ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Measurement of the CP properties of Higgs boson interactions with τ-leptons with the ATLAS detector. Eur. Phys. J. C, 83(7), 563–30pp.
Abstract: A study of the charge conjugation and parity (CP) properties of the interaction between the Higgs boson and tau -leptons is presented. The study is based on a measurement of CP-sensitive angular observables defined by the visible decay products of t -leptons produced in Higgs boson decays. The analysis uses 139 fb(-1) of proton-proton collision data recorded at a centre-of-mass energy of root s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Contributions from CP-violating interactions between the Higgs boson and t -leptons are described by a single mixing angle parameter phi(tau) in the generalised Yukawa interaction. Without constraining the H -> tau tau signal strength to its expected value under the Standard Model hypothesis, the mixing angle ft is measured to be 9 degrees +/- 16 degrees, with an expected value of 0 degrees +/- 28 degrees at the 68% confidence level. The pure CPodd hypothesis is disfavoured at a level of 3.4 standard deviations. The results are compatible with the predictions for the Higgs boson in the Standard Model.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Search for supersymmetry in final states with missing transverse momentum and three or more b-jets in 139 fb-1 of proton-proton collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 83(7), 561–41pp.
Abstract: A search for supersymmetry involving the pair production of gluinos decaying via off-shell third-generation squarks into the lightest neutralino ((chi) over tilde (0)(1)) is reported. It exploits LHC proton-proton collision data at a centre-ofmass energy root s = 13 TeV with an integrated luminosity of 139 fb(-1) collected with the ATLAS detector from 2015 to 2018. The search uses events containing large missing transverse momentum, up to one electron or muon, and several energetic jets, at least three of which must be identified as containing b-hadrons. Both a simple kinematic event selection and an event selection based upon a deep neural network are used. No significant excess above the predicted background is found. In simplified models involving the pair production of gluinos that decay via off-shell top (bottom) squarks, gluino masses less than 2.44 TeV (2.35 TeV) are excluded at 95% CL for a massless (chi) over tilde (0)(1). Limits are also set on the gluino mass in models with variable branching ratios for gluino decays to b (b) over bar (chi) over tilde (0)(1), t (t) over bar (chi) over tilde (0)(1) and t (b) over bar (chi) over tilde (-)(iota) / (t) over barb (chi) over tilde (0)(1).
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Search for an axion-like particle with forward proton scattering in association with photon pairs at ATLAS. J. High Energy Phys., 07(7), 234–38pp.
Abstract: A search for forward proton scattering in association with light-by-light scattering mediated by an axion-like particle is presented, using the ATLAS Forward Proton spectrometer to detect scattered protons and the central ATLAS detector to detect pairs of outgoing photons. Proton-proton collision data recorded in 2017 at a centre-of-mass energy of root s = 13TeV were analysed, corresponding to an integrated luminosity of 14.6 fb(-1). A total of 441 candidate events were selected. A search was made for a narrow resonance in the diphoton mass distribution, corresponding to an axion-like particle (ALP) with mass in the range 150-1600 GeV. No excess is observed above a smooth background. Upper limits on the production cross section of a narrow resonance are set as a function of the mass, and are interpreted as upper limits on the ALP production coupling constant, assuming 100% decay branching ratio into a photon pair. The inferred upper limit on the coupling constant is in the range 0.04-0.09TeV-1 at 95% confidence level.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Charged-hadron production in pp, p+Pb, Pb+Pb, and Xe+Xe collisions at √s_NN=5 TeV with the ATLAS detector at the LHC. J. High Energy Phys., 07(7), 074–58pp.
Abstract: This paper presents measurements of charged-hadron spectra obtained in pp, p+Pb, and Pb+Pb collisions at root s or root s(NN) = 5.02TeV, and in Xe+Xe collisions at root s(NN) = 5.44TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb(-1), 28 nb(-1), 0.50 nb(-1), and 3 μb(-1), respectively. The nuclear modification factors RpPb and R-AA are obtained by comparing the spectra in heavy-ion and pp collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor RpPb shows a moderate enhancement above unity with a maximum at p(T) approximate to 3 GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct p(T)-dependence with a local maximum at p(T) approximate to 2 GeV and a local minimum at p(T) approximate to 7 GeV. This dependence is more distinguishable in more central collisions. No significant eta |-dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe RAA better in central collisions and in the pT range from about 10 to 100 GeV.
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ATLAS and CMS Collaborations(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo Gimenez, V., et al. (2023). Combination of inclusive top-quark pair production cross-section measurements using ATLAS and CMS data at √s=7 and 8 TeV. J. High Energy Phys., 07(7), 213–64pp.
Abstract: A combination of measurements of the inclusive top-quark pair production cross-section performed by ATLAS and CMS in proton-proton collisions at centre-of-mass energies of 7 and 8TeV at the LHC is presented. The cross-sections are obtained using top-quark pair decays with an opposite-charge electron-muon pair in the final state and with data corresponding to an integrated luminosity of about 5 fb(-1) at root s = 7 TeV and about 20 fb(-1) at root s = 8TeV for each experiment. The combined cross-sections are determined to be 178.5 +/- 4.7 pb at root s = 7 TeV and 243.3(-5.9)(+6.0) pb at root s = 8TeV with a correlation of 0.41, using a reference top-quark mass value of 172.5 GeV. The ratio of the combined crosssections is determined to be R-8/7 = 1.363 +/- 0.032. The combined measured cross-sections and their ratio agree well with theory calculations using several parton distribution function (PDF) sets. The values of the top-quark pole mass (with the strong coupling fixed at 0.118) and the strong coupling (with the top-quark pole mass fixed at 172.5 GeV) are extracted from the combined results by fitting a next-to-next-to-leading-order plus next-to-next-to-leading-log QCD prediction to the measurements. Using a version of the NNPDF3.1 PDF set containing no top-quark measurements, the results obtained are m(t)(pole) = 173.4(-2.0)(+1.8) GeV and alpha s(m(Z)) = 0.1170(-0.0018)(+0.0021).
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Measurement of exclusive pion pair production in proton-proton collisions at √s=7 TeV with the ATLAS detector. Eur. Phys. J. C, 83(7), 627–28pp.
Abstract: The exclusive production of pion pairs in the process pp -> pp pi(+)pi(-) has been measured at root s = 7 TeV with the ATLAS detector at the LHC, using 80 μb(-1) of low-luminosity data. The pion pairs were detected in the ATLAS central detector while outgoing protons were measured in the forwardATLASALFAdetector system. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC. Across-sectionmeasurement is performed in two kinematic regions defined by the proton momenta, the pion rapidities and transverse momenta, and the pion-pion invariant mass. Cross-section values of 4.8 +/- 1.0 (stat)(-0.2) (+0.3)(syst) μb and 9 +/- 6 (stat)(-2)(+2) (syst) μb are obtained in the two regions; they are compared with theoretical models and provide a demonstration of the feasibility of measurements of this type.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Integrated and differential fiducial cross-section measurements for the vector boson fusion production of the Higgs boson in the H → WW* → eνμν decay channel at 13 TeV with the ATLAS detector. Phys. Rev. D, 108(7), 072003–52pp.
Abstract: The vector-boson production cross section for the Higgs boson decay in the H -> WW* -> e nu μnu channel is measured as a function of kinematic observables sensitive to the Higgs boson production and decay properties as well as integrated in a fiducial phase space. The analysis is performed using the proton-proton collision data collected by the ATLAS detector in Run 2 of the LHC at root s = 13 TeV center-of-mass energy, corresponding to an integrated luminosity of 139 fb(-1). The different flavor final state is studied by selecting an electron and a muon originating from a pair of W bosons and compatible with the Higgs boson decay. The data are corrected for the effects of detector inefficiency and resolution, and the measurements are compared with different state-of-the-art theoretical predictions. The differential cross sections are used to constrain anomalous interactions described by dimension-six operators in an effective field theory.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2012). A Particle Consistent with the Higgs Boson Observed with the ATLAS Detector at the Large Hadron Collider. Science, 338(6114), 1576–1582.
Abstract: Nearly 50 years ago, theoretical physicists proposed that a field permeates the universe and gives energy to the vacuum. This field was required to explain why some, but not all, fundamental particles have mass. Numerous precision measurements during recent decades have provided indirect support for the existence of this field, but one crucial prediction of this theory has remained unconfirmed despite 30 years of experimental searches: the existence of a massive particle, the standard model Higgs boson. The ATLAS experiment at the Large Hadron Collider at CERN has now observed the production of a new particle with a mass of 126 giga-electron volts and decay signatures consistent with those expected for the Higgs particle. This result is strong support for the standard model of particle physics, including the presence of this vacuum field. The existence and properties of the newly discovered particle may also have consequences beyond the standard model itself.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Measurement of the top quark mass with the template method in the t(t)over-bar -> lepton plus jets channel using ATLAS data. Eur. Phys. J. C, 72(6), 2046–30pp.
Abstract: The top quark mass has been measured using the template method in the t (t) over bar -> lepton + jets channel based on data recorded in 2011 with the ATLAS detector at the LHC. The data were taken at a proton-proton centre-of-mass energy of root s = 7 TeV and correspond to an integrated luminosity of 1.04 fb(-1). The analyses in the e + jets and μ+ jets decay channels yield consistent results. The top quark mass is measured to be m(top) = 174.5 +/- 0.6(stat) +/- 2.3(syst) GeV.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Measurement of t(t)over-bar production with a veto on additional central jet activity in pp collisions at root s=7 TeV using the ATLAS detector. Eur. Phys. J. C, 72(6), 2043–24pp.
Abstract: A measurement of the jet activity in t (t) over bar events produced in proton-proton collisions at a centre-of-mass energy of 7 TeV is presented, using 2.05 fb(-1) of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. The t (t) over bar events are selected in the dilepton decay channel with two identified b-jets from the top quark decays. Events are vetoed if they contain an additional jet with transverse momentum above a threshold in a central rapidity interval. The fraction of events surviving the jet veto is presented as a function of this threshold for four different central rapidity interval definitions. An alternate measurement is also performed, in which events are vetoed if the scalar transverse momentum sum of the additional jets in each rapidity interval is above a threshold. In both measurements, the data are corrected for detector effects and compared to the theoretical models implemented in MC@NLO, POWHEG, ALPGEN and SHERPA. The experimental uncertainties are often smaller than the spread of theoretical predictions, allowing deviations between data and theory to be observed in some regions of phase space.
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