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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2023). Search for lepton-flavour violation in high-mass dilepton final states using 139 fb-1 of pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 082–50pp.
Abstract: A search is performed for a heavy particle decaying into different-flavour, dilepton final states, using 139 fb(-1) of proton-proton collision data at root s = 13TeV collected in 2015-2018 by the ATLAS detector at the Large Hadron Collider. Final states with electrons, muons and hadronically decaying tau leptons are considered (e mu, e tau or μtau). No significant excess over the Standard Model predictions is observed. Upper limits on the production cross-section are set as a function of the mass of a Z' boson, a supersymmetric tau-sneutrino, and a quantum black-hole. The observed 95% CL lower mass limits obtained on a typical benchmark model Z' boson are 5.0TeV (e mu), 4.0TeV (e tau), and 3.9TeV (mu tau), respectively.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2025). Test of lepton flavour universality in W-boson decays into electrons and τ-leptons using pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 05(5), 038–49pp.
Abstract: A measurement of the ratio of the branching fractions, R-tau/e = B(W -> tau nu)/B(W -> e nu), is performed using a sample of W bosons originating from top-quark decays to final states containing tau-leptons or electrons. This measurement uses pp collisions at root s = 13 TeV, collected by the ATLAS experiment at the Large Hadron Collider during Run 2, corresponding to an integrated luminosity of 140 fb(-1). The W -> tau nu(tau) (with tau -> e nu(e)nu(tau)) and W -> e nu(e) decays are distinguished using the differences in the impact parameter distributions and transverse momentum spectra of the electrons. The measured ratio of branching fractions R-tau/e = 0.975 +/- 0.012 (stat.) +/- 0.020 (syst.), is consistent with the Standard Model assumption of lepton flavour universality in W-boson decays.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2012). Measurement of the b-hadron production cross section using decays to D*(+)mu X- final states in pp collisions at root s=7 TeV with the ATLAS detector. Nucl. Phys. B, 864(3), 341–381.
Abstract: The b-hadron production cross section is measured with the ATLAS detector in pp collisions at root s = 7 TeV, using 3.3 pb(-1) of integrated luminosity, collected during the 2010 LHC run. The b-hadrons are selected by partially reconstructing D*(+)mu X- final states. Differential cross sections are measured as functions of the transverse momentum and pseudorapidity. The measured production cross section for a b-hadron with p(T) > 9 GeV and vertical bar eta vertical bar < 2.5 is 32.7 +/- 0.8(stat.)(-6.8)(+4.5)(syst.) μb, higher than the next-to-leading-order QCD predictions but consistent within the experimental and theoretical uncertainties.
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Bonilla, C., Lamprea, J. M., Peinado, E., & Valle, J. W. F. (2018). Flavour-symmetric type-II Dirac neutrino seesaw mechanism. Phys. Lett. B, 779, 257–261.
Abstract: We propose a Standard Model extension with underlying A(4) flavour symmetry where small Dirac neutrino masses arise from a Type-II seesaw mechanism. The model predicts the “golden” flavour-dependent bottom-tau mass relation, requires an inverted neutrino mass ordering and non-maximal atmospheric mixing angle. Using the latest neutrino oscillation global fit[ 1] we derive restrictions on the oscillation parameters, such as a correlation between delta(CP) and m(nu lightest).
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Felix-Beltran, O., Gonzalez-Canales, F., Hernandez-Sanchez, J., Moretti, S., Noriega-Papaqui, R., & Rosado, A. (2015). Analysis of the quark sector in the 2HDM with a four-zero Yukawa texture using the most recent data on the CKM matrix. Phys. Lett. B, 742, 347–352.
Abstract: In this Letter we analyse, in the context of the general 2-Higgs Doublet Model, the structure of the Yukawa matrices, (Y) over tilde (q)(1,2), by assuming a four-zero texture ansatz for their definition. In this framework, we obtain compact expressions for (Y) over tilde (q)(1,2), which are reduced to the Cheng and Sher ansatz with the difference that they are obtained naturally as a direct consequence of the invariants of the fermion mass matrices. Furthermore, in order to avoid large flavour violating effects coming from charged Higgs exchange, we consider the main flavour constraints on the off-diagonal terms of Yukawa texture ((chi) over tilde (q)(j))(kl) (k not equal l). We perform a chi(2)-fit based on current experimental data on the quark masses and the Cabibbo-KobayashiMaskawa mixing matrix V-CKM. Hence, we obtain the allowed ranges for the parameters (Y) over tilde (q)(1,2) at 1 sigma for several values of tan beta. The results are in complete agreement with the bounds obtained taking into account constraints on Flavour Changing Neutral Currents reported in the literature.
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Hernandez, P. (2012). CP violation in the neutrino sector: The new frontier. C. R. Phys., 13(2), 186–192.
Abstract: The discovery of neutrino masses has revealed a new flavour sector in the Standard Model. Just like the quark flavour sector, it contains a seed of CP violation, resulting in an asymmetric behaviour of matter and antimatter. It is argued that this new source of leptonic CP violation may be discovered in more precise neutrino oscillation experiments involving neutrino beams with energies in the GeV range that will be sent to distances of a few thousand kilometres.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2025). Observation of the decay B0s → K0p(p)over-bar and measurement of the B0(s) → K0p(p)over-bar branching fractions. J. High Energy Phys., 07(7), 121–25pp.
Abstract: A study of the charmless baryonic decays B-(s)(0) -> K(0)p (p) over bar is presented, where B-(s)(0) denotes either a B-0 or a B-s(0) meson. The analysis is based on proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb(-1). The decay B-s(0) -> K(0)p (p) over bar is observed for the first time, with a measured branching fraction of (9.14 +/- 1.69 +/- 0.90 +/- 0.33 +/- 0.20) x 10(-7) and a significance of 5.6 sigma. The uncertainties respectively account for statistical and systematic contributions, the precision of the branching fraction of the normalisation channel B-0 -> K-0 pi(+)pi(-) and the fragmentation fraction ratio f(s)/f(d). The branching fraction determined for B-0 -> K(0)p (p) over bar is (2.82 +/- 0.08 +/- 0.12 +/- 0.10) x 10(-6), which is the most precise measurement to date.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2024). Study of the rare decay J/ψ → μ+μ-μ+μ-. J. High Energy Phys., 12(12), 062–25pp.
Abstract: The rare electromagnetic J/psi -> mu(+)mu(-)mu(+)mu(-) decay is observed with a significance greatly exceeding the discovery threshold, using proton-proton collision data collected by the LHCb experiment during 2016-2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb(-1). The rate of this decay is measured relative to that of the J/psi -> mu(+)mu(-) mode. Using the QED model for the four-muon decay in the efficiency estimation, its branching fraction is determined to be B(J/psi -> mu(+)mu(-)mu(+)mu(-)) = (1.13 +/- 0.10 +/- 0.05 +/- 0.01) x 10(-6), where the uncertainties are statistical, systematic and due to the uncertainty on the branching fraction of the J/psi -> mu(+)mu(-) decay.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Martinez-Vidal, F., et al. (2024). Analysis of Λ0b →pK- μ+ μ- decays. J. High Energy Phys., 12(12), 147–33pp.
Abstract: The differential branching fraction and angular coefficients of Lambda(0)(b) -> pK(-) mu(+) mu(-)decays are measured in bins of the dimuon mass squared and dihadron mass. The analysis is performed using a data set corresponding to 9 fb(-1) of integrated luminosity collected with the LHCb detector between 2011 and 2018. The data are consistent with receiving contributions from a mixture of. resonances with different spin-parity quantum numbers. The angular coefficients show a pattern of vector-axial vector interference that is a characteristic of the type of flavour-changing neutral-current transition relevant for these decays.
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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. (2021). Searches for 25 rare and forbidden decays of D+ and Ds+ mesons. J. High Energy Phys., 06(6), 044–24pp.
Abstract: A search is performed for rare and forbidden charm decays of the form D+-> hl+l -/+, where h(+/-) is a pion or kaon and l((')+/-) is an electron or muon. The measurements are performed using proton-proton collision data, corresponding to an integrated luminosity of 1.6 fb(-1), collected by the LHCb experiment in 2016. No evidence is observed for the 25 decay modes that are investigated and 90 % confidence level limits on the branching fractions are set between 1.4 x 10(-8) and 6.4 x 10(-6). In most cases, these results represent an improvement on existing limits by one to two orders of magnitude.
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