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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). Measurement of longitudinal flow decorrelations in Pb plus Pb collisions at root s(NN)=2.76 and 5.02 TeV with the ATLAS detector. Eur. Phys. J. C, 78(2), 142–37pp.
Abstract: Measurements of longitudinal flow correlations are presented for charged particles in the pseudorapidity range vertical bar eta vertical bar < 2.4 using 7 μb(-1) and 470 μb(-1) of Pb+Pb collisions at root s(NN) = 2.76 and 5.02 TeV, respectively, recorded by the ATLAS detector at the LHC. It is found that the correlation between the harmonic flow coefficients v(n) measured in two separated eta intervals does not factorise into the product of single-particle coefficients, and this breaking of factorisation, or flow decorrelation, increases linearly with the eta separation between the intervals. The flow decorrelation is stronger at 2.76 TeVthan at 5.02 TeV. Higher-order moments of the correlations are also measured, and the corresponding linear coefficients for the kth-moment of the v(n) are found to be proportional to k for v(3), but not for v(2). The decorrelation effect is separated into contributions from the magnitude of v(n) and the event-plane orientation, each as a function of eta. These two contributions are found to be comparable. The longitudinal flow correlations are also measured between v(n) of different order in n. The decorrelations of v(2) and v(3) are found to be independent of each other, while the decorrelations of v(4) and v(5) are found to be driven by the nonlinear contribution from v(2)(2) and v(2)v(3), respectively.
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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 doubly charged Higgs boson production in multi-lepton final states with the ATLAS detector using proton-proton collisions at root s=13 TeV. Eur. Phys. J. C, 78(3), 199–34pp.
Abstract: A search for doubly charged Higgs bosons with pairs of prompt, isolated, highly energetic leptons with the same electric charge is presented. The search uses a proton-proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to 36.1 fb(-1) of integrated luminosity recorded in 2015 and 2016 by the ATLAS detector at the LHC. This analysis focuses on the decays H-+/-+/- -> e(+/-)e(+/-), H-+/-+/- -> e(+/-)mu(+/-) and H-+/-+/- -> mu(+/-)mu(+/-), fitting the dilepton mass spectra in several exclusive signal regions. No significant evidence of a signal is observed and corresponding limits on the production cross-section and consequently a lower limit on m(H-+/-+/-) are derived at 95% confidence level. With l(+/-)l(+/-) = e(+/-)e(+/-)/mu(+/-)mu(+/-)/e(+/-)mu(+/-), the observed lower limit on the mass of a doubly charged Higgs boson only coupling to left-handed leptons varies from 770 to 870GeV (850GeV expected) for B(H-+/-+/- -> l(+/-)l(+/-)) = 100% and both the expected and observed mass limits are above 450GeV for B(H-+/-+/- -> l(+/-)l(+/-)) = 10% and any combination of partial branching ratios.
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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 dark matter produced in association with bottom or top quarks in root s=13 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 78(1), 18–36pp.
Abstract: A search for weakly interacting massive dark matter particles produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and missing transverse momentum are considered. The analysis uses 36.1 fb(-1) of proton proton collision data recorded by the ATLAS experiment at root s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are interpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour-neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross-section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour-charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., et al. (2019). Search for pair production of Higgs bosons in the b(b)over-barb(b)over-bar final state using proton-proton collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 01(1), 030–49pp.
Abstract: A search for Higgs boson pair production in the bbbb final state is carried out with up to 36.1 fb(-1) of LHC proton-proton collision data collected at s=13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. Two analyses are carried out, each implementing a particular technique for the event reconstruction that targets Higgs bosons reconstructed as pairs of jets or single boosted jets. The resonance mass range covered is 260-3000 GeV. The analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to bbbb are set in each model. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction.
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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 new phenomena in high-mass final states with a photon and a jet from pp collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 78(2), 102–25pp.
Abstract: A search is performed for new phenomena in events having a photon with high transverse momentum and a jet collected in 36.7 fb(-1) of proton-proton collisions at a centre-of-mass energy of root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The invariant mass distribution of the leading photon and jet is examined to look for the resonant production of new particles or the presence of new high-mass states beyond the Standard Model. No significant deviation from the background-only hypothesis is observed and cross-section limits for generic Gaussian-shaped resonances are extracted. Excited quarks hypothesized in quark compositeness models and high-mass states predicted in quantum black hole models with extra dimensions are also examined in the analysis. The observed data exclude, at 95% confidence level, the mass range below 5.3 TeV for excited quarks and 7.1 TeV (4.4 TeV) for quantum black holes in the Arkani-Hamed-Dimopoulos-Dvali (Randall-Sundrum) model with six (one) extra dimensions.
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