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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Estrada, E. J., Gonzalez-Solis, S., Guevara, A., & Roig, P. (2024). Improved π0, η, η′ transition form factors in resonance chiral theory and their aμHLbL contribution. J. High Energy Phys., 12(12), 203–48pp.
Abstract: Working with Resonance Chiral Theory, within the two resonance multiplets saturation scheme, we satisfy leading (and some subleading) chiral and asymptotic QCD constraints and accurately fit simultaneously the pi 0, eta, eta ' transition form factors, for single and double virtuality. In the latter case, we supplement the few available measurements with lattice data to ensure a faithful description. Mainly due to the new results for the doubly virtual case, we improve over existing descriptions for the eta and eta '. Our evaluation of the corresponding pole contributions to the hadronic light-by-light piece of the muon g – 2 read: a μpi 0-pole=61.9 +/- 0.6-1.5+2.4x10-11, a μeta-pole=15.2 +/- 0.5-0.8+1.1x10-11 and a μeta '-pole=14.2 +/- 0.7-0.9+1.4x10-11, for a total of a μpi 0+eta+eta '-pole=91.3 +/- 1.0-1.9+3.0x10-11, where the first and second errors are the statistical and systematic uncertainties, respectively.
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Flores-Tlalpa, A., Lopez Castro, G., & Roig, P. (2016). Five-body leptonic decays of muon and tau lepton. J. High Energy Phys., 04(4), 185–21pp.
Abstract: We study the five-body decays u(-) -> e(-)e(+)e(-)nu u (nu) over bar (e) and tau(-) -> l(-)l'+l'-nu(tau)(nu) over bar (l) for l, l' = e, u within the Standard Model (SM) and in a general effective field theory description of the weak interactions at low energies. We compute the branching ratios and compare our results with two previous – mutually discrepan – SM calculations. By assuming a general structure for the weak currents we derive the expressions for the energy and angular distributions of the three charged leptons when the decaying lepton is polarized, which will be useful in precise tests of the weak charged current at Belle II. In these decays, leptonic T-odd correlations in triple products of spin and momenta – which may signal time reversal violation in the leptonic sector – are suppressed by the tiny neutrino masses. Therefore, a measurement of such T-violating observables would be associated to neutrinoless lepton flavor violating (LFV) decays, where this effect is not extremely suppressed. We also study the backgrounds that the SM five-lepton lepton decays constitute to searches of LFV L- -> ? l(-)l'+l'(-) decays. Searches at high values of the invariant mass of the l'(+)l'(-) pair look the most convenient way to overcome the background.
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Lopez Aguilar, D. A., Rendon, J., & Roig, P. (2025). CP violation in two-meson Tau decays induced by heavy new physics. J. High Energy Phys., 01(1), 105–24pp.
Abstract: We apply the effective field theory formalism that was used to study CP violation induced by heavy new physics in the tau -> KS pi nu tau decays to the other two-meson tau decay channels. We focus on the rate and the forward-backward asymmetries, that are predicted using current bounds on the complex Wilson coefficients of the effective Lagrangian. We discuss our outcomes for the modes with (K/pi)pi 0 and KKS, that can be studied at Belle-II and a super-tau-charm facility. Our main finding is that current and forthcoming experiments would be sensitive to the maximum allowed CP rate asymmetry in the K +/- KS modes if a precision of 5% is reached on this observable, that can check as well the BaBar anomaly in KS pi. For the pi +/-pi 0 channels, new physics would be difficult to probe at present. Disentangling new sources of CP violation would be most challenging in K +/-pi 0 and the other modes.
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