ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo Gimenez, V., et al. (2022). Measurement of the energy response of the ATLAS calorimeter to charged pions from W-+/- -> tau(+/-) (->pi(+/-)nu(tau))nu(tau) events in Run 2 data. Eur. Phys. J. C, 82(3), 223–31pp.
Abstract: The energy response of the ATLAS calorimeter is measured for single charged pions with transverse momentum in the range 10<p(T) < 300 GeV. The measurement is performed using 139 fb(-1) of LHC proton-proton collision data at root s =13 TeV taken in Run 2 by the ATLAS detector. Charged pions originating from tau-lepton decays are used to provide a sample of high-p(T) isolated particles, where the composition is known, to test an energy regime that has not previously been probed by in situ single-particle measurements. The calorimeter response to single-pions is observed to be overestimated by similar to 2% across a large part of the p(T) spectrum in the central region and underestimated by similar to 4% in the endcaps in the ATLAS simulation. The uncertainties in the measurements are less than or similar to 1% for 15<p(T) <185 GeV in the central region. To investigate the source of the discrepancies, the width of the distribution of the ratio of calorimeter energy to track momentum, the energies per layer and response in the hadronic calorimeter are also compared between data and simulation.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., et al. (2022). Search for flavour-changing neutral-current interactions of a top quark and a gluon in pp collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 82(4), 334–35pp.
Abstract: A search is presented for the production of a single top quark via left-handed flavour-changing neutralcurrent (FCNC) interactions of a top quark, a gluon and an up or charm quark. Two production processes are considered: u + g -> t and c + g -> t. The analysis is based on proton-proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. The data set corresponds to an integrated luminosity of 139 fb(-1). Events with exactly one electron or muon, exactly one b-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: sigma(u + g -> t) x B(t -> Wb) x B(W -> l nu) < 3.0 pb and sigma(c + g -> t) xB(t -> Wb) xB(W -> l nu) < 4.7pb at the 95% confidence level, with B(W -> l nu) = 0.325 being the sum of branching ratios of all three leptonic decay modes of the W boson. Based on the framework of an effective field theory, the cross-section limits are translated into limits on the strengths of the tug and tcg couplings occurring in the theory: vertical bar C-uG(ut)vertical bar Lambda(2) < 0.057 TeV-2 and vertical bar C-uG(ct)vertical bar Lambda(2) < 0.14 TeV-2. These bounds correspond to limits on the branching ratios of FCNC-induced top-quark decays: B(t -> u + g) < 0.61 x 10(-4) and B(t -> c + g) < 3.7 x 10(-4).
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo Gimenez, V., et al. (2022). Search for resonant pair production of Higgs bosons in the b(b)over-barb(b)over-bar final state using pp collisions at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 105(9), 092002–36pp.
Abstract: A search for resonant Higgs boson pair production in the b (b) over barb (b) over bar final state is presented. The analysis uses 126 fb(-1)- 139 fb(-1) of pp collision data at root s = 13 TeV collected with the ATLAS detector at the Large Hadron Collider. The analysis is divided into two channels, targeting Higgs boson decays which are reconstructed as pairs of small-radius jets or as individual large-radius jets. Spin-0 and spin2 benchmark signal models are considered, both of which correspond to resonant HH production via gluon-gluon fusion. The data are consistent with Standard Model predictions. Upper limits are set on the production cross section times branching ratio to Higgs boson pairs of a new resonance in the mass range from 251 GeV to 5 TeV.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., et al. (2022). Search for associated production of a Z boson with an invisibly decaying Higgs boson or dark matter candidates root s=13 TeV with the ATLAS detector. Phys. Lett. B, 829, 137066–25pp.
Abstract: A search for invisible decays of the Higgs boson as well as searches for dark matter candidates, produced together with a leptonically decaying Z boson, are presented. The analysis is performed using proton-proton collisions at a centre-of-mass energy of 13 TeV, delivered by the LHC, corresponding to an integrated luminosity of 139 fb(-1) and recorded by the ATLAS experiment. Assuming Standard Model cross-sections for Z H production, the observed (expected) upper limit on the branching ratio of the Higgs boson to invisible particles is found to be 19% (19%) at the 95% confidence level. Exclusion limits are also set for simplified dark matter models and two-Higgs-doublet models with an additional pseudoscalar mediator.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo Gimenez, V., et al. (2022). Measurement of the nuclear modification factor for muons from charm and bottom hadrons in Pb plus Pb collisions at 5.02 TeV with the ATLAS detector. Phys. Lett. B, 829, 137077–23pp.
Abstract: Heavy-flavour hadron production provides information about the transport properties and microscopic structure of the quark-gluon plasma created in ultra-relativistic heavy-ion collisions. A measurement of the muons from semileptonic decays of charm and bottom hadrons produced in Pb+Pb and pp collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV with the ATLAS detector at the Large Hadron Collider is presented. The Pb+Pb data were collected in 2015 and 2018 with sampled integrated luminosities of 208 μb(-1) and 38 μb(-1), respectively, and pp data with a sampled integrated luminosity of 1.17 pb(-1) were collected in 2017. Muons from heavy-flavour semileptonic decays are separated from the light-flavour hadronic background using the momentum imbalance between the inner detector and muon spectrometer measurements, and muons originating from charm and bottom decays are further separated via the muon track's transverse impact parameter. Differential yields in Pb+Pb collisions and differential cross sections in pp collisions for such muons are measured as a function of muon transverse momentum from 4 GeV to 30 GeV in the absolute pseudorapidity interval vertical bar eta vertical bar < 2. Nuclear modification factors for charm and bottom muons are presented as a function of muon transverse momentum in intervals of Pb+Pb collision centrality. The bottom muon results are the most precise measurement of b quark nuclear modification at low transverse momentum where reconstruction of B hadrons is challenging. The measured nuclear modification factors quantify a significant suppression of the yields of muons from decays of charm and bottom hadrons, with stronger effects for muons from charm hadron decays.
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