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Pablo Roig

Publications and source records attributed to Pablo Roig.

At least 19 recordsLinked to original sources
arXiv2026

Can a Nonstandard Invisible Pair Mimic the Michel Distribution?

We ask whether a measured Michel distribution, apparently in excellent agreement with the Standard Model interpretation of the $\ell_i \to \ell_j ν\barν$ decay, could instead arise from a different invisible sector. Within a general low-energy effective field theory, we analyze lepton decays $\ell_i \to \ell_j + X\bar X$ for electrically neutral, color-singlet, mutually conjugate invisible pairs $X\bar X$ of spin up to $2$, allowing (pseudo)scalar, (axial)vector, and antisymmetric tensor interactions in the lepton current, focusing on the massless limit relevant for exact degeneracies. We formulate a criterion for indistinguishability based on the full set of measurable differential distributions. Under these assumptions, besides the obvious spin $1/2$ case, there is a unique nontrivial solution: a massless complex scalar pair coupled through a purely left-handed vector current exactly reproduces the standard Michel pattern, including its extensions to daughter-lepton polarization and radiative channels. All other cases studied here are distinguishable, in particular because higher-spin invisible pairs produce additional kinematic prefactors. These results isolate the only nonstandard and nontrivial invisible sector that can remain hidden in Michel-type lepton decay measurements. Phenomenologically, a subpercent branching fraction into this channel could bias the conventional muon-lifetime extraction of $G_F$ upward by enough for the corrected value to agree with the CKM-unitarity determination, while increasing the discrepancy with the electroweak fit.

hep-ph↗
arXiv2026

Hadronic vacuum polarization contribution to $a_μ$ from functional methods with strong and electromagnetic isospin breaking

We present a continuum-QCD determination of the leading-order hadronic vacuum-polarization contribution to the anomalous magnetic moment of the muon within the Dyson-Schwinger and Bethe-Salpeter equation framework. The calculation incorporates pion back-reaction, a fully dressed quark-photon vertex with a dynamically generated $ρ$-resonance structure in the timelike region, and both strong and electromagnetic isospin breaking treated self-consistently at the quark level. We obtain $a_μ^{\mathrm{HVP,LO}}(u+d+s+c)|_{\mathrm{ISB}} = 709.7 \times 10^{-10}$, in good agreement with recent lattice-QCD determinations, and find an isospin-breaking shift of $Δa_μ^{\mathrm{HVP,LO}} = 4.5 \times 10^{-10}$ ($0.6\%$), demonstrating that isospin-breaking effects, while quantitatively modest, are not negligible. Including the bottom-quark contribution and an indicative estimate of the systematic uncertainties, we obtain our final result, $a_μ^{\mathrm{HVP,LO}}(u+d+s+c+b)|_{\mathrm{ISB}} = (710.0 \pm 14.5) \times 10^{-10}$.

hep-ph↗
arXiv2025

Generalized structure functions in semileptonic tau decays

In case there are tensor interactions beyond the SM in the low-energy effective Lagrangian, their interference with the SM V-A currents yields a contribution for tau decays into three or more mesons that cannot be included in the famous structure functions introduced by Kühn and Mirkes in their seminal 1992 paper. We present this generalization here, highlighting again the importance of measuring the (generalized) spectral functions, as being model-independent and exploiting maximally the data. This can be particularly relevant for CP and T violation studies with at least three mesons in the final state of the tau decay.

hep-ph↗
arXiv2025

Hadronic contributions to $a_μ$ within Resonance Chiral Theory

We review the recent progress achieved, using Resonance Chiral Theory, in the hadronic contributions to the muon anomalous magnetic moment. These include the hadronic vacuum polarization, either using $e^+e^-$ or $τ$ decays into hadron final states as input; and the hadronic light-by-light part, where in addition to previous results on the lightest pseudoscalar and tensor-poles contributions, we first present the evaluation of the pseudoscalar box using this formalism. We also discuss the scalar, axial-pole and other subleading pieces. The results obtained are consistent with the White Paper 2 values, with comparable precision.

hep-ph↗
arXiv2025

Tensor Meson Pole contributions to the HLbL piece of $a_μ$ within R$χ$T

We compute the tensor meson pole contributions to the Hadronic Light-by-Light piece of $a_μ$ in the purely hadronic region, using Resonance Chiral Theory. Given the differences between the dispersive and holographic groups determinations, we consider timely to present an alternative evaluation. In our approach, the lightest tensor meson nonet and two vector meson resonance nonets are considered in the chiral limit. Disregarding operators with derivatives, only the form factor $\mathcal{F}_1^T$ is non-vanishing, as assumed in the dispersive study. All parameters are determined by imposing a set of short-distance QCD constraints, and the radiative decay widths. In this case, we obtain (in units of $10^{-11}$): $ a_2$-pole: $-\left(1.02(10)_{\rm stat}(^{+0.00}_{-0.12})_{\rm syst}\right)$, $f_2$-pole: $-\left(3.2(3)_{\rm stat}(^{+0.0}_{-0.4})_{\rm syst}\right)$ and $f_2^\prime$-pole: $-\left(0.042(13)_{\rm stat}\right)$, which add up to $a_μ^{a_2+f_2+f_2^\prime \rm -pole}=-\left(4.3^{+0.3}_{-0.5}\right)$, in close agreement with the holographic result when truncated to $\mathcal F_1^T$ only. However, with an ad-hoc extended Lagrangian, that also generates $\mathcal F_3^T$, as in the holographic approach, we have found: $ a_2$-pole: $+0.47(1.43)_{\rm norm}(3)_{\rm stat}(^{+0.06}_{-0.00})_{\rm syst}$, $f_2$-pole: $+1.18(4.18)_{\rm norm}(12)_{\rm stat}(^{+0.24}_{-0.00})_{\rm syst}$ and $f_2^\prime$-pole: $+0.040(78)_{\rm norm}(2)_{\rm stat}$, summing to $a_μ^{a_2+f_2+f_2^\prime \rm - pole}=+1.7(4.4)$, which agree with these recent determinations within uncertainties (dominated by the $\mathcal F_3^T$ normalization). We point out that $RχT$ generates all form factors, the contributions to $a_μ$ of $\mathcal F_{2,4,5}$ cannot be evaluated in the current basis, preventing for the moment a complete calculation of $a_μ^{\rm T-poles}$ within our framework.

hep-ph↗
arXiv2025

Prospects for detecting the rare heavy Higgs decay $H\to hγγ$ through the $H\to b\bar{b}γγ$ channel at the LHC

We study the decay of a heavy CP-even neutral Higgs into an on-shell Standard Model-like Higgs boson and two photons, $H\to hγγ$, in the two-Higgs doublet model. We argue that the decay channel $H\to hγγ$, followed by the decay of the Standard Model Higgs $h\rightarrow b\bar b$, could be observed at the 5$σ$ level at the High-Luminosity LHC for masses of the heavy Higgs up to 950 GeV for the type-II, 650 GeV for the Lepton Specific and the Flipped 2HDMs, and 350 GeV for the type-I. We also discuss the possible role of the decay $H\to hγγ$ in discriminating among different types of 2HDMs and in enhancing the total number of events in the final state $H\rightarrow b\bar b γγ$ compared to the cascade decay $H\to hh$ followed by $h\toγγ$ $h\to b\bar{b}$ with identical final state (although with different kinematical distributions).

hep-ph↗
arXiv2025

Proton-box contribution to $a_μ^{\rm{HLbL}}$

We analyze the proton$\text{-}$box contribution to the hadronic light$\text{-}$by$\text{-}$light part of the muon's anomalous magnetic moment, which is the first reported baryonic contribution to this piece. We follow the quark$\text{-}$loop analysis, incorporating the relevant data$\text{-}$driven and lattice proton form factors. Although the heavy mass expansion would yield a contribution of $\mathcal{O}(10^{-10})$, the damping of the form factors in the regions where the kernel peaks, explains our finding $a_μ^{\rm{p-box}}=1.82 (7)\times 10^{-12}$, two orders of magnitude smaller than the forthcoming uncertainty on the $a_μ$ measurement and on its Standard Model prediction.

hep-ph↗
arXiv2025

Isospin breaking corrections in $2π$ production in tau decays and $e^+e^-$ annihilation: consequences for the muon $g-2$ and CVC tests

We revisit the isospin-breaking corrections relating the $e^+e^-$ hadronic cross-section and the tau decay spectral function, focusing on the di-pion 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 $τ$ 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 ${\rm BR}(τ\to ππν_τ)$ and to the $2π$ hadronic vacuum polarization (HVP) contribution to the muon $g-2$, in agreement with previous determinations and with similar precision. Our results abound in the convenience of using tau data-based results in the updated data-driven prediction of the muon $g-2$ in the Standard Model.

hep-ph↗
arXiv2025

HEFT approach to investigate the muon g-2 anomaly at a muon collider

In a previous work \cite{Buttazzo:2020ibd}, it has been pointed out that heavy new physics (NP) contributions to lepton dipole moments and high-energy cross-sections of lepton pairs into Higgs and $γ/Z$ bosons are connected, if the electroweak symmetry breaking is realized linearly. As a consequence, a multi-TeV muon collider would provide a unique test of NP in the muon $g$-2 through the study of high-energy processes such as $μ^+ μ^-\to h +γ/Z$. Since the analysis involved a SMEFT approach to Higgs-processes, it could also be studied by the more general HEFT formulation. We compute the modification of the high-energy cross-section $μ^+ μ^-\to h +γ/Z$ and $h\to μ^{+} μ^{-} +γ/Z$ decay using the dimension six HEFT Lagrangian and compare them with the SMEFT analysis. We find that, within the current HEFT analysis, there are plausible scenarios where the HEFT approach could lead to a higher sensitivity to test the NP contributions in the muon $g$-2. However, a more precise knowledge of the new HEFT parameters is needed for a definite conclusion, which motivates the search for complementary measurements.

hep-ph↗
arXiv2025

CP violation studies at Super Tau-Charm Facility

Charge-parity ($C\!P$) violation in the tau-charm energy region is a promising area for sensitive tests of Standard Model (SM) predictions and searches for new, beyond the SM physics. A future Tau-Charm Facility that operates at center-of-mass energies between 2.0 and 7.0 GeV, with a peak luminosity of $0.5\times10^{35}$~cm$^{-2}$s$^{-1}$, would provide huge numbers of hadrons and tau ($τ$) leptons that are produced in low-background environments and with well understood kinematic properties. In this report, prospects for unique studies of $C\!P$ violation in the decay of charmed hadrons, and in the production and decay of hyperons and $τ$ leptons at a next-generation tau-charm facility are discussed. In addition, opportunities for improved tests of $CPT$ invariance test in $K^{0}-\bar{K}^{0}$ mixing are presented.

hep-ex↗
arXiv2025

CP violation in two-meson Tau decays induced by heavy new physics

We apply the effective field theory formalism that was used to study CP violation induced by heavy new physics in the $τ\to K_S πν_τ$ 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 $(K/π)π^0$ and $K K_S$, 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 $K^\pm K_S$ modes if a precision of $5\%$ is reached on this observable, that can check as well the BaBar anomaly in $K_Sπ$. For the $π^\pmπ^0$ channels, new physics would be difficult to probe at present. Disentangling new sources of CP violation would be most challenging in $K^\pmπ^0$ and the other modes.

hep-ph↗
arXiv2024

Improved $π^0,η,η^{\prime}$ transition form factors in resonance chiral theory and their $a_μ^{\rm{HLbL}}$ contribution

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 $π^{0},η,η^{\prime}$ 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 $η$ and $η^\prime$. Our evaluation of the corresponding pole contributions to the hadronic light-by-light piece of the muon $g-2$ read: $a_μ^{π^{0}\text{-}\rm{pole}}=\left(61.9\pm0.6^{+2.4}_{-1.5}\right)\times10^{-11}$, $a_μ^{η\text{-}\rm{pole}}=\left(15.2\pm0.5^{+1.1}_{-0.8}\right)\times10^{-11}$ and $a_μ^{η^\prime\text{-}\rm{pole}}=\left(14.2\pm0.7^{+1.4}_{-0.9}\right)\times10^{-11}$, for a total of $a_μ^{π^0+η+η^{\prime}\text{-}\rm{pole}}=\left(91.3\pm1.0^{+3.0}_{-1.9}\right)\times10^{-11}$, where the first and second errors are the statistical and systematic uncertainties, respectively.

hep-ph↗
arXiv2024

Hadronic vacuum polarization contribution to the muon g-2 on Euclidean windows from tau data

We computed for the first time the $τ$ data-driven Euclidean windows for the hadronic vacuum polarization contribution to the muon g-2. We showed that $τ$-based results agree with the available lattice window evaluations and with the full result. On the intermediate window, where all lattice evaluations are rather precise and agree, $τ$-based results are compatible with them. This is particularly interesting, given that the disagreement of the $e^+e^-$ data-driven result with the lattice values in this window is the main cause for their discrepancy, affecting the interpretation of the $a_μ$ measurement in terms of possible new physics.

hep-ph↗
arXiv2024

Tau Data-Based Evaluations of the hadronic vacuum polarization contribution to the muon $g-2$

We review the tau data-driven computation of Euclidean windows for the hadronic vacuum polarization contribution to the muon anomalous magnetic moment (a_mu), which agree with the lattice results, making the difference of the $e^+e^-$ data-driven methods with them more intriguing. This conundrum needs to be solved by next year, when the final FNAL a_mu measurement will be published, in order to extract firm conclusions on possible new physics effects.

hep-ph↗
arXiv2024

$νe\toνe$ scattering with massive Dirac or Majorana neutrinos and general interactions

We calculate the neutrino-electron elastic scattering cross section, extending the results previously obtained in arXiv:1702.05721v2, in the presence of generic new interactions that take into account all the effects caused by finite neutrino masses. We address the potential significance of a heavy neutrino sector during precision measurements, particularly for tau neutrinos scattering with masses in the MeV range, for which the existing upper bounds on $|U_{τ4}|^2$ would result in conceivably measurable contributions. Finally, we comment on the possibility to distinguish between Dirac and Majorana neutrinos, including the analysis of the new emerging parameters and its application to illustrative model-dependent scenarios.

hep-ph↗
arXiv2024

Radiative corrections to the $τ^-\to (P_1P_2)^-ν_τ$ ($P_{1,2}=π, K$) decays

The radiative corrections to the $τ^-\to (P_1P_2)^-ν_τ$ ($P_{1,2}=π, K$) decays are calculated for the first time including the structure-dependent real photon corrections, which are obtained using Resonance Chiral Theory. Our results, whose uncertainty is dominated by the model-dependence of the resummation of the radiative corrections and the missing virtual structure-dependent contributions, allow for precise tests of CKM unitarity, lepton flavour universality and non-standard interactions.

hep-ph↗
arXiv2024

Dirac-Majorana neutrinos distinction in four-body decays

A novel method to differentiate the effects of Dirac and Majorana (D-M) neutrinos in four-body decays has been discussed in arXiv:2106.11785. There, it is concluded that the back-to-back kinematic scenario seems to avoid the constraint imposed by the "practical Dirac-Majorana confusion theorem", as one does not need to fully integrate over neutrino and antineutrino momenta. In this paper, we propose to analyse radiative leptonic lepton-decays ($\ell\to\ell'ν\barνγ$), as an independent alternative process to study the possible Majorana nature of neutrinos. Our approach demonstrates that, in the back-to-back kinematic configuration (for the $\ell'- γ$ and $ν-\barν$ systems, respectively), the distinction between Dirac and Majorana cases disappears when the inaccessible neutrino angle is integrated out, which might appear unexpected considering the claims in arXiv:2106.11785. Our results apply in absence of non-standard interactions, which can enhance generally the sensitivity to the neutrino nature.

hep-ph↗