分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: The results of a search for gluinos in final states with an isolated electron or muon, multiple jets and large missing transverse momentum using proton--proton collision data at a centre-of-mass energy of s√ = 13 TeV are presented. The dataset used was recorded in 2015 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 3.2 fb−1. Six signal selections are defined that best exploit the signal characteristics. The data agree with the Standard Model background expectation in all six signal selections, and the largest deviation is a 2.1 standard deviation excess. The results are interpreted in a simplified model where pair-produced gluinos decay via the lightest chargino to the lightest neutralino. In this model, gluinos are excluded up to masses of approximately 1.6 TeV depending on the mass spectrum of the simplified model, thus surpassing the limits of previous searches.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: A search for squarks and gluinos in final states containing hadronic jets, missing transverse momentum but no electrons or muons is presented. The data were recorded in 2015 by the ATLAS experiment in s√= 13 TeV proton--proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 3.2 fb−1 of analyzed data. Results are interpreted within simplified models that assume R-parity is conserved and the neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 1.51 TeV for a simplified model incorporating only a gluino octet and the lightest neutralino, assuming the lightest neutralino is massless. For a simplified model involving the strong production of mass-degenerate first- and second-generation squarks, squark masses below 1.03 TeV are excluded for a massless lightest neutralino. These limits substantially extend the region of supersymmetric parameter space excluded by previous measurements with the ATLAS detector.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: A measurement of the cross section for the inclusive production of isolated prompt photons in proton--proton collisions at a centre-of-mass energy of s√=8 TeV is presented. The measurement covers the pseudo rapidity ranges |ηγ|<1.37 and 1.56≤|ηγ|<2.37 in the transverse energy range 25
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: The process e+e−→η′J/ψ is observed for the first time with a statistical significance of 8.6σ at center-of-mass energy s√=4.226 GeV and 7.3σ at s√=4.258 GeV using data samples collected with the BESIII detector. The Born cross sections are measured to be (3.7±0.7±0.3) and (3.9±0.8±0.3) pb at s√=4.226 and 4.258 GeV, respectively, where the first errors are statistical and the second systematic. Upper limits at the 90% confidence level of the Born cross sections are also reported at other 12 energy points
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: A search for new physics is performed using events with two isolated same-sign leptons, two or more jets, and missing transverse momentum. The results are based on a sample of proton-proton collisions at a center-of-mass energy of 13 TeV recorded with the CMS detector at the LHC, corresponding to an integrated luminosity of 2.3 inverse-femtobarns. Multiple search regions are defined by classifying events in terms of missing transverse momentum, the scalar sum of jet transverse momenta, the transverse mass associated with a W boson candidate, the number of jets, the number of b quark jets, and the transverse momenta of the leptons in the event. The analysis is sensitive to a wide variety of possible signals beyond the standard model. No excess above the standard model background expectation is observed. Constraints are set on various supersymmetric models, with gluinos and bottom squarks excluded for masses up to 1300 and 680 GeV, respectively, at the 95% confidence level. Upper limits on the cross sections for the production of two top quark-antiquark pairs (119 fb) and two same-sign top quarks (1.7 pb) are also obtained. Selection efficiencies and model independent limits are provided to allow further interpretations of the results.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: A search is presented for the Higgs boson off-shell production in gluon fusion and vector boson fusion processes with the Higgs boson decaying into a WW pair and the W bosons decaying leptonically. The data observed in this analysis are used to constrain the Higgs boson total decay width. The analysis is based on the data collected by the CMS experiment at the LHC, corresponding to integrated luminosities of 4.9 inverse femtobarns at a centre-of-mass energy of 7 TeV and 19.4 inverse femtobarns at 8 TeV, respectively. An observed (expected) upper limit on the off-shell Higgs boson event yield normalised to the standard model prediction of 2.4 (6.2) is obtained at the 95% CL for the gluon fusion process and of 19.3 (34.4) for the vector boson fusion process. Observed and expected limits on the total width of 26 and 66 MeV are found, respectively, at the 95% confidence level (CL). These limits are combined with the previous result in the ZZ channel leading to observed and expected 95% CL upper limits on the width of 13 and 26 MeV, respectively.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: The branching fractions of the Υ(1S) inclusive decays into final states with a J/ψ or a ψ(2S) are measured with improved precision to be $\BR(\Upsilon(1S)\to J/\psi + {\rm anything})=(5.25\pm 0.13(\mathrm{stat.})\pm 0.25(\mathrm{syst.}))\times 10^{-4}$ and $\BR(\Upsilon(1S)\to \psi(2S) + {\rm anything})=(1.23\pm 0.17(\mathrm{stat.})\pm 0.11(\mathrm{syst.}))\times 10^{-4}$. The first search for Υ(1S) decays into XYZ states that decay into a J/ψ or a ψ(2S) plus one or two charged tracks yields no significant signals for XYZ states in any of the examined decay modes, and upper limits on their production rates in Υ(1S) inclusive decays are determined.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: By analyzing 2.93 fb−1 data collected at the center-of-mass energy s√=3.773 GeV with the BESIII detector, we measure the absolute branching fraction of the semileptonic decay D+→K¯0e+νe to be B(D+→K¯0e+νe)=(8.59±0.14±0.21)% using K¯0→K0S→π0π0, where the first uncertainty is statistical and the second systematic. Our result is consistent with previous measurements within uncertainties.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: The cross section for pair production of top quarks (t-tbar) with high transverse momenta is measured in pp collisions, collected with the CMS detector at the LHC with sqrt(s) = 8 TeV in data corresponding to an integrated luminosity of 19.7 inverse-femtobarns. The measurement is performed using lepton+jets events, where one top quark decays semileptonically, while the second top quark decays to a hadronic final state. The hadronic decay is reconstructed as a single, large-radius jet, and identified as a top quark candidate using jet substructure techniques. The integrated cross section and the differential cross sections as a function of top quark pt and rapidity are measured at particle level within a fiducial region related to the detector-level requirements and at parton level. The particle-level integrated cross section is found to be sigma[t-tbar] = 0.499 +/- 0.035 (stat+syst) +/- 0.095 (theory) +/- 0.013 (lumi) pb for top quark pt > 400 GeV. The parton-level measurement is sigma[t-tbar] = 1.44 +/- 0.10 (stat+syst) +/- 0.29 (theory) +/- 0.04 (lumi) pb. The integrated and differential cross section results are compared to predictions from several event generators.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: By analyzing 2.93 fb−1 of data collected at s√=3.773 GeV with the BESIII detector, we measure the absolute branching fraction B(D+→K¯0μ+νμ)=(8.72±0.07stat.±0.18sys.)%, which is consistent with previous measurements within uncertainties but with significantly improved precision. Combining the Particle Data Group values of B(D0→K−μ+νμ), B(D+→K¯0e+νe), and the lifetimes of the D0 and D+ mesons with the value of B(D+→K¯0μ+νμ) measured in this work, we determine the following ratios of partial widths: Γ(D0→K−μ+νμ)/Γ(D+→K¯0μ+νμ)=0.963±0.044 and Γ(D+→K¯0μ+νμ)/Γ(D+→K¯0e+νe)=0.988±0.033.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: A search for narrow resonances decaying into dijet final states is performed on data from proton-proton collisions at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 18.8 inverse femtobarns. The data were collected with the CMS detector using a novel technique called data scouting, in which the information associated with these selected events is much reduced, permitting collection of larger data samples. This technique enables CMS to record events containing jets at a rate of 1 kHz, by collecting the data from the high-level-trigger system. In this way, the sensitivity to low-mass resonances is increased significantly, allowing previously inaccessible couplings of new resonances to quarks and gluons to be probed. The resulting dijet mass distribution yields no evidence of narrow resonances. Upper limits are presented on the resonance cross sections as a function of mass, and compared with a variety of models predicting narrow resonances. The limits are translated into upper limits on the coupling of a leptophobic resonance Z'[B] to quarks, improving on the results obtained by previous experiments for the mass range from 500 to 800 GeV.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton--proton collision data corresponding to an integrated luminosity of 3.2 fb−1 at s√=13 TeV collected in 2015 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons. Several signal regions are considered with increasing missing-transverse-momentum requirements between EmissT>250 GeV and EmissT>700 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with large extra spatial dimensions, pair production of weakly interacting dark-matter candidates, and the production of supersymmetric particles in several compressed scenarios.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: H→eτ, H→μτ, and Z→μτ. The searches are based on the data sample of proton--proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 fb−1 at a centre-of-mass energy of s√=8TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95% confidence level: Br(H→eτ)< 1.04%, Br(H→μτ)< 1.43%, and Br(Z→μτ)< 1.69 × 10−5.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: Measurements of the top--antitop quark pair production charge asymmetry in the dilepton channel are presented using data corresponding to an integrated luminosity of 20.3 fb−1 from pp collisions at a center-of-mass energy of s√=8 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. Inclusive and differential measurements as a function of the invariant mass, transverse momentum, and longitudinal boost of the tt¯ system are performed both in the full phase space and in a fiducial phase space closely matching the detector acceptance. Two observables are studied: AℓℓC based on the selected leptons and Att¯C based on the reconstructed tt¯ final state. The inclusive asymmetries are measured in the full phase space to be AℓℓC=0.008±0.006 and Att¯C=0.021±0.016, which are in agreement with the Standard Model predictions of AℓℓC=0.0064±0.0003 and Att¯C=0.0111±0.0004.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: Two-particle correlations in pPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV are studied as a function of the pseudorapidity separation (Delta eta) of the particle pair at small relative azimuthal angle (abs(Delta phi)< pi/3). The correlations are decomposed into a jet component that dominates the short-range correlations (abs(Delta eta) < 1), and a component that persists at large Delta eta and may originate from collective behavior of the produced system. The events are classified in terms of the multiplicity of the produced particles. Finite azimuthal anisotropies are observed in high-multiplicity events. The second and third Fourier components of the particle-pair azimuthal correlations, V[2] and V[3], are extracted after subtraction of the jet component. The single-particle anisotropy parameters v[2] and v[3] are normalized by their lab frame mid-rapidity value and are studied as a function of eta[cm]. The normalized v[2] distribution is found to be asymmetric about eta[cm] = 0, with smaller values observed at forward pseudorapidity, corresponding to the direction of the proton beam, while no significant pseudorapidity dependence is observed for the normalized v[3] distribution within the statistical uncertainties.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: A search for narrow resonances decaying to an electron and a muon is presented. The e-mu mass spectrum is also investigated for non-resonant contributions from the production of quantum black holes (QBHs). The analysis is performed using data corresponding to an integrated luminosity of 19.7 inverse femtobarns collected in proton-proton collisions at a centre-of-mass energy of 8 TeV with the CMS detector at the LHC. With no evidence for physics beyond the standard model in the invariant mass spectrum of selected e-mu pairs, upper limits are set at 95% confidence level on the product of cross section and branching fraction for signals arising in theories with charged lepton flavour violation. In the search for narrow resonances, the resonant production of a tau sneutrino in R-parity violating supersymmetry is considered. The tau sneutrino is excluded for masses below 1.28 TeV for couplings lambda[132] = lambda[231] = lambda'[311] = 0.01, and below 2.30 TeV for lambda[132] = lambda[231] = 0.07 and lambda'[311] = 0.11. These are the most stringent limits to date from direct searches at high-energy colliders. In addition, the resonance searches are interpreted in terms of a model with heavy partners of the Z boson and the photon. In a framework of TeV-scale quantum gravity based on a renormalization of Newton's constant, the search for non-resonant contributions to the e-mu mass spectrum excludes QBH production below a threshold mass M[th] of 1.99 TeV. In models that invoke extra dimensions, the bounds range from 2.36 TeV for one extra dimension to 3.63 TeV for six extra dimensions. This is the first search for QBHs decaying into the e-mu final state
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: The production of Z bosons with one or two isolated high-energy photons is studied using pp collisions at s√ = 8 TeV. The analyses use a data sample with an integrated luminosity of 20.3 fb−1 collected by the ATLAS detector during the 2012 LHC data taking. The Zγ and Zγγ production cross sections are measured with leptonic (e+e−, μ+μ−, νν¯) decays of the Z boson, in extended fiducial regions defined in terms of the lepton and photon acceptance. They are then compared to cross-section predictions from the Standard Model, where the sources of the photons are radiation off initial-state quarks and radiative Z-boson decay to charged leptons, and from fragmentation of final-state quarks and gluons into photons. The yields of events with photon transverse energy ET> 250 GeV from ℓ+ℓ−γ events and with ET> 400 GeV from νν¯γ events are used to search for anomalous triple gauge-boson couplings ZZγ and Zγγ. The yields of events with diphoton invariant mass mγγ> 200 GeV from ℓ+ℓ−γγ events and with mγγ> 300 GeV from νν¯γγ events are used to search for anomalous quartic gauge-boson couplings ZZγγ and Zγγγ. No deviations from Standard Model predictions are observed and limits are placed on parameters used to describe anomalous triple and quartic gauge-boson couplings.
分类: 物理学 >> 核物理学 提交时间: 2016-08-25
摘要: A search for exclusive or quasi-exclusive gamma-gamma to W+W- production, via pp to p(*) W+W- p(*) to p(*) mu+/- e-/+ at sqrt(s)=8 TeV, is reported using data corresponding to an integrated luminosity of 19.7 inverse femtobarns. Events are selected by requiring the presence of an electron-muon pair with large transverse momentum pt(mu+/- e-/+>) 30 GeV, and no associated charged particles detected from the same vertex. The 8 TeV results are combined with the previous 7 TeV results (obtained for 5.05 inverse femtobarns of data). In the signal region, 13 (2) events are observed over an expected background of 3.9 +/- 0.6 (0.84 +/-0.15) events for 8 (7) TeV, resulting in a combined excess of 3.4 sigma over the background-only hypothesis. The observed yields and kinematic distributions are compatible with the standard model prediction for exclusive and quasi-exclusive gamma-gamma to W+W- production. Upper limits on the anomalous quartic gauge coupling operators a[W;0,C] (dimension-6) and f[M0,1,2,3] (dimension-8), the most stringent to date, are derived from the measured dilepton transverse momentum spectrum.