189 research outputs found
System size dependence of cluster properties from two-particle angular correlations in Cu+Cu and Au+Au collisions at = 200 GeV
We present results on two-particle angular correlations in Cu+Cu and Au+Au
collisions at a center of mass energy per nucleon pair of 200 GeV over a broad
range of pseudorapidity () and azimuthal angle () as a function of
collision centrality. The PHOBOS detector at RHIC has a uniquely-large angular
coverage for inclusive charged particles, which allows for the study of
correlations on both long- and short-range scales. A complex two-dimensional
correlation structure in and emerges, which is
interpreted in the context of a cluster model. The effective cluster size and
decay width are extracted from the two-particle pseudorapidity correlation
functions. The effective cluster size found in semi-central Cu+Cu and Au+Au
collisions is comparable to that found in proton-proton collisions but a
non-trivial decrease of the size with increasing centrality is observed.
Moreover, a comparison between results from Cu+Cu and Au+Au collisions shows an
interesting scaling of the effective cluster size with the measured fraction of
total cross section (which is related to the ratio of the impact parameter to
the nuclear radius, ), suggesting a geometric origin. Further analysis
for pairs from restricted azimuthal regions shows that the effective cluster
size at drops more rapidly toward central
collisions than the size at . The effect of limited
acceptance on the cluster parameters is also addressed, and a correction
is applied to present cluster parameters for full coverage, leading to
much larger effective cluster sizes and widths than previously noted in the
literature. These results should provide insight into the hot and dense medium
created in heavy ion collisions.Comment: 9 pages, 8 figures, Published in Phys. Rev.
Measurement of the ratio of the inclusive 3-jet cross section to the inclusive 2-jet cross section in pp collisions at √s = 7 TeV and first determination of the strong coupling constant in the TeV range
A measurement is presented of the ratio of the inclusive 3-jet cross section to the inclusive 2-jet cross section as a function of the average transverse momentum, ⟨p[subscript T1,2]⟩, of the two leading jets in the event. The data sample was collected during 2011 at a proton–proton centre-of-mass energy of 7 TeV with the CMS detector at the LHC, corresponding to an integrated luminosity of 5.0 fb[subscript −1]. The strong coupling constant at the scale of the Z boson mass is determined to be α[subscript S](M[subscript Z])=0.1148±0.0014 (exp.)±0.0018 (PDF)±0.0050(theory), by comparing the ratio in the range 0.42<⟨p[subscript T1,2]⟩<1.39 TeV to the predictions of perturbative QCD at next-to-leading order. This is the first determination of α[subscript S](M[subscript Z]) from measurements at momentum scales beyond 0.6 TeV. The predicted ratio depends only indirectly on the evolution of the parton distribution functions of the proton such that this measurement also serves as a test of the evolution of the strong coupling constant. No deviation from the expected behaviour is observed.United States. Department of EnergyNational Science Foundation (U.S.
Nucleon-Gold Collisions at 200 AGeV Using Tagged d+Au Interactions in PHOBOS
Forward calorimetry in the PHOBOS detector has been used to study charged
hadron production in d+Au, p+Au and n+Au collisions at sqrt(s_nn) = 200 GeV.
The forward proton calorimeter detectors are described and a procedure for
determining collision centrality with these detectors is detailed. The
deposition of energy by deuteron spectator nucleons in the forward calorimeters
is used to identify p+Au and n+Au collisions in the data. A weighted
combination of the yield of p+Au and n+Au is constructed to build a reference
for Au+Au collisions that better matches the isospin composition of the gold
nucleus. The p_T and centrality dependence of the yield of this improved
reference system is found to match that of d+Au. The shape of the charged
particle transverse momentum distribution is observed to extrapolate smoothly
from pbar+p to central d+Au as a function of the charged particle
pseudorapidity density. The asymmetry of positively- and negatively-charged
hadron production in p+Au is compared to that of n+Au. No significant asymmetry
is observed at mid-rapidity. These studies augment recent results from
experiments at the LHC and RHIC facilities to give a more complete description
of particle production in p+A and d+A collisions, essential for the
understanding the medium produced in high energy nucleus-nucleus collisions.Comment: 17 pages, 18 figure
Search for physics beyond the standard model in events with two leptons, jets, and missing transverse momentum in pp collisions at √s = 8 TeV
A search is presented for physics beyond the standard model in final states with two opposite-sign same-flavor leptons, jets, and missing transverse momentum. The data sample corresponds to an integrated luminosity of 19.4 fb[superscript −1] of proton-proton collisions at √s = 8 TeV collected with the CMS detector at the CERN LHC in 2012. The analysis focuses on searches for a kinematic edge in the invariant mass distribution of the oppositesign same-flavor lepton pair and for final states with an on-shell Z boson. The observations are consistent with expectations from standard model processes and are interpreted in terms of upper limits on the production of supersymmetric particles.United States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio
Measurement of the W boson helicity in events with a single reconstructed top quark in pp collisions at √s = 8 TeV
A measurement of the W boson helicity is presented, where the W boson originates from the decay of a top quark produced in pp collisions. The event selection, optimized for reconstructing a single top quark in the final state, requires exactly one isolated lepton (muon or electron) and exactly two jets, one of which is likely to originate from the hadronization of a bottom quark. The analysis is performed using data recorded at a center-of-mass energy of 8 TeV with the CMS detector at the CERN LHC in 2012. The data sample corresponds to an integrated luminosity of 19.7 fb[superscript −1]. The measured helicity fractions are F [subscript L] = 0.298 ± 0.028 (stat) ± 0.032(syst), F [subscript 0] = 0.720 ± 0.039 (stat) ± 0.037(syst), and F [subscript R] = −0.018 ± 0.019 (stat) ± 0.011(syst). These results are used to set limits on the real part of the tWb anomalous couplings, g [subscript L] and g [subscript R]
Search for a standard model Higgs boson produced in association with a top-quark pair and decaying to bottom quarks using a matrix element method
A search for a standard model Higgs boson produced in association with a top-quark pair and decaying to bottom quarks is presented. Events with hadronic jets and one or two oppositely charged leptons are selected from a data sample corresponding to an integrated luminosity of 19.5 fb[superscript -1] collected by the CMS experiment at the LHC in pp collisions at a centre-of-mass energy of 8 TeV. In order to separate the signal from the larger t[bar over t] + jets background, this analysis uses a matrix element method that assigns a probability density value to each reconstructed event under signal or background hypotheses. The ratio between the two values is used in a maximum likelihood fit to extract the signal yield. The results are presented in terms of the measured signal strength modifier, μ, relative to the standard model prediction for a Higgs boson mass of 125 GeV. The observed (expected) exclusion limit at a 95 % confidence level is μ < 4.2 (3.3), corresponding to a best fit value [^ over μ] = 1.2[+1.6 over −1.5].United States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio
Search for new physics in events with same-sign dileptons and jets in pp collisions at √s = 8 TeV
A search for new physics is performed based on events with jets and a pair of isolated, same-sign leptons. The results are obtained using a sample of proton-proton collision data collected by the CMS experiment at a centre-of-mass energy of 8 TeV at the LHC, corresponding to an integrated luminosity of 19.5 fb[superscript −1]. In order to be sensitive to a wide variety of possible signals beyond the standard model, multiple search regions defined by the missing transverse energy, the hadronic energy, the number of jets and b-quark jets, and the transverse momenta of the leptons in the events are considered. No excess above the standard model background expectation is observed and constraints are set on a number of models for new physics, as well as on the same-sign top-quark pair and quadruple-top-quark production cross sections. Information on event selection efficiencies is also provided, so that the results can be used to confront an even broader class of new physics models.United States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio
Search for the production of dark matter in association with top-quark pairs in the single-lepton final state in proton-proton collisions at √s = 8 TeV
A search is presented for particle dark matter produced in association with a pair of top quarks in pp collisions at a centre-of-mass energy of √s = 8 TeV. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of 19.7 fb[superscript −1]. This search requires the presence of one lepton, multiple jets, and large missing transverse energy. No excess of events is found above the SM expectation, and upper limits are derived on the production cross section. Interpreting the findings in the context of a scalar contact interaction between fermionic dark matter particles and top quarks, lower limits on the interaction scale are set. These limits are also interpreted in terms of the dark matter-nucleon scattering cross sections for the spin-independent scalar operator and they complement direct searches for dark matter particles in the low mass region.United States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio
Measurement of the transverse momentum spectrum of the Higgs boson produced in pp collisions at √s=8 TeV using H → WW decays
The cross section for Higgs boson production in pp collisions is studied using the H → W[superscript +]W[superscript −] decay mode, followed by leptonic decays of the W bosons to an oppositely charged electron-muon pair in the final state. The measurements are performed using data collected by the CMS experiment at the LHC at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.4 fb[superscript −1]. The Higgs boson transverse momentum (p[subscript T]) is reconstructed using the lepton pair p[subscript T] and missing p[subscript T]. The differential cross section times branching fraction is measured as a function of the Higgs boson pTin a fiducial phase space defined to match the experimental acceptance in terms of the lepton kinematics and event topology. The production cross section times branching fraction in the fiducial phase space is measured to be 39 ± 8 (stat) ± 9 (syst) fb. The measurements are found to agree, within experimental uncertainties, with theoretical calculations based on the standard model. Keywords: Hadron-Hadron scattering (experiments), Higgs physicsNational Science Foundation (U.S.)United States. Department of Energ
Search for the associated production of the Higgs boson with a top-quark pair
A search for the standard model Higgs boson produced in association with a top-quark pair (tt¯H) is presented, using data samples corresponding to integrated luminosities of up to 5.1 fb−1 and 19.7 fb−1 collected in pp collisions at center-of-mass energies of 7 TeV and 8 TeV respectively. The search is based on the following signatures of the Higgs boson decay: H → hadrons, H → photons, and H → leptons. The results are characterized by an observed tt¯H signal strength relative to the standard model cross section, μ=σ/σ SM,under the assumption that the Higgs boson decays as expected in the standard model. The best fit value is μ = 2.8 ± 1.0 for a Higgs boson mass of 125.6 GeV.National Science Foundation (U.S.
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