Lami, A., Portoles, J., & Roig, P. (2016). Lepton flavor violation in hadronic decays of the tau lepton in the simplest little Higgs model. Phys. Rev. D, 93(7), 076008–14pp.
Abstract: We study lepton flavor violating hadron decays of the tau lepton within the simplest little Higgs model. Namely we consider tau -> mu(P, V, PP) where P and V are short for a pseudoscalar and a vector meson. We find that, in the most positive scenarios, branching ratios for these processes are predicted to be, at least, four orders of magnitude smaller than present experimental bounds.
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Lami, A., & Roig, P. (2016). H -> ll ' in the simplest little Higgs model. Phys. Rev. D, 94(5), 056001–7pp.
Abstract: Little Higgs models are promising constructs to solve the hierarchy problem affecting the Higgs boson mass for generic new physics. However, their preservation of lepton universality forbids them to account for the H -> tau μCMS hint and at the same time respect (as they do) the severe limits on H -> μe inherited from the nonobservation of μ-> e gamma We compute the predictions of the simplest little Higgs model for the H -> ll' decays and conclude that the measurement of any of these decays at LHC (even with a much smaller rate than currently hinted) would, under reasonable assumptions, disfavor this model. This result is consistent with our earlier observation of very suppressed lepton flavor violating semileptonic tau decays within this model.
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Guevara, A., Lopez Castro, G., & Roig, P. (2022). Improved description of dilepton production in tau(-) -> nu(tau)P(- )decays. Phys. Rev. D, 105(7), 076007–15pp.
Abstract: Recently, the Belle Collaboration reported the first measurements of the tau(-) -> nu(tau)pi(-) e(+) e(-) branching fraction and the spectrum of the pion-dielectron system. In an analysis previous to Belle's results, we evaluated this branching fraction which turned out to be compatible with that reported by Belle, although with a large uncertainty. This is the motivation to seek for improvement on our previous evaluation of tau(-) -> nu(tau)pi(-) l(+) l(-) decays (l = e, mu). In this paper we improve our calculation of the WP-gamma* vertex by including flavor-symmetry breaking effects in the framework of the resonance chiral theory. We impose QCD short-distance behavior to constrain most parameters and data on the pi(-) e(+) e(-) spectrum reported by the Belle Collaboration to fix the remaining free ones. As a result, improved predictions for the branching ratios and hadronic/leptonic spectra are reported, which are in good agreement with observations. Analogous calculations for the strangeness-changing tau(-) -> nu(tau) K- l(+) l(-) transitions are reported for the first time. Albeit one expects the m(pi mu+ mu- )spectrum to be measured in Belle-II and the observables with l = e can be improved, it is rather unlikely that the K channels can be measured due to the suppression factor vertical bar V-ud/V-us vertical bar(2) = 0.05.
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Estrada, E. J., Marquez, J. M., Portillo-Sanchez, D., & Roig, P. (2025). Proton-box contribution to aμHLbL. Phys. Rev. D, 111(9), 093008–9pp.
Abstract: We analyze the proton-box contribution to the hadronic light-by-light part of the muon's anomalous magnetic moment, which is the first reported baryonic contribution to this piece. We follow the quark-loop analysis, incorporating the relevant data-driven and lattice proton form factors. Although the heavy mass expansion would yield a contribution of O & eth;10-10 & THORN;, the damping of the form factors in the regions where the kernel peaks explains our finding μap-box 1/4 1.82 & eth;7 & THORN; x 10-12, 2 orders of magnitude smaller than the forthcoming uncertainty on the a μmeasurement and on its Standard Model prediction.
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Lopez Castro, G., Miranda, A., & Roig, P. (2025). Isospin breaking corrections in 2π production in tau decays and e plus e – annihilation: Consequences for the muon g -2 and conserved vector current tests. Phys. Rev. D, 111(7), 073004–26pp.
Abstract: We revisit the isospin-breaking corrections relating the e+e- hadronic cross section and the tau decay spectral function, focusing on the dipion channel, that gives the dominant contribution to the hadronic vacuum polarization piece of the muon g – 2. We test different types of electromagnetic and weak form factors and show that both, the Gounaris-Sakurai and a dispersive-based approach, describe accurately z lepton and e+e- data (less when KLOE measurements are included in the fits) and comply reasonably well with analyticity constraints. From these results we obtain the isospin-breaking contribution to the conserved vector current (CVC) prediction of the BR(z -> vz) and to the 2hadronic vacuum polarization (HVP) contribution to the muon g – 2, in agreement with previous determinations and with similar precision. Our results abound in the utility of using tau data-based results in the updated data-driven prediction of the muon g – 2 in the Standard Model.
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