ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Measurement of exclusive pion pair production in proton-proton collisions at √s=7 TeV with the ATLAS detector. Eur. Phys. J. C, 83(7), 627–28pp.
Abstract: The exclusive production of pion pairs in the process pp -> pp pi(+)pi(-) has been measured at root s = 7 TeV with the ATLAS detector at the LHC, using 80 μb(-1) of low-luminosity data. The pion pairs were detected in the ATLAS central detector while outgoing protons were measured in the forwardATLASALFAdetector system. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC. Across-sectionmeasurement is performed in two kinematic regions defined by the proton momenta, the pion rapidities and transverse momenta, and the pion-pion invariant mass. Cross-section values of 4.8 +/- 1.0 (stat)(-0.2) (+0.3)(syst) μb and 9 +/- 6 (stat)(-2)(+2) (syst) μb are obtained in the two regions; they are compared with theoretical models and provide a demonstration of the feasibility of measurements of this type.
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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. (2022). Search for long-lived charginos based on a disappearing-track signature using 136 fb(-1) of pp collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 82(7), 606–34pp.
Abstract: A search for long-lived charginos produced either directly or in the cascade decay of heavy prompt gluino states is presented. The search is based on proton- proton collision data collected at a centre-of-mass energy of root s = 13 TeV between 2015 and 2018 with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 136 fb(-1). Long-lived charginos are characterised by a distinct signature of a short and then disappearing track, and are reconstructed using at least four measurements in the ATLAS pixel detector, with no subsequent measurements in the silicon-microstrip tracking volume nor any associated energy deposits in the calorimeter. The final state is complemented by a large missing transverse-momentum requirement for triggering purposes and at least one high-transversemomentum jet. No excess above the expected backgrounds is observed. Exclusion limits are set at 95% confidence level on the masses of the chargino and gluino for different chargino lifetimes. Chargino masses up to 660 (210) GeV are excluded in scenarios where the chargino is a pure wino (higgsino). For charginos produced during the cascade decay of a heavy gluino, gluinos with masses below 2.1 TeV are excluded for a chargino mass of 300 GeV and a lifetime of 0.2 ns.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Search for doubly charged Higgs boson production in multi-lepton final states using 139 fb-1 of proton-proton collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 83(7), 605–32pp.
Abstract: A search for pair production of doubly charged Higgs bosons (H-+/-+/-), each decaying into a pair of prompt, isolated, and 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 an integrated luminosity of 139 fb(-1) recorded by the ATLAS detector during Run 2 of the Large Hadron Collider (LHC). This analysis focuses on same-charge leptonic decays, H-+/-+/- -> l(+/-)l'(+/-) where l, l' = e, mu, tau, in two-, three-, and four-lepton channels, but only considers final states which include electrons or muons. No evidence of a signal is observed. Corresponding upper limits on the production cross-section of a doubly charged Higgs boson are derived, as a function of its mass m(H-+/-+/-), at 95% confidence level. Assuming that the branching ratios to each of the possible leptonic final states are equal, B(H-+/-+/- -> e(+/-)e(+/-)) = B(H-+/-+/- -> e(+/-)mu(+/-)) = B(H-+/-+/- -> mu(+/-)mu(+/-)) = B(H-+/-+/- -> e(+/-)tau(+/-)) = B(H-+/-+/- -> mu(+/-)tau(+/-)) = B(H-+/-+/- -> tau(+/-)tau(+/-)) = 1/6, the observed (expected) lower limit on the mass of a doubly charged Higgs boson is 1080GeV(1065 GeV) within the left-right symmetric type-II seesaw model, which is the strongest limit to date produced by the ATLAS Collaboration. Additionally, this paper provides the first direct test of the Zee-Babu neutrino mass model at the LHC, yielding an observed (expected) lower limit of m(H-+/-+/-) = 900 GeV (880 GeV).
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Search for dark matter produced in association with a single top quark and an energetic W boson in √s=13 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 83(7), 603–38pp.
Abstract: This paper presents a search for dark matter, chi, using events with a single top quark and an energetic W boson. The analysis is based on proton-proton collision data collected with the ATLAS experiment at root s = 13 TeV during LHC Run 2 (2015-2018), corresponding to an integrated luminosity of 139 fb(-1). The search considers final states with zero or one charged lepton (electron or muon), at least one b-jet and large missing transverse momentum. In addition, a result from a previous search considering two-charged-lepton final states is included in the interpretation of the results. The data are found to be in good agreement with the Standard Model predictions and the results are interpreted in terms of 95% confidence-level exclusion limits in the context of a class of dark matter models involving an extended two-Higgs-doublet sector together with a pseudoscalar mediator particle. The search is particularly sensitive to on-shell production of the charged Higgs boson state, H-+/-, arising from the two-Higgs-doublet mixing, and its semi-invisible decays via the mediator particle, a: H-+/- -> W(+/-)a(-> chi chi). Signal models with H-+/- masses up to 1.5 TeV and a masses up to 350 GeV are excluded assuming a tan beta value of 1. For masses of a of 150 (250) GeV, tan beta values up to 2 are excluded for H-+/- masses between 200 (400) GeV and 1.5 TeV. Signals with tan beta values between 20 and 30 are excluded for H-+/- masses between 500 and 800 GeV.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Measurement of the CP properties of Higgs boson interactions with τ-leptons with the ATLAS detector. Eur. Phys. J. C, 83(7), 563–30pp.
Abstract: A study of the charge conjugation and parity (CP) properties of the interaction between the Higgs boson and tau -leptons is presented. The study is based on a measurement of CP-sensitive angular observables defined by the visible decay products of t -leptons produced in Higgs boson decays. The analysis uses 139 fb(-1) of proton-proton collision data recorded at a centre-of-mass energy of root s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Contributions from CP-violating interactions between the Higgs boson and t -leptons are described by a single mixing angle parameter phi(tau) in the generalised Yukawa interaction. Without constraining the H -> tau tau signal strength to its expected value under the Standard Model hypothesis, the mixing angle ft is measured to be 9 degrees +/- 16 degrees, with an expected value of 0 degrees +/- 28 degrees at the 68% confidence level. The pure CPodd hypothesis is disfavoured at a level of 3.4 standard deviations. The results are compatible with the predictions for the Higgs boson in the Standard Model.
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