ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2018). Search for charged Higgs bosons decaying via H-+/- -> tau(+/-)nu(tau) in the tau plus jets and tau plus lepton final states with 36 fb(-1) of pp collision data recorded at root s=13 TeV with the ATLAS experiment. J. High Energy Phys., 09(9), 139–48pp.
Abstract: Charged Higgs bosons produced either in top-quark decays or in association with a top-quark, subsequently decaying via H-+/-! -> tau(+/-)nu(tau), are searched for in 36.1 fb(-1) of proton-proton collision data at root s = 13TeV recorded with the ATLAS detector. Depending on whether the top-quark produced together with H-+/- decays hadronically or leptonically, the search targets tau+jets and tau+lepton fi nal states, in both cases with a hadronically decaying tau-lepton. No evidence of a charged Higgs boson is found. For the mass range of m(H)+/- = 90-2000 GeV, upper limits at the 95% con fi dence level are set on the production cross-section of the charged Higgs boson times the branching fraction B (H-+/-->tau(+/-)nu(tau)) in the range 4.2-0.0025 pb. In the mass range 90{160 GeV, assuming the Standard Model cross-section for tit production, this corresponds to upper limits between 0.25% and 0.031% for the branching fraction B (t -> bH(+/-)) x B (H-+/- -> tau(+/-)nu(tau)).
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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 charged Higgs bosons produced in association with a top quark and decaying via H-+/- -> tau nu using pp collision data recorded at root s=13 TeV by the ATLAS detector. Phys. Lett. B, 759, 555–574.
Abstract: Charged Higgs bosons produced in association with a single top quark and decaying via H-+/- -> tau nu are searched for with the ATLAS experiment at the LHC, using proton-proton collision data at root s = 13 TeV corresponding to an integrated luminosity of 3.2 fb(-1). The final state is characterised by the presence of a hadronic tau decay and missing transverse momentum, as well as a hadronically decaying top quark, resulting in the absence of high-transverse-momentum electrons and muons. The data are found to be consistent with the expected background from Standard Model processes. A statistical analysis leads to 95% confidence-level upper limits on the production cross section times branching fraction, sigma (pp -> [b]tH(+/-)) x BR(H-+/- -> tau nu), between 1.9 pb and 15 fb, for charged Higgs boson masses ranging from 200 to 2000 GeV. The exclusion limits for this search surpass those obtained with the proton-proton collision data recorded at root s = 8 TeV.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo, F. L., et al. (2021). Search for charged-lepton-flavour violation in Z-boson decays with the ATLAS detector. Nat. Phys., 17, 819–825.
Abstract: Leptons with essentially the same properties apart from their mass are grouped into three families (or flavours). The number of leptons of each flavour is conserved in interactions, but this is not imposed by fundamental principles. Since the formulation of the standard model of particle physics, the observation of flavour oscillations among neutrinos has shown that lepton flavour is not conserved in neutrino weak interactions. So far, there has been no experimental evidence that this also occurs in interactions between charged leptons. Such an observation would be a sign of undiscovered particles or a yet unknown type of interaction. Here the ATLAS experiment at the Large Hadron Collider at CERN reports a constraint on lepton-flavour-violating effects in weak interactions, searching for Z-boson decays into a tau lepton and another lepton of different flavour with opposite electric charge. The branching fractions for these decays are measured to be less than 8.1 x 10(-6) (e tau) and 9.5 x 10(-6) (mu tau) at the 95% confidence level using 139 fb(-1) of proton-proton collision data at a centre-of-mass energy of root s = 13 TeV and 20.3 fb(-1) at root s = 8 TeV. These results supersede the limits from the Large Electron-Positron Collider experiments conducted more than two decades ago.
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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 chargino and neutralino production in final states with a Higgs boson and missing transverse momentum at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 100(1), 012006–37pp.
Abstract: A search is conducted for the electroweak pair production of a chargino and a neutralino pp -> (chi) over tilde (+/-)(1)(chi) over tilde (0)(2), where the chargino decays into the lightest neutralino and a W boson, (chi) over tilde (+/-)(1) -> (chi) over tilde W-0(1)+/- while the neutralino decays into the lightest neutralino and a Standard Model-like 125 GeV Higgs boson,(chi) over tilde (0)(2) -> (chi) over tilde (0)(1)h. Fully hadronic, semileptonic, diphoton, and multilepton (electrons, muons) final. states with missing transverse momentum are considered in this search. Higgs bosons in the final state are identified by either two jets originating from bottom quarks (h -> b (b) over bar), two photons (h -> gamma gamma), or leptons from the decay modes h -> WW, h -> ZZ or h -> tau tau. The analysis is based on 36.1 fb(-1) of s root s = 13 TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider. Observations are consistent with the Standard Model expectations, and 95% confidence-level limits of up to 680 GeV in (chi) over tilde (+/-)(1)/(chi) over tilde (0)(2) mass are set in the context of a simplified supersymmetric model.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo Gimenez, V., et al. (2021). Search for chargino-neutralino pair production in final states with three leptons and missing transverse momentum in root s=13 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 81(12), 1118–55pp.
Abstract: A search for chargino-neutralino pair production in three-lepton final states with missing transverse momentum is presented. The study is based on a dataset of root s = 13 TeV pp collisions recorded with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 139 fb(-1). No significant excess relative to the Standard Model predictions is found in data. The results are interpreted in simplified models of supersymmetry, and statistically combined with results from a previous ATLAS search for compressed spectra in two-lepton final states. Various scenarios for the production and decay of charginos ((chi) over tilde (+/-)(1)) and neutralinos ((chi) over tilde (0)(2)) are considered. For pure higgsino (chi) over tilde (+/-)(1)(chi) over tilde (0)(2) pair-production scenarios, exclusion limits at 95% confidence level are set on (chi) over tilde (0)(2) masses up to 210 GeV. Limits are also set for pure wino (chi) over tilde (+/-)(1)(chi) over tilde (0)(2) pair production, on (chi) over tilde (0)(2) masses up to 640 GeV for decays via on-shell W and Z bosons, up to 300 GeV for decays via off-shell W and Z bosons, and up to 190 GeV for decays via W and Standard Model Higgs bosons.
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