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 one lepton, missing transverse momentum and a Higgs boson decaying into two b-jets in pp collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 80(8), 691–29pp.
Abstract: The results of a search for electroweakino pair production pp -> (chi) over tilde (+/-)(1) (chi) over tilde (0)(2) in which the chargino ((chi) over tilde (+/-)(1)) decays into a W boson and the lightest neutralino ((chi) over tilde (0)(1)), while the heavier neutralino ((chi) over tilde (0)(2)) decays into the Standard Model 125 GeV Higgs boson and a second (chi) over tilde (0)(1) are presented. The signal selection requires a pair of b-tagged jets consistent with those from a Higgs boson decay, and either an electron or a muon from the W boson decay, together with missing transverse momentum from the corresponding neutrino and the stable neutralinos. The analysis is based on data corresponding to 139 fb(-1) of root s = 13 TeV pp collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. No statistically significant evidence of an excess of events above the Standard Model expectation is found. Limits are set on the direct production of the electroweakinos in simplified models, assuming pure wino cross-sections. Masses of (chi) over tilde (+/-)(1) (chi) over tilde (0)(2) up to 740 GeV are excluded at 95% confidence level for a massless (chi) over tilde (0)(1).
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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 direct top squark pair production in final states with two leptons in root s=13 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 77(12), 898–41pp.
Abstract: The results of a search for direct pair production of top squarks in events with two opposite-charge leptons (electrons or muons) are reported, using 36.1 fb(-1) of integrated luminosity from proton-proton collisions at root s = 13 TeV collected by the ATLAS detector at the Large Hadron Collider. To cover a range of mass differences between the top squark (t) over tilde and lighter supersymmetric particles, four possible decay modes of the top squark are targeted with dedicated selections: the decay (t) over tilde -> b (chi) over tilde (+/-)(1) into a b-quark and the lightest chargino with (chi) over tilde (+/-)(1) W (chi) over tilde (0)(1),the decay (t) over tilde -> t (chi) over tilde (0)(1) into an on-shell top quark and the lightest neutralino, the three-body decay (t) over tilde -> bW (chi) over tilde (0)(1) and the four-body decay (t) over tilde -> bl nu(chi) over tilde (0)(1). No significant excess of events is observed above the Standard Model background for any selection, and limits on top squarks are set as a function of the (t) over tilde and (chi) over tilde (0)(1) masses. The results exclude at 95% confidence level (t) over tilde masses up to about 720 GeV, extending the exclusion region of supersymmetric parameter space covered by previous searches.
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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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