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NA62 Collaboration(Cortina Gil, E. et al), & Husek, T. (2020). Search for heavy neutral lepton production in K+ decays to positrons. Phys. Lett. B, 807, 135599–9pp.
Abstract: A search for heavy neutral lepton (N) production in K+ -> e(+)N decays using the data sample collected by the NA62 experiment at CERN in 2017-2018 is reported. Upper limits of the extended neutrino mixing matrix element vertical bar U-e4 vertical bar(2) are established at the level of 10(-9) over most of the accessible heavy neutral lepton mass range 144-462 MeV/c(2), with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The vertical bar U-e4 vertical bar(2) range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/c(2).
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NA62 Collaboration(Cortina Gil, E. et al), & Husek, T. (2020). An investigation of the very rare K+ -> pi+ nu nubar decay. J. High Energy Phys., 11(11), 042–57pp.
Abstract: The NA62 experiment reports an investigation of the K+-> pi+nu nu <overbar></mml:mover> mode from a sample of K+ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of (0.389 +/- 0.024) x 10(-10), corresponding to 2.2 events assuming the Standard Model branching ratio of (8.4 +/- 1.0) x 10(-11). Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of 1.78 x 10(-10) for the K+-> pi+nu nu <overbar></mml:mover> branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of (0.48<mml:mo>-0.48<mml:mo>+0.72) x 10(-10), are currently the most precise results worldwide, and are able to constrain some New Physics models that predict large enhancements still allowed by previous measurements.
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Husek, T., Monsalvez-Pozo, K., & Portoles, J. (2021). Lepton-flavour violation in hadronic tau decays and mu-tau conversion in nuclei. J. High Energy Phys., 01(1), 059–48pp.
Abstract: Within the Standard Model Effective Field Theory framework, with operators up to dimension 6, we perform a model-independent analysis of the lepton-flavour-violating processes involving tau leptons. Namely, we study hadronic tau decays and l-tau conversion in nuclei, with l = e, mu. Based on available experimental limits, we establish constraints on the Wilson coefficients of the operators contributing to these processes. Our work paves the way to extract the related information from Belle II and foreseen future experiments.
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Husek, T., Monsalvez-Pozo, K., & Portoles, J. (2022). Constraints on leptoquarks from lepton-flavour-violating tau-lepton processes. J. High Energy Phys., 04(4), 165–31pp.
Abstract: Leptoquarks are ubiquitous in several extensions of the Standard Model and seem to be able to accommodate the universality-violation-driven B-meson-decay anomalies and the (g-2)(mu) discrepancy interpreted as deviations from the Standard Model predictions. In addition, the search for lepton-flavour violation in the charged sector is, at present, a major research program that could also be facilitated by the dynamics generated by leptoquarks. In this article, we consider a rather wide framework of both scalar and vector leptoquarks as the generators of lepton-flavour violation in processes involving the tau lepton. We single out its couplings to leptoquarks, thus breaking universality in the lepton sector, and we integrate out leptoquarks at tree level, generating the corresponding dimension-6 operators of the Standard Model Effective Field Theory. In ref. [1] we obtained model-independent bounds on the Wilson coefficients of those operators contributing to lepton-flavour-violating hadron tau decays and l-tau conversion in nuclei, with l = e, mu. Hence, we use those results to translate the bounds into the couplings of leptoquarks to the Standard Model fermions.
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Baeza-Ballesteros, J., Bijnens, J., Husek, T., Romero-Lopez, F., Sharpe, S. R., & Sjo, M. (2023). The isospin-3 three-particle K-matrix at NLO in ChPT. J. High Energy Phys., 05(5), 187–56pp.
Abstract: The three-particle K-matrix, K-df,K-3, is a scheme-dependent quantity that parametrizes short-range three-particle interactions in the relativistic-field-theory three particle finite-volume formalism. In this work, we compute its value for systems of three pions at maximal isospin through next-to-leading order (NLO) in Chiral Perturbation Theory (ChPT). We compare the values to existing lattice QCD results and find that the agreement between lattice QCD data and ChPT in the first two coefficients of the threshold expansion of K-df,K-3 is significantly improved with respect to leading order once NLO effects are incorporated.
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