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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). Configuration and performance of the ATLAS b-jet triggers in Run 2. Eur. Phys. J. C, 81(12), 1087–45pp.
Abstract: Several improvements to the ATLAS triggers used to identify jets containing b-hadrons (b-jets) were implemented for data-taking during Run 2 of the Large Hadron Collider from 2016 to 2018. These changes include reconfiguring the b-jet trigger software to improve primary-vertex finding and allow more stable running in conditions with high pile-up, and the implementation of the functionality needed to run sophisticated taggers used by the offline reconstruction in an online environment. These improvements yielded an order of magnitude better light-flavour jet rejection for the same b-jet identification efficiency compared to the performance in Run 1 (2011-2012). The efficiency to identify b-jets in the trigger, and the conditional efficiency for b-jets that satisfy offline b-tagging requirements to pass the trigger are also measured. Correction factors are derived to calibrate the b-tagging efficiency in simulation to match that observed in data. The associated systematic uncertainties are substantially smaller than in previous measurements. In addition, b-jet triggers were operated for the first time during heavy-ion data-taking, using dedicated triggers that were developed to identify semileptonic b-hadron decays by selecting events with geometrically overlapping muons and jets.
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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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Guerrero, M., Olmo, G. J., Rubiera-Garcia, D., & Saez-Chillon Gomez, D. (2021). Shadows and optical appearance of black bounces illuminated by a thin accretion disk. J. Cosmol. Astropart. Phys., 08(8), 036–19pp.
Abstract: We study the light rings and shadows of an uniparametric family of spherically symmetric geometries interpolating between the Schwarzschild solution, a regular black hole, and a traversable wormhole, and dubbed as black bounces, all of them sharing the same critical impact parameter. We consider the ray-tracing method in order to study the impact parameter regions corresponding to the direct, lensed, and photon ring emissions, finding a broadening of all these regions for black bounce solutions as compared to the Schwarzschild one. Using this, we determine the optical appearance of black bounces when illuminated by three standard toy models of optically and geometrically thin accretion disks viewed in face-on orientation.
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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). First measurement of the CP-violating phase in B-s(0) -> J/Psi (-> e(+) e(-))phi decays. Eur. Phys. J. C, 81(11), 1026–18pp.
Abstract: A flavour-tagged time-dependent angular analysis of B-s(0) -> J/Psi phi decays is presented where the J/Psi meson is reconstructed through its decay to an e(+)e(-) pair. The analysis uses a sample of pp collision data recorded with the LHCb experiment at centre-of-mass energies of 7 and 8TeV, corresponding to an integrated luminosity of 3 fb(-1). The CP-violating phase and lifetime parameters of the B-s(0) s system are measured to be phi(s) = 0.00 +/- 0.28 +/- 0.07 rad, Delta Gamma(s) = 0.115 +/- 0.045 +/- 0.011 ps(-1) and Delta Gamma(s) = 0.608 +/- 0.018 +/- 0.012 ps(-1) where the first uncertainty is statistical and the second systematic. This is the first time that CP-violating parameters are measured in the B-s(0) -> J/Psi phi decay with an e+e- pair in the final state. The results are consistent with previous measurements in other channels and with the Standard Model predictions.
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Alves, J. M., Botella, F. J., Branco, G. C., Cornet-Gomez, F., & Nebot, M. (2021). The framework for a common origin of delta(CKM) and delta S-PMN. Eur. Phys. J. C, 81(8), 727–11pp.
Abstract: We analyse a possible connection between CP violations in the quark and lepton sectors, parametrised by the CKM and PMNS phases. If one assumes that CP breaking arises from complexYukawa couplings, both in the quark and lepton sectors, the above connection is not possible in general, sinceYukawa couplings in the two sectors have independent flavour structures. We show that both the CKM and PMNS phases can instead be generated by a vacuum phase in a class of two Higgs doublet models, and in this case a connection may be established. This scenario requires the presence of scalar FCNC at tree level, both in the quark and lepton sectors. The appearance of these FCNC is an obstacle and a blessing. An obstacle since one has to analyse which models are able to conform to the strict experimental limits on FCNC, both in the quark and lepton sectors. A blessing, because this class of models is falsifiable since FCNC arise at a level which can be probed experimentally in the near future, specially in the processes h up arrow e(+/-) t +/- and t -> hc. The connection between CP violations in CKM and PMNS is explicitely illustrated in models with Minimal Flavour Violation.
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LHCb Collaboration(Aaij, R. et al), Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., Remon Alepuz, C., & Ruiz Vidal, J. (2021). Branching Fraction Measurements of the Rare B-s(0) -> phi mu(+)mu(-) and B-s(0)-> f(2)' (1525)mu(+)mu(-) Decays. Phys. Rev. Lett., 127(15), 151801–11pp.
Abstract: The branching fraction of the rare B-s(0) -> phi mu(+)mu(-) decay is measured using data collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to integrated luminosities of 1, 2, and 6 fb(-1), respectively. The branching fraction is reported in intervals of q(2), the square of the dimuon invariant mass. In the q(2) region between 1.1 and 6.0 GeV2/c(4) , the measurement is found to lie 3.6 standard deviations below a standard model prediction based on a combination of light cone sum rule and lattice QCD calculations. In addition, the first observation of the rare B-s(0)-> f(2)' (1525)mu(+)mu(-) decay is reported with a statistical significance of 9 standard deviations and its branching fraction is determined.
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Cottin, G., Helo, J. C., Hirsch, M., Titov, A., & Wang, Z. S. (2021). Heavy neutral leptons in effective field theory and the high-luminosity LHC. J. High Energy Phys., 09(9), 039–34pp.
Abstract: Heavy neutral leptons (HNLs) with masses around the electroweak scale are expected to be rather long-lived particles, as a result of the observed smallness of the active neutrino masses. In this work, we study long-lived HNLs in NRSMEFT, a Standard Model (SM) extension with singlet fermions to which we add non-renormalizable operators up to dimension-6. Operators which contain two HNLs can lead to a sizable enhancement of the production cross sections, compared to the minimal case where HNLs are produced only via their mixing with the SM neutrinos. We calculate the expected sensitivities for the ATLAS detector and the future far-detector experiments: AL3X, ANUBIS, CODEX-b, FASER, MATHUSLA, and MoEDAL-MAPP in this setup. The sensitive ranges of the HNL mass and of the active-heavy mixing angle are much larger than those in the minimal case. We study both, Dirac and Majorana, HNLs and discuss how the two cases actually differ phenomenologically, for HNL masses above roughly 100 GeV.
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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 New Phenomena in Final States with Two Leptons and One or No b-Tagged Jets at root s=13 TeV Using the ATLAS Detector. Phys. Rev. Lett., 127(14), 141801–23pp.
Abstract: A search for new phenomena is presented in final states with two leptons and one or no b-tagged jets. The event selection requires the two leptons to have opposite charge, the same flavor (electrons or muons), and a large invariant mass. The analysis is based on the full run-2 proton-proton collision dataset recorded at a center-of-mass energy off root S = 13 TeV by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 139 fb(-1). No significant deviation from the expected background is observed in the data. Inspired by the B-meson decay anomalies, a four-fermion contact interaction between two quarks (b, s) and two leptons (ee or μmu) is used as a benchmark signal model, which is characterized by the energy scale and coupling, Lambda and g(*), respectively. Contact interactions with Lambda/g(*) lower than 2.0 (2.4) TeV are excluded for electrons (muons) at the 95% confidence level, still far below the value that is favored by the B-meson decay anomalies. Model-independent limits are set as a function of the minimum dilepton invariant mass, which allow the results to be reinterpreted in various signal scenarios.
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Fioresi, R., & Lledo, M. A. (2021). Quantum Supertwistors. Symmetry-Basel, 13(7), 1241–16pp.
Abstract: In this paper, we give an explicit expression for a star product on the super-Minkowski space written in the supertwistor formalism. The big cell of the super-Grassmannian Gr(2|0,4|1) is identified with the chiral, super-Minkowski space. The super-Grassmannian is a homogeneous space under the action of the complexification SL(4|1) of SU(2,2|1), the superconformal group in dimension 4, signature (1,3), and supersymmetry N=1. The quantization is done by substituting the groups and homogeneous spaces by their quantum deformed counterparts. The calculations are done in Manin's formalism. When we restrict to the big cell, we can explicitly compute an expression for the super-star product in the Minkowski superspace associated to this deformation and the choice of a certain basis of monomials.
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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 Lepton-Flavor Violation in Z-Boson Decays with tau Leptons with the ATLAS Detector. Phys. Rev. Lett., 127(27), 271801–20pp.
Abstract: A search for lepton-flavor-violating Z -> e tau and Z -> μtau decays with pp collision data recorded by the ATLAS detector at the LHC is presented. This analysis uses 139 fb(-1) of Run 2 pp collisions at root s = 13 TeV and is combined with the results of a similar ATLAS search in the final state in which the tau lepton decays hadronically, using the same data set as well as Run 1 data. The addition of leptonically decaying tau leptons significantly improves the sensitivity reach for Z -> l tau decays. The Z -> l tau branching fractions are constrained in this analysis to B(Z -> e tau) < 7.0 x 10(-6) and B (Z -> μtau) < 7.2 x 10(-6) at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: B(Z -> e tau) < 5.0 x 10(-6) and B(Z -> μtau) < 6.5 x 10(-6) at 95% confidence level.
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