分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: A study of the decays B0→μ+μ− and B0s→μ+μ− has been performed using data corresponding to an integrated luminosity of 25 fb−1 of 7 TeV and 8 TeV proton--proton collisions collected with the ATLAS detector during the LHC Run 1. For B0, an upper limit on the branching fraction is set at B(B0→μ+μ−)<4.2×10−10 at 95% confidence level. For B0s, the branching fraction B(B0s→μ+μ−)=(0.9+1.1−0.8)×10−9 is measured. The results are consistent with the Standard Model expectation with a p-value of 4.8%, corresponding to 2.0 standard deviations.
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: We present a measurement of angular observables, P′4, P′5, P′6, P′8, in the decay B0→K∗(892)0ℓ+ℓ−, where ℓ+ℓ− is either e+e− or μ+μ−. The analysis is performed on a data sample corresponding to an integrated luminosity of 711 fb−1 containing 772×106 BB¯ pairs, collected at the Υ(4S) resonance with the Belle detector at the asymmetric-energy e+e− collider KEKB. Four angular observables, P′4,5,6,8 are extracted in five bins of the invariant mass squared of the lepton system, q2. We compare our results for P′4,5,6,8 with Standard Model predictions including the q2 region in which the LHCb collaboration reported the so-called P′5 anomaly.
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: A search for Higgs boson production in association with a pair of top quarks (tt¯H) is performed, where the Higgs boson decays to bb¯, and both top quarks decay hadronically. The data used correspond to an integrated luminosity of 20.3 fb−1 of pp collisions at s√=8 TeV collected with the ATLAS detector at the Large Hadron Collider. The search selects events with at least six energetic jets and uses a boosted decision tree algorithm to discriminate between signal and Standard Model background. The dominant multijet background is estimated using a dedicated data-driven technique. For a Higgs boson mass of 125 GeV, an upper limit of 6.4 (5.4) times the Standard Model cross section is observed (expected) at 95% confidence level. The best-fit value for the signal strength is μ=1.6±2.6 times the Standard Model expectation for mH=125 GeV. Combining all tt¯H searches carried out by ATLAS at s√=8 and 7 TeV, an observed (expected) upper limit of 3.1 (1.4) times the Standard Model expectation is obtained at 95% confidence level, with a signal strength μ=1.7±0.8.
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: This paper describes a measurement of fiducial and differential cross sections of gluon-fusion Higgs boson production in the H→WW∗→eνμν channel, using 20.3 fb−1 of proton-proton collision data. The data were produced at a centre-of-mass energy of s√=8 TeV at the CERN Large Hadron Collider and recorded by the ATLAS detector in 2012. Cross sections are measured from the observed H→WW∗→eνμν signal yield in categories distinguished by the number of associated jets. The total cross section is measured in a fiducial region defined by the kinematic properties of the charged leptons and neutrinos. Differential cross sections are reported as a function of the number of jets, the Higgs boson transverse momentum, the dilepton rapidity, and the transverse momentum of the leading jet. The jet-veto efficiency, or fraction of events with no jets above a given transverse momentum threshold, is also reported. All measurements are compared to QCD predictions from Monte Carlo generators and fixed-order calculations, and are in agreement with the Standard Model predictions.
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: The process γγ→pp¯K+K− and its intermediate processes are measured for the first time using a 980~fb−1 data sample collected with the Belle detector at the KEKB asymmetric-energy e+e− collider. The production of pp¯K+K− and a Λ(1520)0 (Λ¯(1520)0) signal in the pK−~(p¯K+) invariant mass spectrum are clearly observed. However, no evidence for an exotic baryon near 1540~MeV/c2, denoted as Θ(1540)0~(Θ¯ (1540)0) or Θ(1540)++~(Θ(1540)−−), is seen in the pK−~(p¯K+) or pK+~(p¯K−) invariant mass spectra. Cross sections for γγ→pp¯K+K−, Λ(1520)0p¯K++c.c. and the products $\sigma(\gamma \gamma \to \Theta(1540)^0 \bar{p} K^+ +c.c.)\BR(\Theta(1540)^0 \to p K^{-})$ and$\sigma(\gamma \gamma \to \Theta(1540)^{++} \bar{p} K^- +c.c.)\BR(\Theta(1540)^{++}\to p K^{+})$ are measured. We also determine upper limits on the products of the χc0 and χc2 two-photon decay widths and their branching fractions to pp¯K+K− at the 90\% credibility level.
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: Using a data set of 2.93 fb−1 taken at a center-of-mass energy of s√ = 3.773 GeV with the BESIII detector at the BEPCII collider, we measure the process e+e−→J/ψγ→μ+μ−γ and determine the product of the branching fraction and the electronic width Bμμ⋅Γee=(333.4±2.5stat±4.4sys)~eV. Using the earlier-published BESIII result for Bμμ = (5.973 ± 0.007stat ± 0.037sys)\%, we derive the J/ψ electronic width Γee~= (5.58 ± 0.05stat ± 0.08sys) keV.
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum with the ATLAS experiment at the Large Hadron Collider are reported. The data were collected in proton--proton collisions at a centre-of-mass energy of 13 TeV and correspond to an integrated luminosity of 3.2 fb−1. The observed data are in agreement with the Standard Model expectations. Exclusion limits are presented in models of new phenomena including pair production of dark matter candidates or large extra spatial dimensions. In a simplified model of dark matter and an axial-vector mediator, the search excludes mediator masses of up to 710 GeV for dark matter candidate masses up to 150 GeV. In an effective theory of dark matter production, values of the suppression scale M∗ up to 570 GeV are excluded and the effect of truncation for various coupling values is reported. For the ADD large extra spatial dimension model the search places more stringent limits than earlier searches in the same event topology, excluding MD up to about 2.3 (2.8) TeV for two (six) additional spatial dimensions; the limits are reduced by 20--40% depending on the number of additional spatial dimensions when applying a truncation procedure.
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: For the International Large Detector concept at the planned International Linear Collider, the use of time projection chambers (TPC) with micro-pattern gas detector readout as the main tracking detector is investigated. In this paper, results from a prototype TPC, placed in a 1 T solenoidal field and read out with three independent GEM-based readout modules, are reported. The TPC was exposed to a 6 GeV electron beam at the DESY II synchrotron. The efficiency for reconstructing hits, the measurement of the drift velocity, the space point resolution and the control of field inhomogeneities are presented.
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: Measurements of the W±→ℓ±ν and Z→ℓ+ℓ− production cross sections (where ℓ±=e±,μ±) in proton-proton collisions at s√=13 TeV are presented using data recorded by the ATLAS experiment at the Large Hadron Collider, corresponding to a total integrated luminosity of 81 pb−1 The total inclusive W±-boson production cross sections times the single-lepton-flavour branching ratios are σtotW+=11.83±0.02(stat)±0.32(sys)±0.25(lumi) nb and σtotW−=8.79±0.02(stat)±0.24(sys)±0.18(lumi) nb for W+ and W−, respectively. The total inclusive Z-boson production cross section times leptonic branching ratio, within the invariant mass window 66
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: Charged Higgs bosons produced in association with a single top quark and decaying via H±→τν are searched for with the ATLAS experiment at the LHC, using proton-proton collision data at s√=13TeV corresponding to an integrated luminosity of 3.2 fb−1. The final state is characterised by the presence of a hadronic τ decay and missing transverse momentum, as well as a hadronically decaying top quark, resulting in the absence of high-transverse-momentum electrons and muons. The data are found to be consistent with the expected background from Standard Model processes. A statistical analysis leads to 95% confidence-level upper limits on the production cross section times branching fraction, σ(pp→[b]tH±)×BR(H±→τν), between 1.9 pb and 15 fb, for charged Higgs boson masses ranging from 200 to 2000 GeV. The exclusion limits for this search surpass those obtained with the proton-proton collision data recorded at s√=8 TeV.
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: This paper discusses various observations on beam-induced and cosmic-ray backgrounds in the ATLAS detector during the LHC 2012 proton-proton run. Building on published results based on 2011 data, the correlations between background and residual pressure of the beam vacuum are revisited. Ghost charge evolution over 2012 and its role for backgrounds are evaluated. New methods to monitor ghost charge with beam-gas rates are presented and observations of LHC abort gap population by ghost charge are discussed in detail. Fake jets from colliding bunches and from ghost charge are analysed with improved methods, showing that ghost charge in individual radio-frequency buckets of the LHC can be resolved. Some results of two short periods of dedicated cosmic-ray background data-taking are shown; in particular cosmic-ray muon induced fake jet rates are compared to Monte Carlo simulations and to the fake jet rates from beam background. A thorough analysis of a particular LHC fill, where abnormally high background was observed, is presented. Correlations between backgrounds and beam intensity losses in special fills with very high β∗ are studied.
分类: 物理学 >> 核物理学 提交时间: 2016-08-24
摘要: A search for dark matter particles directly produced in proton-proton collisions recorded by the CMS experiment at the LHC is presented. The data correspond to an integrated luminosity of 18.8 inverse femtobarns, at a center-of-mass energy of 8 TeV. The event selection requires at least two jets and no isolated leptons. The razor variables are used to quantify the transverse momentum balance in the jet momenta. The study is performed separately for events with and without jets originating from b quarks. The observed yields are consistent with the expected backgrounds and, depending on the nature of the production mechanism, dark matter production at the LHC is excluded at 90% confidence level for a mediator mass scale Lambda below 1 TeV. The use of razor variables yields results that complement those previously published.