A measurement of the top quark pair-production cross section in the lepton+jets decay channel is presented. It is based on 4.6 fb−1 of √s=7 TeV pp collision data collected during 2011 by the ATLAS ...experiment at the CERN Large Hadron Collider. A three-class, multidimensional event classifier based on support vector machines is used to differentiate t¯t events from backgrounds. The t¯t production cross section is found to be σt¯t=168.5±0.7(stat) +6.2−5.9(syst) +3.4−3.2(lumi) pb. The result is consistent with the Standard Model prediction based on QCD calculations at next-to-next-to-leading order.
A direct search for the standard model Higgs boson decaying to a pair of charm quarks is presented. Associated production of the Higgs and Z bosons, in the decay mode ZH→ + -cc is studied. A data set ...with an integrated luminosity of 36.1 fb-1 of pp collisions at s=13TeV recorded by the ATLAS experiment at the LHC is used. The H→cc signature is identified using charm-tagging algorithms. The observed (expected) upper limit on σ(pp→ZH)×B(H→cc) is 2.7 (3.9-1.1+2.1) pb at the 95% confidence level for a Higgs boson mass of 125 GeV, while the standard model value is 26 fb.
A search for the narrow structure, X(5568), reported by the D0 Collaboration in the decay sequence X→Bs0π±, Bs0→J/ψφ, is presented. The analysis is based on a data sample recorded with the ATLAS ...detector at the LHC corresponding to 4.9 fb-1 of pp collisions at 7 TeV and 19.5 fb-1 at 8 TeV. No significant signal was found. Upper limits on the number of signal events, with properties corresponding to those reported by D0, and on the X production rate relative to Bs0 mesons, ρX, were determined at 95% confidence level. The results are N(X)<382 and ρX<0.015 for Bs0 mesons with transverse momenta above 10 GeV, and N(X)<356 and ρX<0.016 for transverse momenta above 15 GeV. Limits are also set for potential Bs0π± resonances in the mass range 5550 to 5700 MeV.
The production of a top quark in association with a Z boson is investigated. The proton–proton collision data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy ...of s=13TeV are used, corresponding to an integrated luminosity of 36.1fb-1. Events containing three identified leptons (electrons and/or muons) and two jets, one of which is identified as a b-quark jet are selected. The major backgrounds are diboson, tt¯ and Z+jets production. A neural network is used to improve the background rejection and extract the signal. The resulting significance is 4.2σ in the data and the expected significance is 5.4σ. The measured cross-section for tZq production is 600±170(stat.)±140(syst.)fb.
A search for the standard model Higgs boson produced in association with a top-quark pair, ttH, is presented. The analysis uses 36.1 fb-1 of pp collision data at s=13 TeV collected with the ATLAS ...detector at the Large Hadron Collider in 2015 and 2016. The search targets the H→bb decay mode. The selected events contain either one or two electrons or muons from the top-quark decays, and are then categorized according to the number of jets and how likely these are to contain b-hadrons. Multivariate techniques are used to discriminate between signal and background events, the latter being dominated by tt+jets production. For a Higgs boson mass of 125 GeV, the ratio of the measured ttH signal cross-section to the standard model expectation is found to be μ=0.84-0.61+0.64. A value of μ greater than 2.0 is excluded at 95% confidence level (C.L.) while the expected upper limit is μ<1.2 in the absence of a ttH signal.
A search for long-lived, massive particles predicted by many theories beyond the Standard Model is presented. The search targets final states with large missing transverse momentum and at least one ...high-mass displaced vertex with five or more tracks, and uses 32.8 fb-1 of s=13 TeV pp collision data collected by the ATLAS detector at the LHC. The observed yield is consistent with the expected background. The results are used to extract 95% C.L. exclusion limits on the production of long-lived gluinos with masses up to 2.37 TeV and lifetimes of O(10-2)-O(10) ns in a simplified model inspired by split supersymmetry.
A measurement of W± boson production in Pb+Pb collisions at √sNN=5.02 TeV is reported using data recorded by the ATLAS experiment at the LHC in 2015, corresponding to a total integrated luminosity of ...0.49nb-1. The W± bosons are reconstructed in the electron or muon leptonic decay channels. Production yields of leptonically decaying W± bosons, normalised by the total number of minimum-bias events and the nuclear thickness function, are measured within a fiducial region defined by the detector acceptance and the main kinematic requirements. These normalised yields are measured separately for W+ and W- bosons, and are presented as a function of the absolute value of pseudorapidity of the charged lepton and of the collision centrality. The lepton charge asymmetry is also measured as a function of the absolute value of lepton pseudorapidity. In addition, nuclear modification factors are calculated using the W± boson production cross-sections measured in pp collisions. The results are compared with predictions based on next-to-leading-order calculations with CT14 parton distribution functions as well as with predictions obtained with the EPPS16 and nCTEQ15 nuclear parton distribution functions. No dependence of normalised production yields on centrality and a good agreement with predictions are observed for mid-central and central collisions. For peripheral collisions, the data agree with predictions within 1.7 (0.9) standard deviations for W- (W+) bosons.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A search for excited electrons produced in pp collisions at $\sqrt{s}$ = 13 TeV via a contact interaction $q\overline{q}$ → ee* is presented. The search uses 36.1 fb-1 of data collected in 2015 and ...2016 by the ATLAS experiment at the Large Hadron Collider. Decays of the excited electron into an electron and a pair of quarks ($eq\overline{q}$) are targeted in final states with two electrons and two hadronic jets, and decays via a gauge interaction into a neutrino and a W boson (νW) are probed in final states with an electron, missing transverse momentum, and a large-radius jet consistent with a hadronically decaying W boson. No significant excess is observed over the expected backgrounds. Upper limits are calculated for the pp→ ee*→$eeq\overline{q}$ and pp→ ee*→ eνW production cross sections as a function of the excited electron mass $m_{e*}$ at 95% confidence level. The limits are translated into lower bounds on the compositeness scale parameter Λ of the model as a function of $m_{e*}$. For $m_{e*}$ <0.5 TeV, the lower bound for Λ is 11 TeV. In the special case of $m_{e*}$=Λ, the values of $m_{e*}$< 4.8 TeV are excluded. The presented limits on Λ are more stringent than those obtained in previous searches.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The identification of jets containing b hadrons is important for the physics programme of the ATLAS experiment at the Large Hadron Collider. Several algorithms to identify jets containing b hadrons ...are described, ranging from those based on the reconstruction of an inclusive secondary vertex or the presence of tracks with large impact parameters to combined tagging algorithms making use of multi-variate discriminants. An independent b-tagging algorithm based on the reconstruction of muons inside jets as well as the b-tagging algorithm used in the online trigger are also presented. The b-jet tagging efficiency, the c-jet tagging efficiency and the mistag rate for light flavour jets in data have been measured with a number of complementary methods. The calibration results are presented as scale factors defined as the ratio of the efficiency (or mistag rate) in data to that in simulation. In the case of b jets, where more than one calibration method exists, the results from the various analyses have been combined taking into account the statistical correlation as well as the correlation of the sources of systematic uncertainty.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image).Differential cross sections for the production of at least four jets have been measured in proton-proton collisions at ... ...TeV at the Large Hadron Collider using the ATLAS detector. Events are selected if the four anti-k sub(t) R = 0.4 jets with the largest transverse momentum (p sub(T)) within the rapidity range |y| < 2.8 are well separated ( Delta R sub(4j) super(min)>0.65), all have p sub(T) > 64 GeV, and include at least one jet with p sub(T) > 100 GeV. The dataset corresponds to an integrated luminosity of 20.3 fb super(-1). The cross sections, corrected for detector effects, are compared to leading-order and next-to-leading-order calculations as a function of the jet momenta, invariant masses, minimum and maximum opening angles and other kinematic variables. Figure not available: see fulltext.
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