分类: 物理学 >> 核物理学 提交时间: 2016-09-12
摘要: A search is presented for a new, light boson with a mass of about 1 GeV and decaying promptly to jets of collimated electrons and/or muons (lepton-jets). The analysis is performed with 20.3 fb−1 of data collected by the ATLAS detector at the Large Hadron Collider in proton--proton collisions at a centre-of-mass energy of 8 TeV. Events are required to contain at least two lepton-jets. This study finds no statistically significant deviation from predictions of the Standard Model and places 95\% confidence-level upper limits on the contribution of new phenomena beyond the SM, incuding SUSY-portal and Higgs-portal models, on the number of events with lepton-jets.
分类: 物理学 >> 核物理学 提交时间: 2016-09-09
摘要: The top quark mass was measured in the channels tt¯→lepton+jets and tt¯→dilepton (lepton=e,μ) based on ATLAS data recorded in 2011. The data were taken at the LHC with a proton--proton centre-of-mass energy of s√=7 TeV and correspond to an integrated luminosity of 4.6fb−1. The tt¯→lepton+jets analysis uses a three-dimensional template technique which determines the top quark mass together with a global jet energy scale factor (JSF), and a relative b-to-light-jet energy scale factor (bJSF), where the terms b-jets and light-jets refer to jets originating from b-quarks and u,d,c,s-quarks or gluons, respectively. The analysis of the tt¯→dileptonchannel exploits a one-dimensional template method using the mℓb observable, defined as the average invariant mass of the two lepton+b-jet pairs in each event. The top quark mass is measured to be 172.33±0.75(stat)±1.02(syst) GeV, and 173.79±0.54(stat)±1.30(syst) GeV in the tt¯→lepton+jets and tt¯→dilepton channels, respectively. The combination of the two results yields mtop=172.99±0.48(stat)±0.78(syst) GeV, with a total uncertainty of 0.91 GeV.
分类: 物理学 >> 核物理学 提交时间: 2016-09-12
摘要: Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, tt¯ system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of s√=8 TeV}. The observables have been chosen to emphasize the tt¯ production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb−1, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.
分类: 物理学 >> 核物理学 提交时间: 2016-09-09
摘要: A measurement is presented of the tt¯ inclusive production cross-section in pp collisions at a center-of-mass energy of s√=8 TeV using data collected by the ATLAS detector at the CERN Large Hadron Collider. The measurement was performed in the lepton+jets final state using a data set corresponding to an integrated luminosity of 20.3 fb−1. The cross-section was obtained using a likelihood discriminant fit and b-jet identification was used to improve the signal-to-background ratio. The inclusive tt¯ production cross-section was measured to be 260±1(stat.)+22−23(syst.)±8(lumi.)±4(beam) pb assuming a top-quark mass of 172.5 GeV, in good agreement with the theoretical prediction of 253+13−15 pb. The tt¯→(e,μ)+jets production cross-section in the fiducial region determined by the detector acceptance is also reported.
分类: 物理学 >> 核物理学 提交时间: 2016-08-19
摘要: A measurement of the top quark pair production (t-tbar) cross section in proton-proton collisions at the centre-of-mass energy of 8 TeV is presented using data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.6 inverse-femtobarns. This analysis is performed in the t-tbar decay channels with one isolated, high transverse momentum electron or muon and at least four jets, at least one of which is required to be identified as originating from hadronization of a b quark. The calibration of the jet energy scale and the efficiency of b jet identification are determined from data. The measured t-tbar cross section is 228.5 +/- 3.8 (stat) +/- 13.7 (syst) +/- 6.0 (lumi) pb. This measurement is compared with an analysis of 7 TeV data, corresponding to an integrated luminosity of 5.0 inverse-femtobarns, to determine the ratio of 8 TeV to 7 TeV cross sections, which is found to be 1.43 +/- 0.04 (stat) +/- 0.07 (syst) +/- 0.05 (lumi). The measurements are in agreement with QCD predictions up to next-to-next-to-leading order.