2,210 research outputs found
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Z boson production in Pb+Pb collisions at āSnn = 5.02 TeV measured by the ATLAS experiment
The production yield of Z bosons is measured in the electron and muon decay channels in Pb+Pb collisions at āS = 5.02 TeV with the ATLAS detector. Data from the 2015 LHC run corresponding to an integrated luminosity of 0.49 nb are used for the analysis. The Z boson yield, normalised by the total number of minimum-bias events and the mean nuclear thickness function, is measured as a function of dilepton rapidity and event centrality. The measurements in Pb+Pb collisions are compared with similar measurements made in proton-proton collisions at the same centre-of-mass energy. The nuclear modification factor is found to be consistent with unity for all centrality intervals. The results are compared with theoretical predictions obtained at next-to-leading order using nucleon and nuclear parton distribution functions. The normalised Z boson yields in Pb+Pb collisions lie 1-3Ļ above the predictions. The nuclear modification factor measured as a function of rapidity agrees with unity and is consistent with a next-to-leading-order QCD calculation including the isospin effect. nn -
Search for flavour-changing neutral currents in processes with one top quark and a photon using 81 fbā1 of pp collisions at s=13TeV with the ATLAS experiment
A search for flavour-changing neutral current (FCNC) events via the coupling of a top quark, a photon, and an up or charm quark is presented using 81 fbā1 of protonāproton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Events with a photon, an electron or muon, a b-tagged jet, and missing transverse momentum are selected. A neural network based on kinematic variables differentiates between events from signal and background processes. The data are consistent with the background-only hypothesis, and limits are set on the strength of the tqĪ³ coupling in an effective field theory. These are also interpreted as 95% CL upper limits on the cross section for FCNC tĪ³ production via a left-handed (right-handed) tuĪ³ coupling of 36 fb (78 fb) and on the branching ratio for tāĪ³u of 2.8Ć10ā5 (6.1Ć10ā5). In addition, they are interpreted as 95% CL upper limits on the cross section for FCNC tĪ³ production via a left-handed (right-handed) tcĪ³ coupling of 40 fb (33 fb) and on the branching ratio for tāĪ³c of 22Ć10ā5 (18Ć10ā5)
Measurement of J/Ļ production in association with a W Ā± boson with pp data at 8 TeV
A measurement of the production of a prompt J/Ļ meson in association with a WĀ± boson with WĀ± ā Ī¼Ī½ and J/Ļ ā Ī¼+Ī¼ā is presented for J/Ļ transverse momenta in the range 8.5ā150 GeV and rapidity |yJ/Ļ| < 2.1 using ATLAS data recorded in 2012 at the LHC. The data were taken at a proton-proton centre-of-mass energy of s = 8 TeV and correspond to an integrated luminosity of 20.3 fbā1. The ratio of the prompt J/Ļ plus WĀ± cross-section to the inclusive WĀ± cross-section is presented as a differential measurement as a function of J/Ļ transverse momenta and compared with theoretical predictions using different double-parton-scattering cross-sections. [Figure not available: see fulltext.]
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Measurement of Azimuthal Anisotropy of Muons from Charm and Bottom Hadrons in pp Collisions at sqrt[s]=13āāTeV with the ATLAS Detector.
The elliptic flow of muons from the decay of charm and bottom hadrons is measured in pp collisions at sqrt[s]=13āāTeV using a data sample with an integrated luminosity of 150āāpb^{-1} recorded by the ATLAS detector at the LHC. The muons from heavy-flavor decay are separated from light-hadron decay muons using momentum imbalance between the tracking and muon spectrometers. The heavy-flavor decay muons are further separated into those from charm decay and those from bottom decay using the distance-of-closest-approach to the collision vertex. The measurement is performed for muons in the transverse momentum range 4-7 GeV and pseudorapidity range |Ī·|<2.4. A significant nonzero elliptic anisotropy coefficient v_{2} is observed for muons from charm decays, while the v_{2} value for muons from bottom decays is consistent with zero within uncertainties
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Measurement of WĀ± boson production in Pb+Pb collisions at āsNN=5.02Te with the ATLAS detector
A measurement of WĀ± boson production in Pb+Pb collisions at sNN=5.02Te 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 WĀ± bosons are reconstructed in the electron or muon leptonic decay channels. Production yields of leptonically decaying WĀ± 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 WĀ± 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
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Combination of searches for Higgs boson pairs in pp collisions at s=13TeV with the ATLAS detector
This letter presents a combination of searches for Higgs boson pair production using up to 36.1 fbā1 of protonāproton collision data at a centre-of-mass energy s=13 TeV recorded with the ATLAS detector at the LHC. The combination is performed using six analyses searching for Higgs boson pairs decaying into the bbĀÆbbĀÆ, bbĀÆW+Wā, bbĀÆĻ+Ļā, W+WāW+Wā, bbĀÆĪ³Ī³ and W+WāĪ³Ī³ final states. Results are presented for non-resonant and resonant Higgs boson pair production modes. No statistically significant excess in data above the Standard Model predictions is found. The combined observed (expected) limit at 95% confidence level on the non-resonant Higgs boson pair production cross-section is 6.9 (10) times the predicted Standard Model cross-section. Limits are also set on the ratio (ĪŗĪ») of the Higgs boson self-coupling to its Standard Model value. This ratio is constrained at 95% confidence level in observation (expectation) to ā5.0<ĪŗĪ»<12.0 (ā5.8<ĪŗĪ»<12.0). In addition, limits are set on the production of narrow scalar resonances and spin-2 KaluzaāKlein RandallāSundrum gravitons. Exclusion regions are also provided in the parameter space of the habemus Minimal Supersymmetric Standard Model and the Electroweak Singlet Model
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Search for Magnetic Monopoles and Stable High-Electric-Charge Objects in 13 Tev Proton-Proton Collisions with the ATLAS Detector.
A search for magnetic monopoles and high-electric-charge objects is presented using 34.4āāfb^{-1} of 13 TeV pp collision data collected by the ATLAS detector at the LHC during 2015 and 2016. The considered signature is based upon high ionization in the transition radiation tracker of the inner detector associated with a pencil-shape energy deposit in the electromagnetic calorimeter. The data were collected by a dedicated trigger based on the tracker high-threshold hit capability. The results are interpreted in models of Drell-Yan pair production of stable particles with two spin hypotheses (0 and 1/2) and masses ranging from 200 to 4000 GeV. The search improves by approximately a factor of 5 the constraints on the direct production of magnetic monopoles carrying one or two Dirac magnetic charges and stable objects with electric charge in the range 20ā¤|z|ā¤60 and extends the charge range to 60<|z|ā¤100
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