555 research outputs found
Study of Bc+→J/ψDs+ and Bc+→J/ψDs∗+ decays in pp collisions at √s = 13 TeV with the ATLAS detector
No full textA study of Bc+→J/ψDs+ and Bc+→J/ψDs∗+ decays using 139 fb−1 of integrated luminosity collected with the ATLAS detector from s = 13 TeV pp collisions at the LHC is presented. The ratios of the branching fractions of the two decays to the branching fraction of the Bc+→ J/ψπ+ decay are measured: B(Bc+→J/ψDs+)/B(Bc+→J/ψπ+) = 2.76 ± 0.47 and B(Bc+→J/ψDs∗+)/B(Bc+→J/ψπ+) = 5.33 ± 0.96. The ratio of the branching fractions of the two decays is found to be B(Bc+→J/ψDs∗+)/B(Bc+→J/ψDs∗+) = 1.93 ± 0.26. For the Bc+→J/ψDs∗+ decay, the transverse polarization fraction, Γ±±/Γ, is measured to be 0.70 ± 0.11. The reported uncertainties include both the statistical and systematic components added in quadrature. The precision of the measurements exceeds that in all previous studies of these decays. These results supersede those obtained in the earlier ATLAS study of the same decays with s = 7 and 8 TeV pp collision data. A comparison with available theoretical predictions for the measured quantities is presented. [Figure not available: see fulltext.]
Measurement of J/ψ production in association with a W ± boson with pp data at 8 TeV
No full textA measurement of the production of a prompt J/ψ meson in association with a W ± boson with W ± → μν and J/ψ → μ +μ − is presented for J/ψ transverse momenta in the range 8.5–150 GeV and rapidity |y J/ψ| <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 fb −1. The ratio of the prompt J/ψ plus W ± cross-section to the inclusive W ± cross-section is presented as a differential measurement as a function of J/ψ transverse momenta and compared with theoretical predictions using different double-parton-scattering cross-sections. [Figure not available: see fulltext.]
Measurement of b-quark fragmentation properties in jets using the decay B ± → J/ψK ± in pp collisions at √s = 13 TeV with the ATLAS detector
No full textThe fragmentation properties of jets containing b-hadrons are studied using charged B mesons in 139 fb−1 of pp collisions at s = 13 TeV, recorded with the ATLAS detector at the LHC during the period from 2015 to 2018. The B mesons are reconstructed using the decay of B± into J/ψK±, with the J/ψ decaying into a pair of muons. Jets are reconstructed using the anti-kt algorithm with radius parameter R = 0.4. The measurement determines the longitudinal and transverse momentum profiles of the reconstructed B hadrons with respect to the axes of the jets to which they are geometrically associated. These distributions are measured in intervals of the jet transverse momentum, ranging from 50 GeV to above 100 GeV. The results are corrected for detector effects and compared with several Monte Carlo predictions using different parton shower and hadronisation models. The results for the longitudinal and transverse profiles provide useful inputs to improve the description of heavy-flavour fragmentation in jets. [Figure not available: see fulltext.]
Prompt and non-prompt J/ψ and ψ(2S) suppression at high transverse momentum in 5.02 TeV Pb+Pb collisions with the ATLAS experiment
No full textA measurement of J/ψ and ψ(2S) production is presented. It is based on a data sample from Pb+Pb collisions at sNN−−−√=5.02 TeV and pp collisions at s√=5.02 TeV recorded by the ATLAS detector at the LHC in 2015, corresponding to an integrated luminosity of 0.42 nb−1 and 25 pb−1 in Pb+Pb and pp, respectively. The measurements of per-event yields, nuclear modification factors, and non-prompt fractions are performed in the dimuon decay channel for 9<pμμT<40 GeV in dimuon transverse momentum, and −2<yμμ<2 in rapidity. Strong suppression is found in Pb+Pb collisions for both prompt and non-prompt J/ψ , increasing with event centrality. The suppression of prompt ψ(2S) is observed to be stronger than that of J/ψ , while the suppression of non-prompt ψ(2S) is equal to that of the non-prompt J/ψ within uncertainties, consistent with the expectation that both arise from b-quarks propagating through the medium. Despite prompt and non-prompt J/ψ arising from different mechanisms, the dependence of their nuclear modification factors on centrality is found to be quite similar
Measurement of the CP-violating phase φs in B0s → J/ψφ decays in ATLAS at 13 TeV
No full textA measurement of the B0
s → J/ψφ decay
parameters using 80.5 fb−1 of integrated luminosity collected with the ATLAS detector from 13 TeV proton–proton
collisions at the LHC is presented. The measured parameters include the CP-violating phase φs, the width difference
Δ Γs between the B0
s meson mass eigenstates and the average decay width Γs. The values measured for the physical
parameters are combined with those from 19.2 fb−1 of 7 and
8 TeV data, leading to the following:
φs = −0.087 ± 0.036 (stat.) ± 0.021 (syst.) rad
ΔΓs = 0.0657 ± 0.0043 (stat.) ± 0.0037 (syst.) ps−1
Γs = 0.6703 ± 0.0014 (stat.) ± 0.0018 (syst.) ps−1
Results for φs and ΔΓs are also presented as 68% confidence level contours in the φs–ΔΓs plane. Furthermore the
transversity amplitudes and corresponding strong phases are
measured. φs and ΔΓs measurements are in agreement with
the Standard Model predictions
Observation of an Excess of Dicharmonium Events in the Four-Muon Final State with the ATLAS Detector
No full textA search is made for potential ccc̄ c̄ tetraquarks decaying into a pair of charmonium states in the four ffiffi muon final state using proton-proton collision data at ps 1⁄4 13 TeV, corresponding to an integrated luminosity of 140 fb−1 recorded by the ATLAS experiment at LHC. Two decay channels, J=ψ þ J=ψ → 4μ and J=ψ þ ψ ð2SÞ → 4μ, are studied. Backgrounds are estimated based on a hybrid approach involving Monte Carlo simulations and data-driven methods. Statistically significant excesses with respect to backgrounds dominated by the single parton scattering are seen in the di-J=ψ channel consistent with a narrow resonance at 6.9 GeV and a broader structure at lower mass. A statistically significant excess is also seen in the J=ψ þ ψ ð2SÞ channel. The fitted masses and decay widths of the structures are reported
Measurement of b-quark fragmentation properties in jets using the decay B±→ J/ψK± in pp collisions at s = 13 TeV with the ATLAS detector
No full textThe fragmentation properties of jets containing b-hadrons are studied using charged B mesons in 139 fb−1 of pp collisions at s = 13 TeV, recorded with the ATLAS detector at the LHC during the period from 2015 to 2018. The B mesons are reconstructed using the decay of B± into J/ψK±, with the J/ψ decaying into a pair of muons. Jets are reconstructed using the anti-kt algorithm with radius parameter R = 0.4. The measurement determines the longitudinal and transverse momentum profiles of the reconstructed B hadrons with respect to the axes of the jets to which they are geometrically associated. These distributions are measured in intervals of the jet transverse momentum, ranging from 50 GeV to above 100 GeV. The results are corrected for detector effects and compared with several Monte Carlo predictions using different parton shower and hadronisation models. The results for the longitudinal and transverse profiles provide useful inputs to improve the description of heavy-flavour fragmentation in jets
Searches for exclusive Higgs and Z boson decays into J/psi gamma, psi (2S) gamma, and Upsilon(nS) gamma at root s=13 TeV with the ATLAS detector
No full textSearches for the exclusive decays of the Higgs and Z bosons into a J/psi, psi(2S), or Upsilon(nS) (n = 1, 2, 3) meson and a photon are performed with a pp collision data sample corresponding to an integrated luminosity of 36.1 fb(-1) collected at root s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. No significant excess of events is observed above the expected backgrounds, and 95% confidence- level upper limits on the branching fractions of the Higgs boson decays to J/psi gamma, psi(2S) gamma and Upsilon(nS) gamma of 3.5 x 10(-4), 2.0 x 10(-3), and (4.9, 5.9, 5.7) x 10(-4), respectively, are obtained assuming Standard Model production. The corresponding 95% confidence-level upper limits for the branching fractions of the Z boson decays are 2.3 x 10(-6), 4.5 x 10(-6) and (2.8, 1.7, 4.8) 10(-6), respectively. (C) 2018 The Author. Published by Elsevier B.V
Search for flavour-changing neutral-current interactions of a top quark and a gluon in collisions at TeV with the ATLAS detector
No full textA search is presented for the production of a single top quark via
left-handed flavour-changing neutral-current (FCNC) interactions of a top
quark, a gluon and an up or charm quark. Two production processes are
considered: and . The analysis is based on
proton-proton collision data taken at a centre-of-mass energy of 13TeV with
the ATLAS detector at the LHC. The data set corresponds to an integrated
luminosity of 139 fb. Events with exactly one electron or muon, exactly
one -tagged jet and missing transverse momentum are selected, resembling the
decay products of a singly produced top quark. Neural networks based on
kinematic variables differentiate between events from the two signal processes
and events from background processes. The measured data are consistent with the
background-only hypothesis, and limits are set on the production cross-sections
of the signal processes: pb and pb at the 95% confidence
level, with being the sum of
branching ratios of all three leptonic decay modes of the boson. Based on
the framework of an effective field theory, the cross-section limits are
translated into limits on the strengths of the and couplings
occurring in the theory: TeV and
TeV. These bounds correspond to
limits on the branching ratios of FCNC-induced top-quark decays:
and
.Comment: 51 pages in total, author list starting page 35, 7 figures, 4 tables,
published by Eur. Phys. J. C, All figures including auxiliary figures are
available at
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2018-06
Measurement of the energy asymmetry in production at 13 TeV with the ATLAS experiment and interpretation in the SMEFT framework
No full textA measurement of the energy asymmetry in jet-associated top-quark pair
production is presented using 139 of data collected by the
ATLAS detector at the Large Hadron Collider during collisions at
TeV. The observable measures the different probability of top and
antitop quarks to have the higher energy as a function of the jet scattering
angle with respect to the beam axis. The energy asymmetry is measured in the
semileptonic decay channel, and the hadronically decaying top quark
must have transverse momentum above GeV. The results are corrected for
detector effects to particle level in three bins of the scattering angle of the
associated jet. The measurement agrees with the SM prediction at
next-to-leading-order accuracy in quantum chromodynamics in all three bins. In
the bin with the largest expected asymmetry, where the jet is emitted
perpendicular to the beam, the energy asymmetry is measured to be
, in agreement with the SM prediction of .
Interpreting this result in the framework of the Standard Model effective field
theory (SMEFT), it is shown that the energy asymmetry is sensitive to the
top-quark chirality in four-quark operators and is therefore a valuable new
observable in global SMEFT fits.Comment: 52 pages in total, author list starting page 37, 10 figures, 4
tables, submitted to EPJC. All figures including auxiliary figures are
available at
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2019-2
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