of the ratio of the inclusive 3-jet to 2-jet cross sections as a function of the total jet transverse momentum, H_T, in the range 0.20.5 TeV. This measurement extends to an H_T range that has not been explored before

Azimuthal anisotropy of charged particles at high transverse momenta in PbPb collisions at sqrt(s[NN]) = 2.76 TeV

The azimuthal anisotropy of charged particles in PbPb collisions at nucleon-nucleon center-of-mass energy of 2.76 TeV is measured with the CMS detector at the LHC over an extended transverse momentum (pt) range up to approximately 60 GeV. The data cover both the low-pt region associated with hydrodynamic flow phenomena and the high-pt region where the anisotropies may reflect the path-length dependence of parton energy loss in the created medium. The anisotropy parameter (v2) of the particles is extracted by correlating charged tracks with respect to the event-plane reconstructed by using the energy deposited in forward-angle calorimeters. For the six bins of collision centrality studied, spanning the range of 0-60% most-central events, the observed v2 values are found to first increase with pt, reaching a maximum around pt = 3 GeV, and then to gradually decrease to almost zero, with the decline persisting up to at least pt = 40 GeV over the full centrality range measured.Comment: Replaced with published version. Added journal reference and DO

First measurement of hadronic event shapes in pp collisions at √s = 7 TeV

Hadronic event shapes have been measured in proton–proton collisions at √s = 7 TeV source, with a data sample collected with the CMS detector at the LHC. The sample corresponds to an integrated luminosity of 3.2 pb^(−1). Event-shape distributions, corrected for detector response, are compared with five models of QCD multijet production

Search for heavy stable charged particles in pp collisions at √s = 7 TeV

The result of a search at the LHC for heavy stable charged particles produced in pp collisions at √s = 7 TeV is described. The data sample was collected with the CMS detector and corresponds to an integrated luminosity of 3.1 pb^(-1). Momentum and ionization-energy-loss measurements in the inner tracker detector are used to identify tracks compatible with heavy slow-moving particles. Additionally, tracks passing muon identication requirements are also analyzed for the same signature. In each case, no candidate passes the selection, with an expected background of less than 0:1 events. A lower limit at the 95% condence level on the mass of a stable gluino is set at 398 GeV/c^2, using a conventional model of nuclear interactions that allows charged hadrons containing this particle to reach the muon detectors. A lower limit of 311 GeV/c^2 is also set for a stable gluino in a conservative scenario of complete charge suppression, where any hadron containing this particle becomes neutral before reaching the muon detectors