arXiv subjects

Pablo Roig

Publications and source records attributed to Pablo Roig.

At least 55 records · Page 3Linked to original sources

arXiv2020

Axial-vector exchange contribution to the hadronic light-by-light piece of the muon anomalous magnetic moment

In this work we study the axial contributions to the hadronic light-by-light piece of the muon anomalous magnetic moment using the framework of resonance chiral theory. As a result, we obtain $a_μ^{\textrm{HLbL};A} = \left(0.8^{+3.5}_{-0.1}\right)\cdot 10^{-11}$, that might suggest a smaller value than most recent calculations, underlining the need of future work along this direction. In particular, we find that our results depend critically on the asymptotic behavior of the form factors, and as such, emphasizes the relevance of future experiments for large photon virtualities. In addition, we present general results regarding the involved axial form factors description, comprehensively examining (and relating) the current approaches, that shall be of general interest.

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arXiv2019

Exclusive hadronic tau decays as probes of non-SM interactions

We perform a global analysis of exclusive hadronic tau decays into one and two mesons using the low-energy limit of the Standard Model Effective Field Theory up to dimension six, assuming left-handed neutrinos. A controlled theoretical input on the Standard Model hadronic form factors, based on chiral symmetry, dispersion relations, data and asymptotic QCD properties, has allowed us to set bounds on the New Physics effective couplings using the present experimental data. Our results highlight the importance of semileptonic $τ$ decays in complementing the traditional low-energy probes, such nuclear $β$ decays or semileptonic pion and kaon decays, and the high-energy measurements at LHC scales. This makes yet another reason for considering hadronic tau decays as golden modes at Belle-II.

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arXiv2019

Effective-field theory analysis of the $τ^{-}\to K^{-}(η^{(\prime)},K^{0}) ν_τ$ decays

We analyze the decays $τ^-\to K^-(η^{(\prime)},K^0) ν_τ$ within an effective field theory that includes the most general interactions between Standard Model fields up to dimension six, assuming left-handed neutrinos. In particular, we examine different interesting phenomenological observables i.e. decay spectra and branching ratio, Dalitz plot distributions and the forward-backward asymmetry, to explore the sensitivity of the corresponding decays to the effects of non-standard interactions. A controlled theoretical input on the Standard Model hadronic form factors, based on chiral symmetry, dispersion relations, data and asymptotic QCD properties, has allowed us to set bounds on the New Physics scalar and tensor effective couplings using the measured branching ratios. These are found to be in line with the findings of our series of previous analyses of two-meson tau decays and less precise than the constraints obtained from semileptonic kaon decays. In order to set stringent limits on these couplings, we will use all available experimental data of all possible di-meson tau decays. This is our next step plan, that we hope to be of interest for future experimental analyses of these decays.

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arXiv2019

Effective-field theory analysis of the $τ^{-}\rightarrow (K π)^{-}ν_τ$ decays

We analyze the $τ^-\to(Kπ)^-ν_τ$ decays within an effective field theory description of heavy new physics (NP) modifying the SM left-handed weak charged current and include refined SM input for the participant meson form factors exploiting chiral symmetry, dispersion relations and (lattice) data. We include the leading dimension six operators and work at linear order in the effective couplings. Within this setting we: i) follow the derivation in Phys. Rev. Lett. 120 (2018) no.14, 141803 where it was proved unambiguously that it is impossible to understand the BaBar anomaly in the CP asymmetry measurement within this framework. We allow for reasonable variations of the hadronic input involved and study the associated uncertainty; ii) first show that the anomalous bump present in the published Belle data for the $K_Sπ^-$ invariant mass distribution close to threshold cannot be due to heavy NP; iii) first bind the heavy NP effective couplings using $τ^-\to(Kπ)^-ν_τ$ decays and show that they are competitive with those found in hyperon semileptonic decays (but clearly not with those obtained in Kaon (semi)leptonic decays for NP scalar currents). We put forward that the comparison of the considered tau decays with (semi)leptonic kaon and hyperon decays provide with meaningful tests of lepton universality for (NP) tensor interactions. We also compare the SM predictions with the possible deviations caused by NP in a couple of Dalitz plot distributions, in the forward-backward asymmetry and in the di-meson invariant mass distribution and discuss the most interesting measurements to be performed at Belle-II using these decays data.

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arXiv2019

Semileptonic $τ$ decays: powerful probes of non-standard charged current weak interactions

When looking for heavy ($\mathcal{O}$(few TeV)) New Physics, the most efficient way to benefit from both high and low-energy measurements simultaneously is the use of the Standard Model Effective Field Theory (SMEFT). In this talk I highlight the importance of semileptonic $τ$ decays in complementing, in this respect, the traditional low-energy precision observables and high-energy measurements. This is yet another reason for considering hadronic tau decays as golden channels at Belle-II beyond the unquestionable interest of the CP violation anomaly in $τ\to K_Sπν_τ$ decays, that I also discuss within the effective theory. A couple of new results for $τ^-\to K^-ν_τ$ decays are also included.

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arXiv2019

A dispersive analysis of the pion vector form factor and $τ^{-}\to K^{-}K_{S}ν_τ$ decay

We explore the potential of a combined analysis of the decays $τ^{-}\toπ^{-}π^{0}ν_τ$ and $τ^{-}\to K^{-}K_{S}ν_τ$ in the determination of the $ρ(1450)$ and $ρ(1700)$ resonance properties in the frame of resonance Chiral Theory supplemented by dispersion relations. On the one hand, we take advantage of the very precise data on the modulus squared of the pion vector form factor $|F_{V}^π|^{2}$ obtained by Belle to carry out a very dedicated analysis of the region where these resonances come up into play. Our study provides an improved treatment of the systematic theoretical errors and, as a most important result, we conclude that they dominate over the fit uncertainties in the determination of the $ρ(1450)$ and $ρ(1700)$ pole parameters and tend to be larger than in other determinations quoted in the literature where these errors were ignored or underestimated. The results of our analysis are summarized in numerical tables for the form factor modulus and phase, including both statistical and systematic errors, that can be found as ancillary material of this paper. As a byproduct, we also determine the low-energy observables of the pion vector form factor and the $ρ$-pole position. On the other hand, we benefit from the recent experimental data for the transition $τ^{-}\to K^{-}K_{S}ν_τ$ released by BaBar to perform a first analysis of its decay spectrum and discuss the role of these resonances in this decay. We point out that higher-quality data on the $K^{-}K_{S}$ decay channel will allow compete with the $|F_{V}^π|^{2}$ ones and improve the determination of the $ρ(1450)$ and $ρ(1700)$ resonance parameters as a result of a combined analysis. We hope our study to be of interest for present and future experimental analysis of these decays.

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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 $a_μ^{P,HLbL}=(8.47\pm 0.16_{\rm stat}\pm 0.09_{N_C}{}^{+0.5}_{-0.0_{\rm asym}})10^{-10}$.

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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 $a_μ^{P,HLbL}=(8.47\pm 0.16_{\rm stat}\pm 0.09_{N_C}{}^{+0.5}_{-0.0_{\rm asym}})$

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arXiv2016

$H\to \ell\ell'$ in the Simplest Little Higgs Model

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 $H\toτμ$ CMS hint and at the same time respect (as they do) the severe limits on $H\toμe$ inherited from the non-observation of $μ\to eγ$. We compute the predictions of the Simplest Little Higgs Model for the $H\to \ell\ell'$ 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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arXiv2016

Towards the (Mexican) discovery of second class currents at Belle-II

Within the SM, the yet unmeasured $τ^-\toπ^-η^{(\prime)}ν_τ$ decays are predicted as a suppressed, isospin-violating effect with branching ratios $\lesssim\mathcal{O}(10^{-5})$. However, they can also proceed through other mechanisms (such as charged Higgs exchange) at comparable rates. This has motivated several studies of the corresponding QCD predictions for these second class current processes. In this contribution we discuss the prospects for the discovery of these decays at Belle-II emphasizing the Mexican involvement in this project. Our branching ratio prediction $\sim1.7\cdot10^{-5}$ (decay channel with an $η$ meson) is well within the reach of Belle-II. The branching fraction for the decay channel with an $η^\prime$ meson is expected to be between one and two orders of magnitude more suppressed.

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arXiv2016

Predictions on the second-class current decays $τ^{-}\toπ^{-}η^{(\prime)}ν_τ$

We analyze the second-class current decays $τ^{-}\toπ^{-}η^{(\prime)}ν_τ$ in the framework of Chiral Perturbation Theory with resonances. Taking into account $π^{0}$-$η$-$η^{\prime}$ mixing, the $π^{-}η^{(\prime)}$ vector form factor is extracted, in a model-independent way, using existing data on the $π^{-}π^{0}$ one. For the participant scalar form factor, we have considered different parameterizations ordered according to their increasing fulfillment of analyticity and unitarity constraints. We start with a Breit-Wigner parameterization dominated by the $a_{0}(980)$ scalar resonance and after we include its excited state, the $a_{0}(1450)$. We follow by an elastic dispersion relation representation through the Omnès integral. Then, we illustrate a method to derive a closed-form expression for the $π^{-}η$, $π^{-}η^{\prime}$ (and $K^{-}K^{0}$) scalar form factors in a coupled-channels treatment. Finally, predictions for the branching ratios and spectra are discussed emphasizing the error analysis. An interesting result of this study is that both $τ^{-}\toπ^{-}η^{(\prime)}ν_τ$ decay channels are promising for the soon discovery of second-class currents at Belle-II. We also predict the relevant observables for the partner $η^{(\prime)}_{\ell 3}$ decays, which are extremely suppressed in the Standard Model.

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arXiv2014

Resonance Chiral Lagrangians and alternative approaches to hadronic tau decays

Exclusive semi-leptonic decays of the tau lepton offer a clean probe to study the hadronization of QCD currents in its non-perturbative regime and learn about resonance dynamics, which drives strong interactions in these processes. In this theory outlook, I will use the simplest non-trivial di-pion tau decays to illustrate briefly recent theoretical progress on these analyses and their comparison to data.

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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.

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arXiv2014

Consistent high-energy constraints in the anomalous QCD sector

The anomalous Green function and related form-factors (pi^0 to gamma^* gamma^* and tau^- to X^- nu_tau vector form-factors, with X^-=(KKpi)^-, phi^- gamma, (phi V)^-) are analyzed in this letter in the large-N_C limit. Within the single (vector and pseudoscalar) resonance approximation and the context of Resonance Chiral Theory, we show that all these observables over-determine in a consistent way a unique set of compatible high-energy constraints for the resonance couplings. This result is in agreement with analogous relations found in the even intrinsic-parity sector of QCD like, e.g., F_V^2 = 3 F^2. The antisymmetric tensor formalism is considered for the spin-one resonance fields. Finally, we have also worked out and provide here the relation between the two bases of odd intrinsic-parity Lagrangian operators commonly employed in the literature.

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arXiv2014

Failure of Breit-Wigner and success of dispersive descriptions of the τ^-\to K^-ην_τdecays

The τ^-\to K^-ην_τdecays have been studied using Chiral Perturbation Theory extended by including resonances as active fields. We have found that the treatment of final state interactions is crucial to provide a good description of the data. The Breit-Wigner approximation does not resum them and neglects the real part of the corresponding chiral loop functions, which violates analyticity and leads to a failure in the confrontation with the data. On the contrary, its resummation by means of an Omnès-like exponentiation of through a dispersive representation provides a successful explanation of the measurements. These results illustrate the fact that Breit-Wigner parametrizations of hadronic data, although simple and easy to handle, lack a link with the underlying strong interaction theory and should be avoided. As a result of our analysis we determine the properties of the K*(1410) resonance with a precision competitive to its traditional extraction using τ^-\to (Kπ)^-ν_τdecays, albeit the much limited statistics accumulated for the τ^-\to K^-ην_τchannel. We also predict the soon discovery of the τ^-\to K^-η'ν_τdecays.

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arXiv2014

Prospects for discovery of the τ^- \to π^- \ell^+ \ell^- ν_τdecays

We study the phenomenology of the τ^- \to π^- ν_τ\ell^+ \ell^- decays (\ell = e, μ), predicting the respective branching ratios and di-lepton invariant-mass spectra. In addition to the model-independent (QED) contributions, we investigate the structure-dependent (SD) terms, encoding features of the hadronization of QCD currents. The relevant form factors are evaluated by supplementing Chiral Perturbation Theory with the inclusion of the lightest (axial-)vector resonance multiplet as dynamical fields. The Lagrangian couplings are fully predicted requiring the known QCD asymptotic behavior to the relevant Green functions and associated form factors in the limit of an infinite number of colours. As a consequence we predict that the τ^- \to π^- ν_τe^+ e^- decays should be discovered soon while this is not granted for the \ell = μcase.

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arXiv2013

Hadronic and radiative decays of the tau lepton

This PhD thesis studies some hadronic and radiative decays of the tau lepton using a Chiral Lagrangian including resonance fields. After a theoretical introduction, the decays to the $(πππ)^-$, $(KKπ)^-$ and $η^{(\prime)} π^- π^0$ hadronic states and $τ\to (π/K)^- γν_tau$ are analysed. Some material, which was not included in the corresponding references, is also discussed: The structure functions analysis for the 3 pion modes, the differential decay width as a function of the two-particle invariant masses in the considered three-meson modes and a simplified study of the $τ\to$η^{(\prime)} π^- π^0 ν_tau$ processes within a single-angle mixing scheme for the $η$ and $η^\prime$ mesons.

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arXiv2013

Dispersive analysis of $τ^-\toπ^-π^0ν_τ$ Belle data

We analyse Belle data on the decay $τ^- \to π^- π^0 ν_τ$ using a dispersive representation of the vector form factor which is consistent with chiral symmetry and preserves analyticity and unitarity exactly. We fit the unknown theoretical parameters from the data, determining the values of the related low-energy observables $\left\langle r^2\right\rangle^π_V$ and $c_V^π$. The implementation of isospin breaking effects is also discussed.

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