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Adolfo Guevara

Publications and source records attributed to Adolfo Guevara.

15 recordsLinked to original sources
arXiv2026

Novel method for determining the light quark mass ratio using $η'\toηππ$ decays

We propose a novel approach for extracting symmetry breaking effects from symmetry conserving three-body decays. The method is based on mapping the Dalitz plot to a unit disk, and the difference of the disk distributions of two related decays isolates purely symmetry breaking effects. We demonstrate this method by extracting the fundamental parameter $Q$, an isospin breaking ratio of light quark masses defined as $Q^2\equiv (m_s^2-\hat m^2)/(m_d^2-m_u^2)$ with $\hat m$ the average of up and down quark masses, from the decays $η'\toηπ^+π^-$ and $η'\toηπ^0π^0$. With the Dalitz plot distributions for these two decays reported by BESIII, we illustrate the method and obtain $Q=22.5\pm1.0$, which is consistent with previous determinations and has a comparable uncertainty. With the full BESIII data set, which is eight times larger than the one used here, a more precise determination of $Q$ should become possible. This promising and novel method can be generalized to other three-body decays to extract symmetry breaking effects.

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↗
arXiv2023

On the $η_1(1855)$, $π_1(1400)$ and $π_1(1600)$ as dynamically generated states and their SU(3) partners

In this work, we interpret the newly observed $η_1(1855)$ resonance with exotic $J^{PC}=1^{-+}$ quantum numbers in the $I=0$ sector, reported by the BESIII Collaboration, as a dynamically generated state from the interaction between the lightest pseudoscalar mesons and axial-vector mesons. The interaction is derived from the lowest order chiral Lagrangian from which the Weinberg-Tomozawa term is obtained, describing the transition amplitudes among the relevant channels, which are then unitarized using the Bethe-Salpeter equation, according to the chiral unitary approach. We evaluate the $η_1(1855)$ decays into the $ηη^{\prime}$ and $K\bar{K}^*π$ channels and find that the latter has a larger branching fraction. We also investigate its SU(3) partners, and according to our findings, the $π_1(1400)$ and $π_1(1600)$ structures may correspond to dynamically generated states, with the former one coupled mostly to the $b_1π$ component and the latter one coupled to the $K_1(1270)\bar{K}$ channel. In particular, our result for the ratio $Γ(π_1(1600)\to f_1(1285)π)/ Γ(π_1(1600)\to η^{\prime}π)$ is consistent with the measured value, which supports our interpretation for the higher $π_1$ state. We also report two poles with a mass about 1.7~GeV in the $I=1/2$ sector, which may be responsible for the $K^*(1680)$. We suggest searching for two additional $η_1$ exotic mesons with masses around 1.4 and 1.7~GeV. In particular, the predicted $η_1(1700)$ is expected to have a width around 0.1~GeV and can decay easily into $K\bar Kππ$.

hep-ph↗
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 $|V_{ud}/V_{us}|^2=0.05$.

hep-ph↗
arXiv2021

The $τ\toν_τπe^+e^-$ decay revisited

The recent results from Belle of the $τ^-\toν_τπ^-e^+e^-$ analysis has incentivated us to make a reanalysis of a former work where the Structure Dependent parts are obtained using Resonance Chiral Theory. Here we rely on the same theory accounting for effects that explicitly break such symmetry. A motivation is the involved $Wπγ^\star$ vertex, utterly relevant in computing $τ^- \to ν_τπ^-$ radiative corrections.

hep-ph↗
arXiv2021

Partial wave correlations with uncertainties in the Two Nucleon System

The NN scattering problem is usually analyzed in terms of partial waves and the corresponding coupled channel phase-shifts and mixing angles, but the available experiments induce correlations amongst the corresponding channels with different quantum numbers. Based on the Granada-2013 database we analyze the meaning and impact of those correlations taking into account both the purely statistical ones reflecting the primary experimental data uncertainties as well as the systematic ones exhibiting the ambiguities in the form of the potential representing the unknown nuclear force for distances below 3fm. We also ponder on the implications of fixing nuclear potentials by direct fits to the phase-shifts stemming from a full fledged partial wave analysis (PWA) inferred from experimental data.

nucl-th↗
arXiv2019

Simple explanation of strong suppression of fermionic EFT operators and custodial symmetry breaking

The present approach relies on the SM chiral symmetry breaking pattern $SU(2)_L\otimes SU(2)_R\to SU(2)_{L+R}$, with the EW Goldstone bosons given in a non-linear realization and the Higgs boson described by an EW singlet field. In addition, we assume the presence of new physics heavy states around the TeV scale that do not couple to the SM fermions, only to the SM bosonic sector. However, the mixing between gauge bosons and BSM resonances induces a small indirect interaction between the BSM sector and the SM fermions. This leads to an important suppression of the fermionic operators in the low-energy EW effective theory (bilinear and four-fermion operators) in comparison with the purely bosonic ones. This naturally explains the strong experimental bounds on fermionic operators and why these resonances could not be yet detected: even if energies of the order of the TeV can be reached in present and future accelerators, their production from initial SM fermions yields a very small cross section because of this suppression mechanism. On the other hand, they can leave an imprint in SM boson measurements accessible to future experimental runs (e.g., the oblique $S$ and $T$ parameters). Finally, we compare our results with constraints from collider data.

hep-ph↗
arXiv2018

Chiral symmetry breaking corrections to the pseudoscalar pole contribution of the Hadronic Light-by-Light piece of $a_μ$

We have studied the $P\toγ^\starγ^\star$ form factor in Resonance Chiral Theory, with $P = π^0ηη'$, to compute the contribution of the pseudoscalar pole to the hadronic light-by-light piece of the anomalous magnetic moment of the muon. In this work we allow the leading $U(3)$ chiral symmetry breaking terms, obtaining the most general expression for the form factor up to $\mathcal{O}(m_P^2)$. The parameters of the Effective Field Theory are obtained by means of short distance constraints on the form factor and matching with the expected behavior from QCD. Those parameters that cannot be fixed in this way are fitted to experimental determinations of the form factor within the spacelike region. Chiral symmetry relations among the transition form factors for $π^0,η$ and $η'$ allow for a simultaneous fit to experimental data for the three mesons. This shows an inconsistency between the BaBar $π^0$ data and the rest of the experimental inputs. Thus, we find a total pseudoscalar pole contribution of $a_μ^{P,HLbL}=(8.47\pm 0.16)\cdot 10^{-10}$ for our best fit (that neglecting the BaBar $π^0$ data). Also, a preliminary rough estimate of the impact of NLO in $1/N_C$ corrections and higher vector multiplets (asym) enlarges the uncertainty up to $a_μ^{P,HLbL}=(8.47\pm 0.16_{\rm stat}\pm 0.09_{N_C}{}^{+0.5}_{-0.0_{\rm asym}})10^{-10}$.

hep-ph↗
arXiv2018

Pseudoscalar pole contribution to the hadronic light-by-light piece of $a_μ$

We have studied the $P\toγ^\starγ^\star$ form factor in Resonance Chiral Theory, with $P = π^0,η,η'$, to compute the contribution of the pseudoscalar pole to the hadronic light-by-light piece of the anomalous magnetic moment of the muon. In this work we allow the leading $U(3)$ chiral symmetry breaking terms, obtaining the most general expression for the form factor of order $\mathcal{O}(m_P^2)$. The parameters of the Effective Field Theory are obtained by means of short distance constraints on the form factor and matching with the expected behavior from QCD. Those parameters that cannot be fixed in this way are fitted to experimental determinations of the form factor within the spacelike momentum region of the virtual photon. Chiral symmetry relations among the transition form factors for $π^0,η$ and $η'$ allow for a simultaneous fit to experimental data for the three mesons. This shows an inconsistency between the BaBar $π^0$ data and the rest of the experimental inputs. Thus, we find a total pseudoscalar pole contribution of $a_μ^{P,HLbL}=(8.47\pm 0.16)\cdot 10^{-10}$ for our best fit (neglecting the BaBar $π^0$ data). Also, a preliminary rough estimate of the impact of NLO in $1/N_C$ corrections and higher vector multiplets (asym) enlarges the uncertainty up to $a_μ^{P,HLbL}=(8.47\pm 0.16_{\rm stat}\pm 0.09_{N_C}{}^{+0.5}_{-0.0_{\rm asym}})$

hep-ph↗
arXiv2017

Backgrounds in the search for Second Class Currents in $τ$ decays

As constructed, the Standard Model does not include genuine Second Class Currents, however these can be induced through the breaking of isospin or charge conjugation. The experimental limits from B-Factories are getting closer to the predictions made for induced Second Class Currents through isospin breaking in the processes $τ\toπη^{(\prime)}ν_τ$, therefore a more careful analysis of the background in these decays becomes necessary. In this work we analyze the $τ\toπη^{(\prime)}γν_τ$ decays as background of the non-radiative process. We find that the radiative processes are very important background for the non-radiative ones whenever the photon escapes detection. Photons cannot be completely excluded since they need to be kept for the identification of the eta and eta' in the final state. However, we find that making appropriate cuts in the energy of the photon, this process can be disregarded as an important background.

hep-ph↗
arXiv2017

Low-energy meson phenomenology with Resonance Chiral Lagrangians

The low energy behaviour of Quantum Chromodynamics makes unreliable an expansion in terms of its coupling strength, since nothing guarantees the convergence of such expansion. To overcomer such difficulty one resorts to Lattice QCD or Effective Field Theories. In this thesis we compute hadronic processes at low energies, using Resonance Chiral Lagrangians as the effective theory of QCD. The processes we study are important background on the search for Lepton Flavor Violation, Lepton Universality Violation, Lepton Number Violation, Second Class Currents and study the Hadronic light-by-light contribution to the Anomalous Magnetic Moment of the muon. All such processes are of great interest since all of them are to be studied with an increased precision in the very near future by different experiments.

hep-ph↗
arXiv2016

Hadronic light-by-light contribution to the muon g-2

We have computed the hadronic light-by-light (LbL) contribution to the muon anomalous magnetic moment $a_μ$ in the frame of Chiral Perturbation Theory with the inclusion of the lightest resonance multiplets as dynamical fields (R$χ$T). It is essential to give a more accurate prediction of this hadronic contribution due to the future projects of J-Parc and FNAL on reducing the uncertainty in this observable. We, therefore, computed the pseudoscalar transition form factor and proposed the measurement of the $e^+e^-\toμ^+μ^-π^0$ cross section and dimuon invariant mass spectrum to determine more accurately its parameters. Then, we evaluated the pion exchange contribution to $a_μ$, obtaining $(6.66\pm0.21)\cdot10^{-10}$. By comparing the pion exchange contribution and the pion-pole approximation to the corresponding transition form factor ($π$TFF) we recalled that the latter underestimates the complete $π$TFF by (15-20)\%. Then, we obtained the $η^({}'^)$ TFF, obtaining a total contribution of the lightest pseudoscalar exchanges of $(10.47\pm0.54)\cdot10^{-10}$, in agreement with previous results and with smaller error.

hep-ph↗
arXiv2014

Lightest pseudoscalar exchange contribution to light-by-light scattering piece of the muon g-2

Lightest pseudoscalar (P= pi^0, eta, eta') exchange contribution to the light-by-light (LbL) scattering piece of the muon anomaly, a_mu=(g_mu-2)/2, has been evaluated using a resonance chiral Lagrangian (RχL). Best description of pion transition form-factor (TFF) data is obtained with only tiny violations of one of the relations in the minimal consistent set of short-distance constraints on the anomalous RχL couplings. eta^{(')} TFF, predicted in terms of the pi TFF and the eta-eta^' mixing, are in good agreement with measurements. With this input, we obtain a_mu^{P,LbL} = (10.47\pm0.54)x10^-10, consistent with the reference determinations in the literature, albeit with smaller error.

hep-ph↗
arXiv2014

$W^*γ^*π$ form factors in the $τ^- \to π^-\ell^+\ell^- ν_τ$ decays

We study the disintegration $τ^- \to π^-\ell^+\ell^-ν_τ$, where $l$ denotes either an electron or a muon. This process requires knowledge about the $W^*γ^*π$ interaction, which is modeled using Resonance Chiral Theory. Within this framework we study the sensitivity of the observables of the process, such as the branching ratios and the dilepton mass spectra, to the model dependent contributions. The information of the hadronization of the quark currents is encoded in form factors which are evaluated using the lightest vector and axial-vector multiplets as degrees of freedom. We determine the couplings by matching the asymptotic QCD behavior of the form factors in the limit of an infinite number of colours. The branching ratios we obtain show that the process with $l=e$ should be measured soon in $B$ factories.

hep-ph↗