Measurement of associated production of vector bosons and top quark-antiquark pairs in pp collisions at √s=7 TeV

PubMed ID: 23679709The first measurement of vector-boson production associated with a top quark-antiquark pair in proton-proton collisions at s √ =7  TeV is presented. The results are based on a data set corresponding to an integrated luminosity of 5.0  fb^−1 , recorded by the CMS detector at the LHC in 2011. The measurement is performed in two independent channels through a trilepton analysis of tt ¯ Z events and a same-sign dilepton analysis of tt ¯ V (V=W or Z ) events. In the trilepton channel a direct measurement of the tt ¯ Z cross section σ tt ¯ Z =0.28 [+0.14 −0.11]  (stat) [+0.06 −0.03]  (syst)  pb is obtained. In the dilepton channel a measurement of the tt ¯ V cross section yields σtt¯V=0.43 [+0.17 −0.15]  (stat) [+0.09 −0.07]  (syst)  pb . These measurements have a significance, respectively, of 3.3 and 3.0 standard deviations from the background hypotheses and are compatible, within uncertainties, with the corresponding next-to-leading order predictions of 0.137[+0.012 −0.016] and 0.306 [+0.031 −0.053]   pb

Measurement of the prompt J/psi and psi(2S) polarizations in pp collisions at root s=7 TeV

The polarizations of prompt J/psi and psi(2S) mesons are measured in proton-proton collisions at root s = 7 TeV, using a dimuon data sample collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.9 fb(-1). The prompt J/psi and psi(2S) polarization parameters lambda, lambda, and lambda, as well as the frame-invariant quantity (lambda) over bar. are measured from the dimuon decay angular distributions in three different polarization frames. The J/psi results are obtained in the transverse momentum range 14 < P-T < 70 GeV, in the rapidity intervals |y| < 0.6 and 0.6 < |y| < 1.2. The corresponding psi(2S) results cover 14 < PT < 50 GeV and include a third rapidity bin, 1.2 < |y| < 1.5. No evidence of large polarizations is seen in these kinematic regions, which extend much beyond those previously explored. (C) 2013 CERN. Published by Elsevier B.V. All rights reserved

Measurement of the W+W- and ZZ production cross sections in pp collisions at root s=8 TeV

The W+W- and ZZ production cross sections are measured in proton-proton collisions at root s = 8 TeV with the CMS experiment at the LHC in data samples corresponding to an integrated luminosity of up to 5.3 fb(-1). The measurements are performed in the leptonic decay modes W+W- -> l'vl '' v and ZZ -> 2l2l', where l = e, mu and l'(l '') = e, mu, tau. The measured cross sections sigma (pp -> W+W-) =, 69.9 +/- 2.8 (stat.) +/- 5.6 (syst.) 3.1 +/- (lum.) pb and sigma (pp -> ZZ) = 8.4 +/- 1.0 (stat) +/- 0.7 (syst) +/- 0.4 (lum.) pb, for both Z bosons produced in the mass region 60 < m(Z) < 120 GeV, are consistent with standard model predictions. These are the first measurements of the diboson production cross sections at root s = 8 TeV. (C) 2013 CERN. Published by Elsevier B.V. All rights reserved

Search for Z ' resonances decaying to t(t)over-bar in dilepton plus jets final states in pp collisions at root s=7 TeV

A search for resonances decaying to top quark-antiquark pairs is performed using a dilepton+jets data sample recorded by the CMS experiment at the LHC in pp collisions at √s=7 TeV corresponding to an integrated luminosity of 5.0 fb-1. No significant deviations from the standard model background are observed. Upper limits are presented for the production cross section times branching fraction of top quark-antiquark resonances for masses from 750 to 3000 GeV. In particular, the existence of a leptophobic topcolor particle Z′ is excluded at the 95% confidence level for resonance masses MZ ′&lt;1.3 TeV for ΓZ′=0.012MZ ′, and M&lt;1.9 TeV for ΓZ′=0.10MZ ′. © 2013 CERN

Search for supersymmetry in hadronic final states with missing transverse energy using the variables AlphaT and b-quark multiplicity in pp collisions at s=8\sqrt{s} = 8 TeV

An inclusive search for supersymmetric processes that produce final states with jets and missing transverse energy is performed in pp collisions at a centre-of-mass energy of 8 TeV. The data sample corresponds to an integrated luminosity of 11.7 inverse femtobarns collected by the CMS experiment at the LHC. In this search, a dimensionless kinematic variable, AlphaT, is used to discriminate between events with genuine and misreconstructed missing transverse energy. The search is based on an examination of the number of reconstructed jets per event, the scalar sum of transverse energies of these jets, and the number of these jets identified as originating from bottom quarks. No significant excess of events over the standard model expectation is found. Exclusion limits are set in the parameter space of simplified models, with a special emphasis on both compressed-spectrum scenarios and direct or gluino-induced production of third-generation squarks

Search for third-generation leptoquarks and scalar bottom quarks in pp collisions at root s=7 TeV

Results are presented from a search for third-generation leptoquarks and scalar bottom quarks in a sample of proton-proton collisions at √s=7Tev collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.7 fb-1. A scenario where the new particles are pair produced and each decays to a b quark plus a tau neutrino or neutralino is considered. The number of observed events is found to be in agreement with the standard model prediction. Upper limits are set at 95% confidence level on the production cross sections. Leptoquarks with masses below ~450 GeV are excluded. Upper limits in the mass plane of the scalar quark and neutralino are set such that scalar bottom quark masses up to 410 GeV are excluded for neutralino masses of 50 GeV. © 2012 CERN for the benefit of CMS collaboration

Study of the underlying event at forward rapidity in pp collisions at root s=0.9, 2.76, and 7 TeV

The underlying event activity in proton-proton collisions at forward pseudorapidity (-6.6 < eta < -5.2) is studied with the CMS detector at the LHC, using a novel observable: the ratio of the forward energy density, dE/d(eta), for events with a charged-particle jet produced at central pseudorapidity (vertical bar eta(jet)vertical bar < 2) to the forward energy density for inclusive events. This forward energy density ratio is measured as a function of the central jet transverse momentum, p(T), at three different pp centre-of-mass energies (root s = 0.9, 2.76, and 7 TeV). In addition, the root s evolution of the forward energy density is studied in inclusive events and in events with a central jet. The results are compared to those of Monte Carlo event generators for pp collisions and are discussed in terms of the underlying event. Whereas the dependence of the forward energy density ratio on jet p(T), at each root s separately can be well reproduced by some models, all models fail to simultaneously describe the increase of the forward energy density with root s in both inclusive events and in events with a central jet

Study of the Mass and Spin-Parity of the Higgs Boson Candidate via Its Decays to Z Boson Pairs

A study is presented of the mass and spin-parity of the new boson recently observed at the LHC at a mass near 125 GeV. An integrated luminosity of 17: 3 fb(-1), collected by the CMS experiment in proton-proton collisions at center-of-mass energies of 7 and 8 TeV, is used. The measured mass in the ZZ channel, where both Z bosons decay to e or mu pairs, is 126: 2 +/- 0.6(stat) +/- 0. 2(syst) GeV. The angular distributions of the lepton pairs in this channel are sensitive to the spin-parity of the boson. Under the assumption of spin 0, the present data are consistent with the pure scalar hypothesis, while disfavoring the pure pseudoscalar hypothesis. DOI: 10.1103/PhysRevLett.110.08180

Search in leptonic channels for heavy resonances decaying to long-lived neutral particles

A search is performed for heavy resonances decaying to two long-lived massive neutral particles, each decaying to leptons. The experimental signature is a distinctive topology consisting of a pair of oppositely charged leptons originating at a separated secondary vertex. Events were collected by the CMS detector at the LHC during pp collisions at √s = 7 TeV, and selected from data samples corresponding to 4.1 (5.1) fb-1 of integrated luminosity in the electron (muon) channel. No significant excess is observed above standard model expectations, and an upper limit is set with 95% confidence level on the production cross section times the branching fraction to leptons, as a function of the long-lived massive neutral particle lifetime. Copyright CERN