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Pablo Roig

Publications and source records attributed to Pablo Roig.

At least 37 records · Page 2Linked to original sources
arXiv2024

$τ$ data-driven evaluation of Euclidean windows for the hadronic vacuum polarization

We compute for the first time the $τ$ data-driven Euclidean windows for the hadronic vacuum polarization contribution to the muon $g-2$. We show 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.

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arXiv2024

Data-driven approximations to the Hadronic Light-by-Light scattering contribution to the muon (g-2)

We review recent progress on the numerical determination of the Hadronic Light-by-Light contribution to the anomalous magnetic moment of the muon. We advocate for a slight increase of the White Paper number for its Standard Model prediction, to $(102\pm17)\times10^{-11}$, accounting for a revised contribution from axial-vector mesons and short-distance constraints. This $\sim10\%$ larger result seems to be supported by the most recent lattice QCD evaluations.

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arXiv2023

Tau data-driven evaluation of the Hadronic Vacuum Polarization

Windows in Euclidean time have become a standard tool for comparing lattice QCD and data-driven computations of the hadronic vacuum polarization (HVP) contribution to the muon $g-2$. Here we review our results, obtained using isospin-rotated $τ^-\toπ^-π^0ν_τ$ data instead of $e^+e^-\toπ^+π^-$ measurements, and compare them to other approaches. Consistency of the tau-based and lattice results hints to underestimated uncertainties in the $e^+e^-$ data. If that is the case, the theory prediction of the muon $g-2$ would only lie at $\sim 2σ$ from its measured value.

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arXiv2023

Lepton Flavor Violation from diphoton effective interactions

We consider charged lepton flavor violating transitions mediated by the diphoton effective interactions $\ell_i\ell_jγγ$ and explore which processes can probe them better. Our analysis includes single and double radiative decays, $\ell_i\to\ell_jγ(γ)$, as well as $\ell_i\to\ell_j$ conversions in nuclei for all possible flavor combinations, which we compute for the first time for $\ell\toτ$ conversions in this framework. We find that currently the best limits are provided by the loop-induced $\ell_i\to\ell_jγ$ processes, while the best future sensitivities come from $μ\to e$ conversion in aluminum and from potential $τ\to \ellγγ$ searches at Belle II or at the Super Tau Charm Facility. We also motivate the search for $μ\to eγγ$ at the Mu3e experiment as a complementary probe of these operators.

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arXiv2023

Impact of ATLAS constraints on effective dark matter-standard model interactions with spin-one mediators

We complement a previous work \cite{Fortuna:2020wwx} using an EFT framework of dark matter and standard model interactions, with spin-one mediators, exploring a wider dark matter mass range, up to $6.4$ TeV. We use again bounds from different experiments: relic density, direct detection experiments and indirect detection limits from the search of gamma-ray emissions and positron fluxes. Besides, in this paper we add collider constraints by the ATLAS Collaboration in monojet analysis. Moreover, here we tested our previous results in the light of the aforementioned ATLAS data, which turn out to be the most restrictive forlight dark matter masses (as expected), $m_{\rm DM}<M_Z/2$. We obtain a larger range of solutions for the operators of dimension 5, OP1 and OP4, where masses above $43$ GeV and $30$ GeV (but for the $Z$ resonance region, $\sim (M_Z\pmΓ_Z)/2$), respectively, are allowed. In contrast, the operator of dimension 6, OP3, has viable solutions for masses $\gtrsim 190$ GeV. For the combination of OP1\&OP3 we obtain solutions (for masses larger than $140$ or $325$ GeV) that depend on the relative sign between the operators.

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arXiv2023

Michel parameters in the presence of massive Dirac and Majorana neutrinos

We analyse the effects of Dirac and Majorana neutrinos on leptonic decays of muon and tau leptons using the most general four-lepton effective interaction Hamiltonian of dimension six. We calculate the specific energy and angular distribution of the final charged lepton, including the polarizations of the initial and final charged leptons. We discuss the new generalized Michel parameters and focus on the effects of the heavy neutrino masses that would lead to sizable contributions on scenarios where the new sterile neutrinos have non-negligible mixing. Specifically, the most promising scenario is found for the case of $τ$ decay with one heavy final-state neutrino with a mass around $10^2 - 10^3$ MeV, where the linear term suppression could be of order $10^{-4}$, low enough to be measured in current and forthcoming experiments.

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arXiv2022

Indirect upper limits on $\ell_i\to\ell_jγγ$ from $\ell_i\to\ell_jγ$

We perform an effective field theory analysis to correlate the charged lepton flavor violating processes $\ell_i\to\ell_jγγ$ and $\ell_i\to\ell_jγ$. Using the current upper bounds on the rate for $\ell_i\to\ell_jγ$, we derive model-independent upper limits on the rates for $\ell_i\to\ell_jγγ$. Our indirect limits are about three orders of magnitude stronger than the direct bounds from current searches for $μ\to eγγ$, and four orders of magnitude better than current bounds for $τ\to\ellγγ$. We also stress the relevance of Belle II or a Super Tau Charm Facility to discover the rare decay $τ\to\ellγγ$.

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arXiv2022

Lepton flavor violation within the Simplest Little Higgs model

We carry out an exhaustive analysis of lepton flavor violating processes within the Simplest Little Higgs model. Its discovery could be expected from either $μ\to e$ conversion in nuclei, $μ\to eγ$ or $μ\to3e$ decays. Then, the tau sector could help discriminating this model not only via $τ\to\ellγ$ ($\ell=μ,e$) and $τ\to3\ell$ decays, but also by means of $\ell\toτ$ conversion in nuclei, which is promising in this respect. Although the model violates slightly custodial symmetry, acommodating the recent CDF $M_W$ measurement is in tension with electroweak precision data.

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arXiv2022

Lepton Flavour Violation in Hadron Decays of the Tau Lepton within the Littlest Higgs Model with T-parity

We first study the hadronic lepton flavor violating tau decays within the littlest Higgs model with T-parity (including one or two pseudoscalars, or a vector resonance). We consider the case where only T-odd particles and partner fermions contribute, and also its extension including Majorana neutrinos coming from an inverse seesaw. In both cases our mean values lie only one order of magnitude below current upper limits, strengthening the case of searching for these decays in the quest for new physics.

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arXiv2022

Pion and Kaon box contribution to $a_μ^{\text{HLbL}}$

We present an evaluation of the $π^\pm$ and $K^\pm$ box contributions to the hadronic light-by-light piece of the muon's anomalous magnetic moment, $a_μ$. The calculation of the corresponding electromagnetic form factors (EFFs) is performed within a Dyson-Schwinger equations (DSE) approach to quantum chromodynamics. These form factors are calculated in the so-called rainbow-ladder (RL) truncation, following two different evaluation methods and, subsequently, in a further improved approximation scheme which incorporates meson cloud effects. The results are mutually consistent, indicating that in the domain of relevance for $a_μ$ the obtained EFFs are practically equivalent. Our analysis yields the combined estimates of $a_μ^{π^\pm-box}=-(15.6\pm 0.2)\times 10^{-11}$ and $a_μ^{K^\pm-\text{box}}=-(0.48\pm 0.02)\times 10^{-11}$, in full agreement with results previously obtained within the DSE formalism and other contemporary estimates.

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arXiv2022

Dyson-Schwinger equations and the muon g-2

We present a brief introduction to the Dyson-Schwinger equations (DSEs) approach to hadron and high-energy physics. In particular, how this formalism is applied to calculate the electromagnetic form factors $γ^* γ^* \to \textbf{P}^0$ and $γ^* \textbf{P}^\pm \to \textbf{P}^\pm$ (with $\textbf{P}^\pm$ and $\textbf{P}^0$ charged and neutral ground-state pseudoscalar mesons, respectively) is discussed. Subsequently, the corresponding contributions of those form factors to the muon anomalous magnetic moment ($g-2$) are estimated. We look forward to promoting the DSE approach to address theoretical aspects of the muon $g-2$, highlighting some calculations that could be carried out in the future.

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arXiv2022

Improved description of di-lepton production in $τ^-\toν_τP^-$ decays

Recently, the Belle collaboration reported the first measurements of the $τ^- \toν_τπ^-e^+e^-$ 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 improvements on our previous evaluation of $τ^- \to ν_τπ^-\ell^+\ell^-$ decays ($\ell=e,\,μ$). In this paper we improve our calculation of the $WP^-γ^*$ vertex by including flavour symmetry breaking effects in the framework of the Resonance Chiral Theory. We impose QCD short-distance behaviour to constrain most parameters and data on the $π^-e^+e^-$ spectrum reported by Belle to fix the remaining free ones. As a result, improved predictions for the branching ratios and hadronic/leptonic spectra are reported, in good agreement with observations. Analogous calculations for the strangeness-changing $τ^- \to ν_τK^-\ell^+\ell^-$ transitions are reported for the first time. Albeit one expects the $m_{πμ^+μ^-}$ spectrum to be measured in Belle-II and the observables with $\ell=e$ can be improved, it is rather unlikely that the $K$ channels can be measured due to the suppression factor $|V_{ud}/V_{us}|^2=0.05$.

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arXiv2022

Lepton flavor violation in the Littlest Higgs Model with T parity realizing an inverse seesaw

We study lepton flavor violation (LFV) within the Littlest Higgs Model with T parity (LHT) realizing an inverse seesaw (ISS) mechanism of type I. With respect to the traditional LHT, there appear new $\mathcal{O}$(\textcolor{black}{10} TeV) Majorana neutrinos, driving LFV. For $τ\to\ell\ell'\bar{\ell}"$ (including wrong-sign, $\ell=e, μ$) decays and $μ\to e$ conversion in Ti, we get typical rates only one order of magnitude below present bounds ($\ell\to\ell'γ$ can reach the current upper limit) and for $Z\to\barτ\ell$, $μ\to e e \bar{e}$ and conversion in Au, results are within two orders of magnitude from present limits. Correlations among modes are drastically different to the traditional LHT and other models, which would ease the confrontation of this scenario to eventual measurements of LFV processes involving charged leptons.

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arXiv2022

Axial-vector exchange contribution to the Hyperfine Splitting

We revisit the contribution of axial-vector mesons to the hyperfine splitting of muonic hydrogen. We focus our attention on the doubly-virtual asymptotic behavior of the relevant form factors of axial-vector mesons, together with their coupling to nucleons based on resonance saturation and short-distance constraints. Among others, we find significant differences with respect to previous studies, including an opposite sign and a $\sim50\%$ effect of the doubly-virtual high-energy behavior.

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arXiv2021

The interplay of transverse degrees of freedom and axial-vector mesons with short-distance constraints in g-2

We revisit well-known short-distance constraints relating the hadronic light-by light Green's function to the $\langle VVA \rangle$ one, that have been a subject of debate over the past years in the context of the muon $(g-2)$. Specifically, we identify a relation among the longitudinal and transverse degrees of freedom that is enforced by the axial anomaly that, by contrast, has not received attention in the past. Such relation allows, among other things, to overcome the problem of basis ambiguities when describing axial-vector mesons transition form factors, but further applications are discussed as well, with special focus on the role of axial-vector mesons in the HLbL contribution to the muon $(g-2)$. Our results should also contribute to a better understanding of the, so far, controversial interplay among short-distance constraints with longitudinal and transverse degrees of freedom, such as axial-vector mesons. This is key to confront the theoretical and experimental result for the muon $(g-2)$ that, currently, exhibits a $4.2σ$ tension.

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arXiv2021

Flavor violating muon decay into an electron and a light gauge boson

We analyze the flavor violating muon decay $μ\rightarrow eχ$, where $χ$ is a massive gauge boson, with emphasis in the regime where $χ$ is ultralight. We first study this process from an effective field theory standpoint in terms of form factors. We then present two explicit models where $μ\rightarrow eχ$ is generated at tree level and at the one-loop level. We also comment on the prospects of observing the process $μ\rightarrow eχ$ in view of the current limits on $μ\rightarrow 3e$ from the SINDRUM collaboration.

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arXiv2020

Effective field theory analysis of dark matter-standard model interactions with spin one mediators

We analyze interactions between dark matter and standard model particles with spin one mediators in an effective field theory framework. In this paper, we are considering dark particles masses in the range from a few MeV to the mass of the $Z$ boson. We use bounds from different experiments: $Z$ invisible decay width, relic density, direct detection experiments, and indirect detection limits from the search of gamma-ray emissions and positron fluxes. We obtain solutions corresponding to operators with antisymmetric tensor mediators that fulfill all those requirements within our approach.

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