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Pere Masjuan

Publications and source records attributed to Pere Masjuan.

At least 55 records · Page 3Linked to original sources
arXiv2016

Updated pseudoscalar contributions to the hadronic light-by-light of the muon (g-2)

In this work, we present our recent results on a new and alternative data-driven determination for the hadronic light-by-light pseudoscalar-pole contribution to the muon $(g-2)$. Our approach is based on Canterbury approximants, a rational approach to describe the required transition form factors, which provides a systematic and model-independent framework beyond traditional large-$N_c$ approaches. As a result, we obtain a competitive determination with errors according to future $(g-2)$ experiments including, for the first time, a well-defined systematic uncertainty.

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arXiv2016

The $η^\prime$ transition form factor from space- and time-like experimental data

The $η^\prime$ transition form factor is reanalyzed in view of the recent BESIII first observation of the Dalitz decay $η^\prime\toγe^+e^-$ in both space- and time-like regions at low and intermediate energies using the Padé approximants method. The present analysis provides a suitable parameterization for reproducing the measured form factor in the whole energy region and allows to extract the corresponding low-energy parameters together with a prediction of its values at the origin, related to $Γ_{η^\prime\toγγ}$, and the asymptotic limit. The $η$-$η^\prime$ mixing is reassessed within a mixing scheme compatible with the large-$N_c$ chiral perturbation theory at next-to-leading order, with particular attention to the OZI-rule--violating parameters. The $J/ψ$, $Z\toη^{(\prime)}γ$ decays are also considered and predictions reported.

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arXiv2016

Phenomenology of bivariate approximants: the pi0 to e+e- case and its impact on the electron and muon g-2

The current 3$σ$ discrepancy between experiment and Standard Model predictions for $π^0 \to e^+e^-$ is reconsidered using the Padé Theory for bivariate functions, the Canterbury approximants. This method provides a model-independent data-driven approximation to the decay as soon as experimental data for the doubly virtual $π^0$ transition form factor are available. It also implements the correct QCD constraints of the form factor both at low- and high-energies. We reassess the Standard Model result including, for the first time, a systematic error. Our result, BR$(π^0 \to e^+e^-)=6.23(5)\times 10^{-8}$, still represents a discrepancy larger than $2σ$, unsurmountable with our present knowledge of the Standard Model, and would claim New Physics if the experimental result is confirmed by a new measurement. Our method also provides the adequate tool to extract the doubly virtual form factor from experimental data in a straightforward manner. This measurement would further shrink our error and establish once and for all the New Physics nature of the discrepancy. In addition, we remark the challenge this discrepancy poses in the evaluation of the hadronic light-by-light scattering contribution to the $(g-2)_μ$, specially confronted with the foreseen accuracy of the forthcoming $(g-2)_μ$ experiments.

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arXiv2016

On the uncertainty estimates of the $σ$-pole determination by Padé approximants

We discuss the determination of the $f_0(500)$ (or $σ$) resonance by analytic continuation through Padé approximants of the $ππ$-scattering amplitude from the physical region to the pole in the complex energy plane. The aim is to analyze the uncertainties of the method, having in view the fact that analytic continuation is an ill-posed problem in the sense of Hadamard. Using as input a class of admissible parameterizations of the scalar-isoscalar $ππ$ partial wave, which satisfy with great accuracy the same set of dispersive constraints, we find that the Roy-type integral representations lead to almost identical pole positions for all of them, while the predictions of the Padé approximants have a larger spread, being sensitive to features of the input parameterization that are not controlled by the dispersive constraints. Our conservative conclusion is that the $σ$-pole determination by Padé approximants is consistent with the prediction of Roy-type equations, but has an uncertainty almost a factor two larger.

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arXiv2015

Pseudoscalar transition form factors: $(g-2)$ of the muon, pseudoscalar decays into lepton pairs, and the $η-η'$ mixing

We present our model-independent and data-driven method to describe pseudoscalar meson transition form factors in the space- and (low-energy) time-like regions. The method is general and conforms a toolkit applicable to any other form factor, of one and two variables, with the potential to include both high- and low-energy QCD constraints altogether. The method makes use of analyticity and unitary properties of form factors, it is simple, systematic and can be improved upon by including new data. In the present discussion, the method is used to show the impact of experimental data for precision calculations in the low-energy sector of the Standard Model. In particular, due to its relevance for New Physics searches, we have considered the hadronic light-by-light scattering contribution to the anomalous magnetic moment of the muon (the pseudoscalar exchange contribution), the pseudoscalar decays into lepton pairs, and the determination of the mixing parameters of the $η$ and $η'$ system. For all of them we provide the most updated results in a data-driven manner.

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arXiv2015

Ballpark prediction for the hadronic light-by-light contribution to the muon (g-2)_μ

Using the momentum dependence of the dressed quark mass and the well-known formulae for the mass dependent quark-loop contribution to the light-by-light scattering insertions, we compute the hadronic light-by-light contribution to the muon anomalous magnetic moment. The relevant momentum running in the quark loop is calculated from the $π^0$ exchange contribution to the light- by-light scattering. Special emphasis on the reconstruction of the pseudoscalar transition form factor is made, and the $π^0$ contribution to the hadronic light-by-light is, as well, updated.

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arXiv2015

$η$-$η'$ mixing in large-$N_c$ chiral perturbation theory: discussion, phenomenology, and prospects

A systematic study of the $η$-$η'$ mixing in Large-$N_c$ chiral perturbation theory is presented with special emphasis on the role of the next-to-next-to-leading-order contributions in the combined momentum, quark-mass, and $1/N_c$ expansions. At this order, loop corrections as well as OZI-rule-violating pieces need to be included. Mixing angles as well as pseudoscalar decay constants are discussed within this framework. The results are compared with recent phenomenological approaches.

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arXiv2015

Pseudoscalar decays into lepton pairs from rational approximants

The pseudoscalar decays into lepton pairs $P\rightarrow\overline{\ell}\ell$ are analyzed with the machinery of Canterbury approximants, an extension of Padé approximants to bivariate functions. This framework provides an ideal model-independent approach to implement all our knowledge of the pseudoscalar transition form factors driving these decays, can be used for data analysis, and allows to include experimental data and theoretical constraints in an easy way, and determine a systematic error. We find that previous theoretical estimates for these branching ratios have underestimated their theoretical uncertainties. From our updated results, the existing experimental discrepancies for $π^0\rightarrow e^+e^-$ and $η\rightarrow μ^+μ^-$ channels cannot be explained unless the doubly-virtual transition form factors behavior -not yet measured- are out of theoretical expectations, which is an interesting result both for anomalous magnetic moments of leptons, and for physics beyond the standard model.

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arXiv2015

Pseudoscalar-exchange contribution to $(g-2)_μ$ from rational approximants

We present our recent results on the hadronic light-by-light pseudoscalar-exchange contribution to the anomalous magnetic moment of the muon using rational approximants. Our work provides a generalization of Padé approximants to the bivariate case to describe the most general doubly-virtual transition form factor, which is required in the calculation. This method provides a powerful tool to systematically implement the known experimental data on transition form factors in the space-like region and low energies in a model-independent way. Given the lack of experimental data on the doubly-virtual transition form factor, we make use of the pseduoscalar decays into lepton pairs. We find an interesting and puzzling situation which calls for new experimental measurements to clarify the present state.

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arXiv2014

Large-Nc Regge spectroscopy

This talk, dedicated to Eef van Beveren on the occasion of his birthday, reviews some of our results concerning the hadron spectroscopy, Regge trajectories, and the large-Nc meson-dominance of hadronic form factors.

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arXiv2014

$η$ and $η'$ transition form factors from Padé approximants

We employ a systematic and model-independent method to extract, from space- and time-like data, the $η$ and $η'$ transition form factors (TFFs) obtaining the most precise determination for their low-energy parameters and discuss the $Γ_{η\rightarrowγγ}$ impact on them. Using TFF data alone, we also extract the $η-η'$ mixing parameters, which are compatible to those obtained from more sophisticated and input-demanding procedures.

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arXiv2014

Precise determination of resonance pole parameters through Padé approximants

In this work, we present a precise and model--independent method to extract resonance pole parameters from phase-shift scattering data. These parameters are defined from the associated poles in the second Riemann sheet, unfolded by the analytic continuation to the complex pole using Padé approximants. Precise theoretical parameterizations of pion-pion scattering phase shifts based on once-- and twice-- subtracted dispersion relations are used as input, whose functional form allows us to show the benefit and accuracy of the method. In particular, we extract from these parametrization the pole positions of the $f_0(500)$ at $\sqrt{s}=(453\pm 15) - i(297 \pm 15)$ MeV, the $ρ(770)$ at $\sqrt{s}=(761.4\pm 1.2)-i(71.8 \pm 1.0)$ MeV, and the pole of the $f_2(1270)$, located at $\sqrt{s}=(1267.3\pm 1.7)-i(95.0\pm 2.3)$ MeV. The couplings of the resonances to two pions are also determined with high precision, obtaining respectively, $3.8\pm 0.4$ GeV, $5.92\pm 0.15$ and $4.41\pm 0.23$ GeV$^{-1}$. Special attention is dedicated to the systematic treatment of the theoretical and statistical uncertainties, together with their comparison with previous determinations.

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arXiv2014

Pseudoscalar Transition Form Factors from Rational Approximants

The $π^0$, $η$, and $η^\prime$ transition form factors in the space-like region are analyzed at low and intermediate energies in a model-independent way through the use of rational approximants. Slope and curvature parameters as well as their values at infinity are extracted from experimental data. These results are suited for constraining hadronic models such as the ones used for the hadronic light-by-light scattering piece of the anomalous magnetic moment of the muon, and for the mixing parameters of the $η- η^\prime$ system.

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arXiv2014

Hadron form factors and large-Nc phenomenology

The half width rule provides a way to consider 1/Nc corrections to hadronic models containing resonances. Consequences of such ideas for hadron form factors and Regge trajectories are explored, with special emphasis on the possibility to describe the spectrum of light and heavy unflavored vector mesons in a universal way.

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arXiv2014

Heavy neutrino decays at MiniBooNE

It has been proposed that a sterile neutrino ν_h with m_h \approx 50 MeV and a dominant decay mode (ν_h -> νγ) may be the origin of the experimental anomaly observed at LSND. We define a particular model that could also explain the MiniBooNE excess consistently with the data at other neutrino experiments (radiative muon capture at TRIUMF, T2K, or single photon at NOMAD). The key ingredients are (i) its long lifetime (τ_h\approx 3-7x10^{-9} s), which introduces a 1/E dependence with the event energy, and (ii) its Dirac nature, which implies a photon preferably emitted opposite to the beam direction and further reduces the event energy at MiniBooNE. We show that these neutrinos are mostly produced through electromagnetic interactions with nuclei, and that T2K observations force BR(ν_h -> ν_τγ) \le 0.01 \approx BR(ν_h -> ν_μγ). The scenario implies then the presence of a second sterile neutrino ν_{h'} which is lighter, longer lived and less mixed with the standard flavors than ν_h. Since such particle would be copiously produced in air showers through (ν_h -> ν_{h'}γ) decays, we comment on the possible contamination that its photon-mediated elastic interactions with matter could introduce in dark matter experiments.

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arXiv2013

Reply to "Comment on 'Systematics of radial and angular-momentum Regge trajectories of light non-strange qqbar-states' "

In his Comment, D. Bugg argues against our usage of the PDG collection of light non-strange states together with the half-width rule to analyze the linearity of radial and angular-moment Regge trajectories in the large-N_c limit. After taking into account his observations on our choice of data, the radial Regge trajectories are again analyzed. We still find that our conclusion on the lack of universality between radial- and angular-momentum Regge trajectories is valid.

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