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 type-III seesaw heavy leptons in leptonic final states in pp collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 82(11), 988–30pp.
Abstract: A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton-proton collision data at a centre-of-mass energy of 13 TeV, corresponding to 139 fb(-1) of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on final states with three or four electrons or muons from the possible decays of new heavy leptons via intermediate electroweak bosons. No significant deviations above the Standard Model expectation are observed; upper and lower limits on the heavy lepton production cross-section and masses are derived respectively. These results are then combined for the first time with the ones already published by ATLAS using the channel with two leptons in the final state. The observed lower limit on the mass of the type-III seesaw heavy leptons combining two, three and four lepton channels together is 910 GeV at the 95% confidence level.
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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 type-III seesaw heavy leptons in dilepton final states in pp collisions at root s=13TeV with the ATLAS detector. Eur. Phys. J. C, 81(3), 218–30pp.
Abstract: A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton-proton collision data at a centre-of-mass energy of 13TeV, corresponding to 139 fb-1 of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on the final state with two light leptons (electrons or muons) of different flavour and charge combinations, with at least two jets and large missing transverse momentum. No significant excess over the Standard Model expectation is observed. The results are translated into exclusion limits on heavy-lepton masses, and the observed lower limit on the mass of the type-III seesaw heavy leptons is 790GeV at 95% confidence level.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., et al. (2023). Search for pair-production of vector-like quarks in pp collision events at root s=13 TeV with at least one leptonically decaying Z boson and a third-generation quark with the ATLAS detector. Phys. Lett. B, 843, 138019–25pp.
Abstract: A search for the pair-production of vector-like quarks optimized for decays into a Z boson and a third-generation Standard Model quark is presented, using the full Run 2 dataset corresponding to 139 fb-1 of pp collisions at & RADIC;s = 13 TeV, collected in 2015-2018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a Z boson with high transverse momentum, reconstructed from a pair of same-flavour leptons with opposite-sign charges, as well as by the presence of b-tagged jets and high-transverse-momentum large-radius jets reconstructed from calibrated smaller-radius jets. Events with exactly two or at least three leptons are used, which are further categorized by the presence of boosted W, Z, and Higgs bosons and top quarks. The categorization is performed using a neural-network-based boosted object tagger to enhance the sensitivity to signal relative to the background. No significant excess above the background expectation is observed and exclusion limits at 95% confidence level are set on the masses of the vector-like partners T and B of the top and bottom quarks, respectively. The limits depend on the branching ratio configurations and, in the case of 100% branching ratio for T-+ Zt and 100% branching ratio for B-+ Zb, this search sets the most stringent limits to date, allowing mT > 1.60 TeV and mB > 1.42 TeV, respectively.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Evidence for the production of three massive vector bosons with the ATLAS detector. Phys. Lett. B, 798, 134913–22pp.
Abstract: A search for the production of three massive vector bosons in proton-proton collisions is performed using data at root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider in the years 2015-2017, corresponding to an integrated luminosity of 79.8 fb(-1). Events with two same-sign leptons l (electrons or muons) and at least two reconstructed jets are selected to search for WWW -> l nu l nu qq. Events with three leptons without any same-flavour opposite-sign lepton pairs are used to search for WWW > l nu l nu l nu, while events with three leptons and at least one same-flavour opposite-sign lepton pair and one or more reconstructed jets are used to search for WWZ -> l nu qqll. Finally, events with four leptons are analysed to search for WWZ > l nu l nu ll and WZZ > qqllll. Evidence for the joint production of three massive vector bosons is observed with a significance of 4.1 standard deviations, where the expectation is 3.1 standard deviations.
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LHCb Collaboration(Aaij, R. et al), Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., Remon Alepuz, C., et al. (2021). Search for the rare decay B-0 -> J/psi phi. Chin. Phys. C, 45(4), 043001–14pp.
Abstract: A search for the rare decay B-0 -> J/psi phi, is performed using pp collision data collected with the LHCb detector at centre-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of 9 fb(-1). No significant signal of the decay is observed and an upper limit of 1.1 x 10(-7) at 90% confidence level is set on the branching fraction.
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LHCb Collaboration(Aaij, R. et al), Jaimes Elles, S. J., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., Rebollo De Miguel, M., et al. (2023). Search for the rare decays W+ → Ds+γ and Z → D0 γ at LHCb. Chin. Phys. C, 47(9), 093002–13pp.
Abstract: A search for the rare decays W+ -> D-s(+)gamma and Z -> D-0 gamma and is performed using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13TeV, corresponding to an integrated luminosity of 2.0fb(-1). No significant signal is observed for either decay mode and upper limits on their branching fractions are set using W+ -> mu(+)nu and Z ->mu(+)mu(-)decays as normalization channels. The upper limits are and at 95% confidence level for W+ -> D-s(+)gamma and Z -> D-0 gamma the and decay modes, respectively. This is the first reported search for Z -> D-0 gamma the decay, while the upper limit on the Z -> D-0 gamma branching fraction improves upon the previous best limit.
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BABAR Collaboration(Lees, J. P. et al), Martinez-Vidal, F., & Oyanguren, A. (2019). Search for B- -> Lambda(p)over-bar nu(nu)over-bar with the BABAR experiment. Phys. Rev. D, 100(11), 111101–8pp.
Abstract: A search for the rare flavor-changing neutral current process B- -> Lambda(p) over bar nu(nu) over bar using data from the BABAR experiment has been performed. A total of 424 fb(-1) of e(+) e(-) collision data collected at the center-of-mass energy of the Upsilon(4S) resonance is used in this study, corresponding to a sample of (471 +/- 3) x 10(6) B (B) over bar pairs. Signal B- -> Lambda(p) over bar nu(nu) over bar candidates arc identified by fast fully reconstructing a B+ decay in one of many possible exclusive decays to hadronic final states, then examining detector activity that is not associated with this reconstructed B+ decay for evidence of a signal B- -> Lambda(p) over bar nu(nu) over bar decay. The data yield is found to be consistent with the expected background contribution under a null signal hypothesis, resulting in an upper limit of B(B- -> Lambda(p) over bar nu(nu) over bar) < 3.0 x 10(-5) at the 90% confidence level.
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LHCb Collaboration(Aaij, R. et al), Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., Rebollo De Miguel, M., Remon Alepuz, C., et al. (2023). Search for the rare hadronic decay Bs0 → p(p)over-bar. Phys. Rev. D, 108(1), 012007–12pp.
Abstract: A search for the rare hadronic decay B-s(0) -> p (p) over bar is performed using proton-proton collision data recorded by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 6 fb(-1). No evidence of the decay is found and an upper limit on its branching fraction is set at B(B-s(0) -> p (p) over bar) < 4.4(5.1)x 10(-9) at 90% (95%) confidence level; this is currently the world's best upper limit. The decay mode B-0 -> p<(p)over bar> is measured with very large significance, confirming the first observation by the LHCb experiment in 2017. The branching fraction is determined to be B(B-0 -> p (p) over bar) = (1.27 +/- 0.15 +/- 0.05 +/- 0.04) 10(-8), where the first uncertainty is statistical, the second is systematic and the third is due to the external branching fraction of the normalization channel B-0 -> K+pi(-). The combination of the two LHCb measurements of the B-0 -> p (p) over bar branching fraction yields B(B-0 -> p (p) over bar) (1.27 +/- 0.13 +/- 0.05 +/- 0.03) x 10(-8).
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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., et al. (2019). Search for the rare decay B+ -> mu(+) mu(-) mu(+)nu(mu). Eur. Phys. J. C, 79(8), 675–12pp.
Abstract: A search for the rare leptonic decay B +. μ+ μ- μ+.mu is performed using proton- proton collision data corresponding to an integrated luminosity of 4.7 fb – 1 collected by the LHCb experiment. The search is carried out in the region where the lowest of the two μ+ μ- mass combinations is below 980 MeV/ c2. The data are consistent with the background- only hypothesis and an upper limit of 1.6x10 – 8 at 95% confidence level is set on the branching fraction in the stated kinematic region.
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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 squarks and gluinos in final states with jets and missing transverse momentum using 139 fb(-1) of root s=13 TeV pp collision data with the ATLAS detector. J. High Energy Phys., 02(2), 143–64pp.
Abstract: A search for the supersymmetric partners of quarks and gluons (squarks and gluinos) in final states containing jets and missing transverse momentum, but no electrons or muons, is presented. The data used in this search were recorded by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of root s = 13 TeV during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb(-1). The results are interpreted in the context of various R-parity-conserving models where squarks and gluinos are produced in pairs or in association and a neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 2.30 TeV for a simplified model containing only a gluino and the lightest neutralino, assuming the latter is massless. For a simplified model involving the strong production of mass-degenerate first- and second-generation squarks, squark masses below 1.85 TeV are excluded if the lightest neutralino is massless. These limits extend substantially beyond the region of supersymmetric parameter space excluded previously by similar searches with the ATLAS detector.
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