ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Search for supersymmetry in a final state containing two photons and missing transverse momentum in root s=13 TeV pp collisions at the LHC using the ATLAS detector. Eur. Phys. J. C, 76(9), 517–23pp.
Abstract: A search has been made for supersymmetry in a final state containing two photons and missing transverse momentum using the ATLAS detector at the Large Hadron Collider. The search makes use of 3.2 fb-1 of proton-proton collision data collected at a centre-of-mass energy of 13 TeV in 2015. Using a combination of data-driven and Monte-Carlo-based approaches, the Standard Model background is estimated to be 0.27(-0.10)(+0.22) events. No events are observed in the signal region; considering the expected background and its uncertainty, this observation implies a model-independent 95 % CL upper limit of 0.93 fb (3.0 events) on the visible cross section due to physics beyond the Standard Model. In the context of a generalized model of gauge-mediated supersymmetry breaking with a bino-like next-to-lightest super-symmetric particle, this leads to a lower limit of 1650 GeV on the mass of a degenerate octet of gluino states, independent of the mass of the lighter bino-like neutralino.
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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). Search for bottom squark pair production in proton-proton collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 76(10), 547–25pp.
Abstract: The result of a search for pair production of the supersymmetric partner of the Standard Model bottom quark ((b) over tilde (1)) is reported. The search uses 3.2 fb(-1) of pp collisions at root s = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider in 2015. Bottom squarks are searched for in events containing large missing transverse momentum and exactly two jets identified as originating from b-quarks. No excess above the expected Standard Model background yield is observed. Exclusion limits at 95 % confidence level on the mass of the bottom squark are derived in phenomenological supersymmetric R-parity-conserving models in which the (b) over tilde (1) is the lightest squark and is assumed to decay exclusively via (b) over tilde (1) -> b (chi) over tilde (0)(1), where (chi) over tilde (0)(1) is the lightest neutralino. The limits significantly extend previous results; bottom squark masses up to 800 (840) GeV are excluded for the. (chi) over tilde (0)(1) mass below 360 (100) GeV whilst differences in mass above 100 GeV between the (b) over tilde (1) and the (chi) over tilde (0)(1) are excluded up to a (b) over tilde (1) mass of 500 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. (2016). Search for the Higgs boson produced in association with a W boson and decaying to four b-quarks via two spin-zero particles in pp collisions at 13 TeV with the ATLAS detector. Eur. Phys. J. C, 76(11), 605–31pp.
Abstract: This paper presents a dedicated search for exotic decays of the Higgs boson to a pair of new spin-zero particles, H -> aa, where the particle a decays to b-quarks and has a mass in the range of 20-60 GeV. The search is performed in events where the Higgs boson is produced in associationwith aW boson, giving rise to a signature of a lepton (electron or muon), missing transverse momentum, and multiple jets from b-quark decays. The analysis is based on the full dataset of pp collisions at root s = 13 TeV recorded in 2015 by theATLAS detector at theCERNLargeHadron Collider, corresponding to an integrated luminosity of 3.2 fb(-1). No significant excess of events above the Standard Model prediction is observed, and a 95% confidence-level upper limit is derived for the product of the production cross section for pp -> WH times the branching ratio for the decay H -> aa -> 4b. The upper limit ranges from 6.2 pb for an a-boson mass m(a) = 20 GeV to 1.5 pb for m(a) = 60 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. (2017). Measurement of forward-backward multiplicity correlations in lead-lead, proton-lead, and proton-proton collisions with the ATLAS detector. Phys. Rev. C, 95(6), 064914–30pp.
Abstract: Two-particle pseudorapidity correlations are measured in root s(NN) = 2.76 TeV Pb + Pb, root s(NN) = 5.02 TeV p+Pb, and root s = 13 TeV pp collisions at the Large Hadron Collider (LHC), with total integrated luminosities of approximately 7 μb(-1), 28 nb(-1), and 65 nb(-1), respectively. The correlation function CN(eta(1),eta(2))is measured as a function of event multiplicity using charged particles in the pseudorapidity range |eta| < 2.4. The correlation function contains a significant short-range component, which is estimated and subtracted. After removal of the short-range component, the shape of the correlation function is described approximately by 1 + < a(1)(2)>(1/2) eta(1) eta(2) in all collision systems over the full multiplicity range. The values of < a(1)(2)>(1/2) are consistent for the opposite-charge pairs and same-charge pairs, and for the three collision systems at similar multiplicity. The values of < a(1)(2)>(1/2) and the magnitude of the short-range component both follow a power-law dependence on the event multiplicity. The short-range component in p + Pb collisions, after symmetrizing the proton and lead directions, is found to be smaller at a given eta than in pp collisions with comparable multiplicity.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). The performance of the jet trigger for the ATLAS detector during 2011 data taking. Eur. Phys. J. C, 76(10), 526–47pp.
Abstract: The performance of the jet trigger for the ATLAS detector at the LHC during the 2011 data taking period is described. During 2011 the LHC provided proton-proton collisions with a centre-of-mass energy of 7 TeV and heavy ion collisions with a 2.76 TeV per nucleon-nucleon collision energy. The ATLAS trigger is a three level system designed to reduce the rate of events from the 40 MHz nominal maximum bunch crossing rate to the approximate 400 Hz which can be written to offline storage. The ATLAS jet trigger is the primary means for the online selection of events containing jets. Events are accepted by the trigger if they contain one or more jets above some transverse energy threshold. During 2011 data taking the jet trigger was fully efficient for jets with transverse energy above 25 GeV for triggers seeded randomly at Level 1. For triggers which require a jet to be identified at each of the three trigger levels, full efficiency is reached for offline jets with transverse energy above 60 GeV. Jets reconstructed in the final trigger level and corresponding to offline jets with transverse energy greater than 60 GeV, are reconstructed with a resolution in transverse energy with respect to offline jets, of better than 4 % in the central region and better than 2.5 % in the forward direction.
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