A search for supersymmetry with R-parity conservation in proton-proton collisions at a centre-of-mass energy of 7 TeV is presented. The data correspond to an integrated luminosity of 35 inverse picobarns collected by the CMS experiment at the LHC. The search is performed in events with jets and significant missing transverse energy, characteristic of the decays of heavy, pair-produced squarks and gluinos. The primary background, from standard model multijet production, is reduced by several orders of magnitude to a negligible level by the application of a set of robust kinematic requirements. With this selection, the data are consistent with the standard model backgrounds, namely t t-bar, W + jet and Z + jet production, which are estimated from data control samples. Limits are set on the parameters of the constrained minimal supersymmetric extension of the standard model. These limits extend those set previously by experiments at the Tevatron and LEP colliders

The results of comprehensive studies of missing transverse energy as measured by the CMS detector in pp collisions at a centre-of-mass energy of 7 TeV are presented. Three missing transverse energy reconstruction algorithms are deployed for various physics analyses. The scale and resolution for missing transverse energy are validated using vector boson and dijet events, and severe mismeasurements due to the detector are studied. We also parametrize the effects of multiple pp interactions within the same bunch crossings on the scale and resolution. A tool, called missing transverse energy significance, based on particle resolutions in each event is also presented.

A measurement is presented of the charged hadron multiplicity in hadronic PbPb collisions, as a function of pseudorapidity and centrality, at a collision energy of 2.76 TeV per nucleon pair. The data sample is collected using the CMS detector and a minimum-bias trigger, with the CMS solenoid off. The number of charged hadrons is measured both by counting the number of reconstructed particle hits and by forming hit doublets of pairs of layers in the pixel detector. The two methods give consistent results. The charged hadron multiplicity density, dN ch /dη| η=0 , for head-on collisions is found to be 1612 ± 55, where the uncertainty is dominated by systematic effects. Comparisons of these results to previous measurements and to various models are also presented. Keywords: Hadron-Hadron ScatteringOpen Access, Copyright CERN, for the benefit of the CMS collaboration doi:10.1007/JHEP08(2011)141 The CMS collaboration 21 JHEP08(2011)141 IntroductionQuantum chromodynamics (QCD), the theory of strong interactions, predicts a phase transition at high temperature between hadronic and deconfined matter [1]. Strongly interacting matter under extreme conditions can be studied experimentally using ultrarelativistic collisions of heavy nuclei. The field entered a new era in November 2010 when the Large Hadron Collider (LHC) produced the first PbPb collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV. This represents an increase of more than one order of magnitude over the highest-energy nuclear collisions previously achieved in the laboratory. The multiplicity of charged particles produced in the central-rapidity region is a key observable characterising the properties of the quark-gluon matter created in these collisions [2]. Nuclei are extended objects, and their collisions occur at various impact parameters, referred to as "centralities". The studies of the dependence of the charged particle density on the type of colliding nuclei, on the centre-of-mass energy, and on the collision geometry are important for understanding the relative contributions of hard scattering and soft processes to particle production and provide insight into the partonic structure of the nuclei.-1 - JHEP08(2011)141In this paper we report measurements of the multiplicity density dN ch /dη of primary charged hadrons. The analysis is based on the 2.76 TeV-per-nucleon PbPb collision data recorded by the Compact Muon Solenoid (CMS) detector in December 2010, in runs without magnetic field. The pseudorapidity is defined as η = − ln[tan(θ/2)] with θ the polar angle with respect to the counterclockwise beam direction (the z axis). The number of primary charged hadrons N ch is defined as all charged hadrons produced in an event including decay products of particles with proper lifetimes less than 1 cm.A detailed description of the CMS experiment can be found in ref. [3]. The pixel tracker used for the analysis covers the region |η| < 2.5 and a full 2π in azimuth, with 66M detector channels out of which 97.5% were functional during data...

A search for exclusive or semi-exclusive γγ production, pp → p ( * ) + γγ + p ( * ) (where p * stands for a diffractively-dissociated proton), and the observation of exclusive and semi-exclusive e + e − production, pp → p ( * ) + e + e − + p ( * ) , in proton-proton collisions at √ s = 7 TeV, are presented. The analysis is based on a data sample corresponding to an integrated luminosity of 36 pb −1 recorded by the CMS experiment at the LHC at low instantaneous luminosities. Candidate γγ or e + e − events are selected by requiring the presence of two photons or a positron and an electron, each with transverse energy E T > 5.5 GeV and pseudorapidity |η| < 2.5, and no other particles in the region |η| < 5.2. No exclusive or semi-exclusive diphoton candidates are found in the data. An upper limit on the cross section for the reaction pp → p ( * ) + γγ + p ( * ) , within the above kinematic selections, is set at 1.18 pb at 95% confidence level. Seventeen exclusive or semi-exclusive dielectron candidates are observed, with an estimated background of 0.85 ± 0.28 (stat.) events, in agreement with the QED-based prediction of 16.3 ± 1.3 (syst.) events.

The tt production cross section (σ tt ) is measured in proton-proton collisions at √ s = 7 TeV in data collected by the CMS experiment, corresponding to an integrated luminosity of 2.3 fb −1 . The measurement is performed in events with two leptons (electrons or muons) in the final state, at least two jets identified as jets originating from b quarks, and the presence of an imbalance in transverse momentum. The measured value of σ tt for a top-quark mass of 172.5 GeV is 161.9 ± 2.5 (stat.) +5.1 −5.0 (syst.) ± 3.6 (lumi.) pb, consistent with the prediction of the standard model. Keywords: Hadron-Hadron ScatteringOpen Access, Copyright CERN, for the benefit of the CMS collaboration doi:10.1007/JHEP11(2012)067 The CMS collaboration 23 JHEP11(2012)067 IntroductionA precise measurement of the tt production cross section (σ tt ) in pp collisions at the Large Hadron Collider (LHC) is important for several reasons. At LHC energies, tt production is dominated by gluon-gluon fusion, representing a benchmark for other processes of the standard model (SM) initiated through the same mechanism, such as the production of Higgs bosons. In addition, a precise value of σ tt can provide constraints on parton distribution functions (PDF) and be used to check the validity of perturbative calculations in quantum chromodynamics (QCD). It can also benefit searches for physics beyond the SM, as tt production is often a major source of background. The precision of initial measurements of σ tt in 2010 at √ s = 7 TeV [1, 2] has been improved in refs. [3][4][5][6][7][8], and measurements of electroweak single-top-quark production are available in refs. [9, 10]. By now, many top-quark events have been collected at the LHC, with studies proceeding on a variety of top-quark production and decay channels [5,[11][12][13], as well as on searches for deviations relative to predictions of the SM.-1 - JHEP11(2012)067This work presents a measurement of σ tt in dilepton final states that improves upon a previous measurement [5] of the Compact Muon Solenoid (CMS) experiment [14]. It is based on more refined event selections and analysis techniques, an improved estimation of systematic uncertainties, and with a data sample approximately sixty times larger. Starting from the nearly 100% decay of both top quarks through the electroweak transition t → Wb, we focus on events in the dilepton final states e + e − , µ + µ − , and e ± µ ∓ , where both W bosons decay leptonically (W → ν ), but with contributions from W → τ ν τ arising only when the τ leptons decay into ν ν τ ( = e or µ) states. A cross-section measurement based on τ leptons that decay into hadrons and a neutrino can be found in ref. [15]. The final states contain two leptons of opposite electric charge, their accompanying neutrinos from the W-boson decays, and two jets of particles resulting from the hadronization of the b quarks. These modes correspond to (6.45 ± 0.11)% [16] of all tt decay channels, including the partial contributions from τ leptons.This measurement is based on a da...