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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 doubly charged scalar bosons decaying into same-sign W boson pairs with the ATLAS detector. Eur. Phys. J. C, 79(1), 58–30pp.
Abstract: A search for doubly charged scalar bosons decaying into W boson pairs is presented. It uses a data sample from proton-proton collisions corresponding to an integrated luminosity of 36.1fb-1 collected by the ATLAS detector at the LHC at a centre-of-mass energy of 13TeV in 2015 and 2016. This search is guided by a model that includes an extension of the Higgs sector through a scalar triplet, leading to a rich phenomenology that includes doubly charged scalar bosons H +/-+/-. Those bosons are produced in pairs in proton-proton collisions and decay predominantly into electroweak gauge bosons H +/-+/- W +/- W +/-. Experimental signatures with several leptons, missing transverse energy and jets are explored. No significant deviations from the Standard Model predictions are found. The parameter space of the benchmark model is excluded at 95% confidence level for H +/-+/- bosons with masses between 200 and 220 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. (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., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2020). Search for displaced vertices of oppositely charged leptons from decays of long-lived particles in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 801, 135114–21pp.
Abstract: A search for long-lived particles decaying into an oppositely charged lepton pair, μmu, ee, or e mu, is presented using 32.8 fb(-1) of pp collision data collected at root s = 13 TeV by the ATLAS detector at the LHC. Candidate leptons are required to form a vertex, within the inner tracking volume of ATLAS, displaced from the primary pp interaction region. No lepton pairs with an invariant mass greater than 12 GeV are observed, consistent with the background expectations derived from data. The detection efficiencies for generic resonances with lifetimes (c tau) of 100-1000 mm decaying into a dilepton pair with masses between 0.1-1.0 TeV are presented as a function of p(T) and decay radius of the resonances to allow the extraction of upper limits on the cross sections for theoretical models. The result is also interpreted in a supersymmetric model in which the lightest neutralino, produced via squark-antisquark production, decays into l(+) l'(-) nu(l, l' = e, mu) with a finite lifetime due to the presence of R-parity violating couplings. Cross-section limits are presented for specific squark and neutralino masses. For a 700 GeV squark, neutralinos with masses of 50-500 GeV and mean proper lifetimes corresponding to c tau values between 1 mm to 6 m are excluded. For a 1.6 TeV squark, c tau values between 3 mm to 1 m are excluded for 1.3 TeV neutralinos.
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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. (2017). Search for direct top squark pair production in events with a Higgs or Z boson, and missing transverse momentum in root s=13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 08(8), 006–46pp.
Abstract: A search for direct top squark pair production resulting in events with either a same-flavour opposite-sign dilepton pair with invariant mass compatible with a Z boson or a pair of jets compatible with a Standard Model (SM) Higgs boson (h) is presented. Requirements on the missing transverse momentum, together with additional selections on leptons, jets, jets identified as originating from b-quarks are imposed to target the other decay products of the top squark pair. The analysis is performed using proton-proton collision data at root s = 13 TeV collected with the ATLAS detector at the LHC in 20152016, corresponding to an integrated luminosity of 36.1 fb(-1). No excess is observed in the data with respect to the SM predictions. The results are interpreted in two sets of models. In the first set, direct production of pairs of lighter top squarks ((t) over tilde (1)) with long decay chains involving Z or Higgs bosons is considered. The second set includes direct pair production of the heavier top squark pairs ((t) over tilde (2)) decaying via (t) over tilde (2) -> Z (t) over tilde (1) or (t) over tilde (2) -> h (t) over tilde (1). The results exclude at 95% confidence level (t) over tilde (2) and (t) over tilde (1) masses up to about 800 GeV, extending the exclusion region of supersymmetric parameter space covered by previous LHC searches.
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