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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2018). Observation of Centrality-Dependent Acoplanarity for Muon Pairs Produced via Two-Photon Scattering in Pb plus Pb Collisions at root s(NN)=5.02 TeV with the ATLAS Detector. Phys. Rev. Lett., 121(21), 212301–21pp.
Abstract: This Letter presents a measurement of gamma gamma -> mu(+)mu(-)- production in Pb + Pb collisions recorded by the ATLAS detector at the Large Hadron Collider at root s(NN) = 5.02 TeV with an integrated luminosity of 0.49 nb(-1). The azimuthal angle and transverse momentum correlations between the muons are measured as a function of collision centrality. The muon pairs are produced from gamma gamma through the interaction of the large electromagnetic fields of the nuclei. The contribution from background sources of muon pairs is removed using a template fit method. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of muon pair production in ultraperipheral collisions. The angular correlations are observed to broaden significantly in central collisions. The modifications arc qualitatively consistent with rescattering of the muons while passing through the hot matter produced in the collision.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Measurement of the Inelastic Proton-Proton Cross Section at root s=13 TeV with the ATLAS Detector at the LHC. Phys. Rev. Lett., 117(18), 182002–19pp.
Abstract: This Letter presents a measurement of the inelastic proton-proton cross section using 60 μb(-1) of pp collisions at a center-of-mass energy root s of 13 TeV with the ATLAS detector at the LHC. Inelastic interactions are selected using rings of plastic scintillators in the forward region (2.07 <vertical bar eta vertical bar < 3.86) of the detector. A cross section of 68.1 +/- 1.4 mb is measured in the fiducial region. xi = M-X(2) > s > 10(-6), where M-X is the larger invariant mass of the two hadronic systems separated by the largest rapidity gap in the event. In this xi range the scintillators are highly efficient. For diffractive events this corresponds to cases where at least one proton dissociates to a system with M-X > 13 GeV. The measured cross section is compared with a range of theoretical predictions. When extrapolated to the full phase space, a cross section of 78.1 +/- 2.9 mb is measured, consistent with the inelastic cross section increasing with center-of-mass energy.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo, F. L., et al. (2018). Probing the Quantum Interference between Singly and Doubly Resonant Top-Quark Production in pp Collisions at root s=13 TeV with the ATLAS Detector. Phys. Rev. Lett., 121(15), 152002–20pp.
Abstract: This Letter presents a normalized differential cross-section measurement in a fiducial phase-space region where interference effects between top-quark pair production and associated production of a single top quark with a W boson and a b-quark are significant. Events with exactly two leptons (ee, μmu, or e mu) and two b-tagged jets that satisfy a multiparticle invariant mass requirement are selected from 36.1 fb(-1) of protonproton collision data taken at root s = 13 TeV with the ATLAS detector at the LHC in 2015 and 2016. The results are compared with predictions from simulations using various strategies for the interference. The standard prescriptions for interference modeling are significantly different from each other but are within 2 sigma of the data. State-of-the-art predictions that naturally incorporate interference effects provide the best description of the data in the measured region of phase space most sensitive to these effects. These results provide an important constraint on interference models and will guide future model development and tuning.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., Ferrer, A., et al. (2015). Search for a Heavy Neutral Particle Decaying to e mu, e tau, or μtau in pp Collisions at root s=8 TeV with the ATLAS Detector. Phys. Rev. Lett., 115(3), 031801–18pp.
Abstract: This Letter presents a search for a heavy neutral particle decaying into an opposite-sign different-flavor dilepton pair, e(+/-) mu(-/+), e(+/-) tau(-/+), or mu(+/-) tau(-/+) using 20.3 fb(-1) of pp collision data at root s = 8 TeV collected by the ATLAS detector at the LHC. The numbers of observed candidate events are compatible with the standard model expectations. Limits are set on the cross section of new phenomena in two scenarios: the production of (nu) over tilde (tau) in R-parity-violating supersymmetric models and the production of a lepton-flavor-violating Z' vector boson.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2018). Search for Higgs boson decays into pairs of light (pseudo)scalar particles in the gamma gamma jj final state in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 782, 750–767.
Abstract: This Letter presents a search for exotic decays of the Higgs boson to a pair of new (pseudo) scalar particles, H -> aa, where the a particle has a mass in the range 20-60 GeV, and where one of the a bosons decays into a pair of photons and the other to a pair of gluons. The search is performed in event samples enhanced in vector-boson fusion Higgs boson production by requiring two jets with large invariant mass in addition to the Higgs boson candidate decay products. The analysis is based on the full dataset of pp collisions at root s = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 36.7 fb(-1). The data are in agreement with the Standard Model predictions and an upper limit at the 95% confidence level is placed on the production cross section times the branching ratio for the decay H -> aa -> gamma gamma gg. This limit ranges from 3.1 pb to 9.0 pb depending on the mass of the a boson.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for heavy charged long-lived particles in proton-proton collisions at root s=13 TeV using an ionisation measurement with the ATLAS detector. Phys. Lett. B, 788, 96–116.
Abstract: This Letter presents a search for heavy charged long-lived particles produced in proton-proton collisions at root s= 13 TeV at the LHC using a data sample corresponding to an integrated luminosity of 36.1 fb(-1) collected by the ATLAS experiment in 2015 and 2016. These particles are expected to travel with a velocity significantly below the speed of light, and therefore have a specific ionisation higher than any high-momentum Standard Model particle of unit charge. The pixel subsystem of the ATLAS detector is used in this search to measure the ionisation energy loss of all reconstructed charged particles which traverse the pixel detector. Results are interpreted assuming the pair production of R-hadrons as composite colourless states of a long-lived gluino and Standard Model partons. No significant deviation from Standard Model background expectations is observed, and lifetime-dependent upper limits on R-hadron production cross-sections and gluino masses are set, assuming the gluino always decays to two quarks and a 100 GeV stable neutralino. R-hadrons with lifetimes above 1.0 ns are excluded at the 95% confidence level, with lower limits on the gluino mass ranging between 1290 GeV and 2060 GeV. In the case of stable R-hadrons, the lower limit on the gluino mass at the 95% confidence level is 1890 GeV.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2019). Search for light resonances decaying to boosted quark pairs and produced in association with a photon or a jet in proton-proton collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 788, 316–335.
Abstract: This Letter presents a search for new light resonances decaying to pairs of quarks and produced in association with a high-p(T) photon or jet. The dataset consists of proton-proton collisions with an integrated luminosity of 36.1 fb(-1) at a centre-of-mass energy of root s = 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. Resonance candidates are identified as massive large-radius jets with substructure consistent with a particle decaying into a quark pair. The mass spectrum of the candidates is examined for local excesses above background. No evidence of a new resonance is observed in the data, which are used to exclude the production of a lepto-phobic axial-vector Z' boson. (C) 2018 The Author(s). Published by Elsevier B.V.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Search for heavy resonances decaying to a Z boson and a photon in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 764, 11–30.
Abstract: This Letter presents a search for new resonances with mass larger than 250 GeV, decaying to a Z boson and a photon. The dataset consists of an integrated luminosity of 3.2 fb(-1) of pp collisions collected at root s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The Z bosons are identified through their decays either to charged, light, lepton pairs (e(+) e(-), mu(+) mu(-)) or to hadrons. The data are found to be consistent with the expected background in the whole mass range investigated and upper limits are set on the production cross section times decay branching ratio to Z gamma of a narrow scalar boson with mass between 250 GeV and 2.75 TeV.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for the Production of a Long-Lived Neutral Particle Decaying within the ATLAS Hadronic Calorimeter in Association with a Z Boson from pp Collisions at root s=13 TeV. Phys. Rev. Lett., 122(15), 151801–19pp.
Abstract: This Letter presents a search for the production of a long-lived neutral particle (Z(d)) decaying within the ATLAS hadronic calorimeter, in association with a standard model (SM) Z boson produced via an intermediate scalar boson, where Z -> l(+) l(-) (l = e, mu). The data used were collected by the ATLAS detector during 2015 and 2016 pp collisions with a center-of-mass energy of root s = 13 TeV at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 +/- 0.8 fb(-1). No significant excess of events is observed above the expected background. Limits on the production cross section of the scalar boson times its decay branching fraction into the long-lived neutral particle are derived as a function of the mass of the intermediate scalar boson, the mass of the long-lived neutral particle, and its c tau from a few centimeters to one hundred meters. In the case that the intermediate scalar boson is the SM Higgs boson, its decay branching fraction to a long-lived neutral particle with a c tau approximately between 0.1 and 7 m is excluded with a 95% confidence level up to 10% for m(zd) between 5 and 15 GeV.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., Costa, M. J., et al. (2016). Evidence for single top-quark production in the s-channel in proton-proton collisions at root s=8 TeV with the ATLAS detector using the Matrix Element Method. Phys. Lett. B, 756, 228–246.
Abstract: This Letter presents evidence for single top-quark production in the s-channel using proton-proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The analysis is performed on events containing one isolated electron or muon, large missing transverse momentum and exactly two b-tagged jets in the final state. The analysed data set corresponds to an integrated luminosity of 20.3 fb(-1). The signal is extracted using a maximum-likelihood fit of a discriminant which is based on the matrix element method and optimized in order to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and W boson production in association with heavy-flavour jets. The measurement leads to an observed signal significance of 3.2 standard deviations and a measured cross-section of sigma s = 4.8 +/- 0.8(stat.)(-1.3)(+1.6)(syst.) pb, which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.
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