ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2015). Measurement of the production and lepton charge asymmetry of W bosons in Pb plus Pb collisions at root s(NN)=2.76 TeV with the ATLAS detector. Eur. Phys. J. C, 75(1), 23–30pp.
Abstract: A measurement of W boson production in lead-lead collisions at root s(NN) = 2.76 TeV is presented. It is based on the analysis of data collected with the ATLAS detector at the LHC in 2011 corresponding to an integrated luminosity of 0.14 nb(-1) and 0.15 nb(-1) in the muon and electron decay channels, respectively. The differential production yields and lepton charge asymmetry are each measured as a function of the average number of participating nucleons < N-part > and absolute pseudorapidity of the charged lepton. The results are compared to predictions based on next-to-leading-order QCD calculations. These measurements are, in principle, sensitive to possible nuclear modifications to the parton distribution functions and also provide information on scaling of W boson production in multi-nucleon systems.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., et al. (2019). Measurement of W-+/- boson production in Pb plus Pb collisions at root s(NN)=5.02 TeV with the ATLAS detector. Eur. Phys. J. C, 79(11), 935–32pp.
Abstract: A measurement of W-+/- boson production in Pb+Pb collisions at root s(NN) = 5.02 TeV is reported using data recorded by the ATLAS experiment at the LHC in 2015, corresponding to a total integrated luminosity of 0.49 nb(-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.
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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). Model-independent search for the presence of new physics in events including H → γγ with √s=13 TeV pp data recorded by the ATLAS detector at the LHC. J. High Energy Phys., 07(7), 176–51pp.
Abstract: A model-independent search for new physics leading to final states containing a Higgs boson, with a mass of 125.09 GeV, decaying to a pair of photons is performed with 139 fb(-1) of p root s = 13TeV pp collision data recorded by the ATLAS detector at the Large Hadron Collider at CERN. This search examines 22 final states categorized by the objects that are produced in association with the Higgs boson. These objects include isolated electrons or muons, hadronically decaying iota -leptons, additional photons, missing transverse momentum, and hadronic jets, as well as jets that are tagged as containing a b-hadron. No significant excesses above Standard Model expectations are observed and limits on the production cross section at 95% confidence level are set. Detector efficiencies are reported for all 22 signal regions, which can be used to convert detector-level cross-section limits reported in this paper to particle-level cross-section constraints.
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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. (2021). Search for exotic decays of the Higgs boson into long-lived particles in pp collisions at root s=13 TeV using displaced vertices in the ATLAS inner detector. J. High Energy Phys., 11(11), 229–41pp.
Abstract: A novel search for exotic decays of the Higgs boson into pairs of long-lived neutral particles, each decaying into a bottom quark pair, is performed using 139 fb(-1)of root s = 13 TeV proton-proton collision data collected with the ATLAS detector at the LHC. Events consistent with the production of a Higgs boson in association with a leptonically decaying Z boson are analysed. Long-lived particle (LLP) decays are reconstructed from inner-detector tracks as displaced vertices with high mass and track multiplicity relative to Standard Model processes. The analysis selection requires the presence of at least two displaced vertices, effectively suppressing Standard Model backgrounds. The residual background contribution is estimated using a data-driven technique. No excess over Standard Model predictions is observed, and upper limits are set on the branching ratio of the Higgs boson to LLPs. Branching ratios above 10% are excluded at 95% confidence level for LLP mean proper lifetimes c tau as small as 4 mm and as large as 100 mm. For LLP masses below 40 GeV, these results represent the most stringent constraint in this lifetime regime.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2014). A neural network clustering algorithm for the ATLAS silicon pixel detector. J. Instrum., 9, P09009–34pp.
Abstract: A novel technique to identify and split clusters created by multiple charged particles in the ATLAS pixel detector using a set of artificial neural networks is presented. Such merged clusters are a common feature of tracks originating from highly energetic objects, such as jets. Neural networks are trained using Monte Carlo samples produced with a detailed detector simulation. This technique replaces the former clustering approach based on a connected component analysis and charge interpolation. The performance of the neural network splitting technique is quantified using data from proton-proton collisions at the LHC collected by the ATLAS detector in 2011 and from Monte Carlo simulations. This technique reduces the number of clusters shared between tracks in highly energetic jets by up to a factor of three. It also provides more precise position and error estimates of the clusters in both the transverse and longitudinal impact parameter resolution.
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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). Determination of the Strange-Quark Density of the Proton from ATLAS Measurements of the W -> lv and Z -> ll Cross Sections. Phys. Rev. Lett., 109(1), 012001–17pp.
Abstract: A QCD analysis is reported of ATLAS data on inclusive W-+/- and Z boson production in pp collisions at the LHC, jointly with ep deep-inelastic scattering data from HERA. The ATLAS data exhibit sensitivity to the light quark sea composition and magnitude at Bjorken x similar to 0:01. Specifically, the data support the hypothesis of a symmetric composition of the light quark sea at low x. The ratio of the strange-to-down sea quark distributions is determined to be 1:00(-0:28)(+0.25) at absolute four-momentum transfer squared Q(2) = 1: 9 GeV2 and x = 0: 023.
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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. (2013). Search for a light charged Higgs boson in the decay channel H+ -> c(s)over-bar in t(t)over-bar events using pp collisions at root s=7 TeV with the ATLAS detector. Eur. Phys. J. C, 73(6), 2465–20pp.
Abstract: A search for a charged Higgs boson (H+) in t (t) over bar decays is presented, where one of the top quarks decays via t -> H(+)b, followed by H+ -> two jets (c (s) over bar). The other top quark decays to Wb, where the W boson then decays into a lepton (e/mu) and a neutrino. The data were recorded in pp collisions at root s = 7 TeV by the ATLAS detector at the LHC in 2011, and correspond to an integrated luminosity of 4.7 fb(-1). With no observation of a signal, 95 % confidence level (CL) upper limits are set on the decay branching ratio of top quarks to charged Higgs bosons varying between 5 % and 1 % for H+ masses between 90 GeV and 150 GeV, assuming B(H+ -> c (s) over bar) = 100 %.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Search for a Charged Higgs Boson Produced in the Vector-Boson Fusion Mode with Decay H-+/- -> W(+/-)Z using pp Collisions at root S=8 TeV with the ATLAS Experiment. Phys. Rev. Lett., 114(23), 231801–18pp.
Abstract: A search for a charged Higgs boson, H-+/-, decaying to a W-+/- boson and a Z boson is presented. The search is based on 20.3 fb(-1) of proton-proton collision data at a center-of-mass energy of 8 TeV recorded with the ATLAS detector at the LHC. The H-+/- boson is assumed to be produced via vector-boson fusion and the decays W-+/- -> q (q') over bar and Z -> e(+)e(-)/mu(+)mu(-) are considered. The search is performed in a range of charged Higgs boson masses from 200 to 1000 GeV. No evidence for the production of an H+ boson is observed. Upper limits of 31-1020 fb at 95% C.L. are placed on the cross section for vector-boson fusion production of an H-+/- boson times its branching fraction to W(+/-)Z. The limits are compared with predictions from the Georgi-Machacek Higgs triplet model.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Search for a light charged Higgs boson in t → H±b decays, with H± → cb, in the lepton plus jets final state in proton-proton collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 09(9), 004–52pp.
Abstract: A search for a charged Higgs boson, H-+/-, produced in top-quark decays, t -> H(+/-)b, is presented. The search targets H-+/- decays into a bottom and a charm quark, H-+/- -> cb. The analysis focuses on a selection enriched in top-quark pair production, where one top quark decays into a leptonically decaying W boson and a bottom quark, and the other top quark decays into a charged Higgs boson and a bottom quark. This topology leads to a lepton-plus-jets final state, characterised by an isolated electron or muon and at least four jets. The search exploits the high multiplicity of jets containing b-hadrons, and deploys a neural network classifier that uses the kinematic differences between the signal and the background. The search uses a dataset of proton-proton collisions collected at a centre-of-mass energy root s = 13TeV between 2015 and 2018 with the ATLAS detector at CERN's Large Hadron Collider, amounting to an integrated luminosity of 139 fb(-1). Observed (expected) 95% confidence-level upper limits between 0.15% (0.09%) and 0.42% (0.25%) are derived for the product of branching fractions B( t -> H-+/- b) x B( H +/- -> cb) for charged Higgs boson masses between 60 and 160 GeV, assuming the SM production of the top-quark pairs.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., et al. (2020). Search for direct production of electroweakinos in final states with missing transverse momentum and a Higgs boson decaying into photons in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 005–46pp.
Abstract: A search for a chargino-neutralino pair decaying via the 125 GeV Higgs boson into photons is presented. The study is based on the data collected between 2015 and 2018 with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 139 fb(-1) of pp collisions at a centre-of-mass energy of 13 TeV. No significant excess over the expected background is observed. Upper limits at 95% confidence level for a massless (chi) over tilde (0)(1) are set on several electroweakino production cross-sections and the visible cross-section for beyond the Standard Model processes. In the context of simplified supersymmetric models, 95% confidence-level limits of up to 310 GeV in m((chi) over tilde (+/-)(1)/(chi) over tilde (0)(2)), where m((chi) over tilde (0)(1)) = 0.5 GeV, are set. Limits at 95% confidence level are also set on the (chi) over tilde (+/-)(1)(chi) over tilde (0)(2) cross-section in the mass plane of m((chi) over tilde (+/-)(1)/(chi) over tilde (0)(2)) and m((chi) over tilde (0)(1)), and on scenarios with gravitino as the lightest supersymmetric particle. Upper limits at the 95% confidence-level are set on the higgsino production cross-section. Higgsino masses below 380 GeV are excluded for the case of the higgsino fully decaying into a Higgs boson and a gravitino.
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