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 long-lived particles produced in pp collisions at root s=13 TeV that decay into displaced hadronic jets in the ATLAS muon spectrometer. Phys. Rev. D, 99(5), 052005–36pp.
Abstract: A search for the decay of neutral, weakly interacting, long-lived particles using data collected by the ATLAS detector at the LHC is presented. The analysis in this paper uses 36.1 fb(-1) of proton-proton collision data at root s =13 TeV recorded in 2015-2016. The search employs techniques for reconstructing vertices of long-lived particles decaying into jets in the muon spectrometer exploiting a two-vertex strategy and a novel technique that requires only one vertex in association with additional activity in the detector that improves the sensitivity for longer lifetimes. The observed numbers of events are consistent with the expected background and limits for several benchmark signals are determined.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo, F. L., et al. (2019). Search for long-lived particles in final states with displaced dimuon vertices in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 99(1), 012001–32pp.
Abstract: A search is performed for a long-lived particle decaying into a final state that includes a pair of muons of opposite-sign electric charge, using proton-proton collision data collected at root s = 13 TeV by the ATLAS detector at the Large Hadron Collider corresponding to an integrated luminosity of 32.9 fb(-1). No significant excess over the Standard Model expectation is observed. Limits at 95% confidence level on the lifetime of the long-lived particle are presented in models of new phenomena including gauge-mediated supersymmetry or decay of the Higgs boson, H, to a pair of dark photons, Z(D). Lifetimes in the range c tau = 1-2400 cm are excluded, depending on the parameters of the model. In the supersymmetric model, the lightest neutralino is the next-to-lightest supersymmetric particle, with a relatively long lifetime due to its weak coupling to the gravitino, the lightest supersymmetric particle. The lifetime limits are determined for very light gravitino mass and various assumptions for the neutralino mass in the range 300-1000 GeV. In the dark photon model, the lifetime limits are interpreted as exclusion contours in the plane of the coupling between the Z(D) and the Standard Model Z boson versus the Z(D) mass (in the range 20-60 GeV), for various assumptions for the H -> Z(D)Z(D) branching fraction.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., Castillo Gimenez, V., et al. (2020). Search for long-lived neutral particles produced in pp collisions at root s=13 TeV decaying into displaced hadronic jets in the ATLAS inner detector and muon spectrometer. Phys. Rev. D, 101(5), 052013–29pp.
Abstract: A search is presented for pair production of long-lived neutral particles using 33 fb(-1) off root s = 13 TcV proton-proton collision data, collected during 2016 by the ATLAS detector at the LHC. This search focuses on a topology in which one long-lived particle decays in the ATLAS inner detector and the other decays in the muon spectrometer. Special techniques are employed to reconstruct the displaced tracks and vertices in the inner detector and in the muon spectrometer. One event is observed that passes the full event selection, which is consistent with the estimated background. Limits are placed on scalar boson propagators with masses from 125 GeV to 1000 GeV decaying into pairs of long-lived hidden-sector scalars with masses from 8 GeV to 400 GeV. The limits placed on several low-mass scalars extend previous exclusion limits in the range of proper lifetimes c tau from 5 cm to 1 m.
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T2K Collaboration(Abe, K. et al), Antonova, M., Cervera-Villanueva, A., Fernandez, P., Izmaylov, A., & Novella, P. (2019). Search for light sterile neutrinos with the T2K far detector Super-Kamiokande at a baseline of 295 km. Phys. Rev. D, 99(7), 071103–10pp.
Abstract: We perform a search for light sterile neutrinos using the data from the T2K far detector at a baseline of 295 km, with an exposure of 14.7(7.6) x 10(20) protons on target in neutrino (antineutrino) mode. A selection of neutral-current interaction samples is also used to enhance the sensitivity to sterile mixing. No evidence of sterile neutrino mixing in the 3 + 1 model was found from a simultaneous fit to the charged-current muon, electron and neutral-current neutrino samples. We set the most stringent limit on the sterile oscillation amplitude sin(2)theta(24 )for the sterile neutrino mass splitting Delta m(41)(2 )< 3 x 10(-3 )eV(2)/c(4).
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BABAR Collaboration(Lees, J. P. et al), Martinez-Vidal, F., & Oyanguren, A. (2012). Search for lepton-number violating processes in B+ -> h(-)l(+)l(+) decays. Phys. Rev. D, 85(7), 071103–8pp.
Abstract: We have searched for the lepton-number violating processes B+ -> h(-)l(+)l(+) with h(-) = K-/pi(-) and l(+) = e(+)/mu(+), using a sample of 471 +/- 3 million B (B) over bar events collected with the BABAR detector at the PEP-II e(+)e(-) collider at the SLAC National Accelerator Laboratory. We find no evidence for these decays and place 90%-confidence-level upper limits on their branching fractions B(B+ -> pi(-)e(+)e(+)) < 2.3 x 10(-8), B(B+ -> K(-)e(+)e(+)) < 3.0 x 10(-8), B(B+ -> pi(-)mu(+)mu(+)) < 10.7 x 10(-8), and B(B+ -> K-mu(+)mu(+)) < 6.7 x 10(-8).
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BABAR Collaboration(Lees, J. P. et al), Martinez-Vidal, F., Oyanguren, A., & Villanueva-Perez, P. (2014). Search for lepton-number violating B+ -> X(-)l(+)l '(+) decays. Phys. Rev. D, 89(1), 011102–8pp.
Abstract: We report on a search for eleven lepton-number violating processes B+ -> X(-)l(+)l'(+) with X- = K-, pi(-), rho(-), K*(-), or D- and l(+)/l'(+) = e(+) or mu(+), using a sample of 471 +/- 3 million B (B) over bar events collected with the BABAR detector at the PEP-II e(+)e(-) collider at the SLAC National Accelerator Laboratory. We find no evidence for any of these modes and place 90% confidence level upper limits on their branching fractions in the range (1.5-26) x 10(-7).
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo, F. L., et al. (2018). Search for lepton-flavor-violating decays of the Z boson into a r lepton and a light lepton with the ATLAS detector. Phys. Rev. D, 98(9), 092010–31pp.
Abstract: Direct searches for lepton flavor violation in decays of the Z boson with the ATLAS detector at the LHC are presented. Decays of the Z boson into an electron or muon and a hadronically decaying r lepton are considered. The searches are based on a data sample of proton-proton collisions collected by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb(-1) at a center-of-mass energy of root s = 13 TeV. No statistically significant excess of events above the expected background is observed, and upper limits on the branching ratios of lepton-flavor-violating decays are set at the 95% confidence level: B(Z -> e tau) < 5.8 x 10(-5) and B(Z -> μtau) < 2.4 x 10(-5). This is the first limit on B(Z -> e tau) with ATLAS data. The upper limit on 13(Z -> μtau) is combined with a previous ATLAS result based on 20.3 fb(-1) of proton protoncollision data at a center-of-mass energy of root s = 8 TeV and the combined upper limit at 95% confidence level is B(Z -> μtau) < 1.3 x 10(-5).
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BABAR Collaboration(Lees, J. P. et al), Martinez-Vidal, F., & Oyanguren, A. (2020). Search for lepton-flavor-violating decays D-0 -> X(0)e(+/-)mu(-/+). Phys. Rev. D, 101(11), 112003–11pp.
Abstract: We present a search for seven lepton-flavor-violating neutral charm meson decays of the type D-0 -> X(0)e(+/-)mu(-/+), where X-0 represents a pi(0), K-S(0), (K) over bar*(0), rho(0), phi, omega, or eta meson. The analysis is based on 468 fb(-1) of e(+)e(-) annihilation data collected at or close to the Upsilon(4S) resonance with the BABAR detector at the SLAC National Accelerator Laboratory. No significant signals are observed, and we establish 90% confidence level upper limits on the branching fractions in the range (5.0-22.5) x 10(-7). The limits are between 1 and 2 orders of magnitude more stringent than previous measurements.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo, F. L., et al. (2018). Search for lepton-flavor violation in different-flavor, high-mass final states in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 98(9), 092008–34pp.
Abstract: A search is performed for a heavy particle decaying into different-flavor, dilepton pairs (e mu, e tau or μtau), using 36.1 fb(-1) of proton-proton collision data at root s = 13 TeV collected in 2015-2016 by the ATLAS detector at the Large Hadron Collider. No excesses over the Standard Model predictions are observed. Bayesian lower limits at the 95% credibility level are placed on the mass of a Z' boson, the mass of a supersymmetric tau-sneutrino, and on the threshold mass for quantum black-hole production. For the Z' and sneutrino models, upper cross-section limits are converted to upper limits on couplings, which are compared with similar limits from low-energy experiments and which are more stringent for the e tau and μtau modes.
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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 K0S(L)→μ+μ−μ+μ− decays at LHCb. Phys. Rev. D, 108, L031102–12pp.
Abstract: A search for K0S(L)→μ+μ−μ+μ− decays is performed using proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.1 fb−1. No evidence for signal is found. The 90% confidence level upper limits are the first set for both decays and are B(K0S→μ+μ−μ+μ−)<5.1×10−12 and B(K0L→μ+μ−μ+μ−)<2.3×10−9.
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