arXiv subjects

Emilio Royo

Publications and source records attributed to Emilio Royo.

7 recordsLinked to original sources

arXiv2025

Assessment of the $a_{2}(1320)$ tensor-meson contribution to $\eta/\eta^{\prime}\to\pi^{0}\gamma\gamma$ decays

In light of the recent measurement of the $\eta\to\pi^{0}\gamma\gamma$ decay by the KLOE-2 Collaboration, a previous analysis including vector- and scalar-meson exchange contributions using the VMD and L$\sigma$M frameworks, respectively, is extended in the present study to incorporate the effects of the $a_{2}(1320)$ tensor meson within a chiral context. Although the individual contribution of the $a_{2}$ is negligible, its destructive interference with the vector-meson resonances is found to be significant, representing approximately $18\%$ of the total signal and substantially affecting the diphoton invariant-mass distribution, especially at low $m_{\gamma\gamma}^{2}$ values. The total decay rate is calculated to be $\Gamma(\eta\to\pi^{0}\gamma\gamma)=0.154(22)$ eV, which corresponds to a branching ratio of ${\rm BR}(\eta\to\pi^{0}\gamma\gamma)=1.17(17)\times10^{-4}$. This result is approximately $5\sigma$ below the reported value of the PDG, $2.55(22)\times10^{-4}$, while it is in very good agreement with the KLOE-2 measurement, $0.98(11_{\rm stat})(14_{\rm syst})\times10^{-4}$. In contrast, the total contribution of the $a_{2}$ is found to be negligible in $\eta^{\prime}\to\pi^{0}\gamma\gamma$, as this process is completely dominated by the exchange of an on-shell $\omega$ vector resonance.

hep-ph↗

arXiv2022

Sensitivity of the $\eta^{(\prime)}\to\pi^{0}\gamma\gamma$ and $\eta^{\prime}\to\eta\gamma\gamma$ decays to a sub-GeV leptophobic $U(1)_{B}$ boson

The sensitivity of the rare decays $\eta^{(\prime)}\to\pi^{0}\gamma\gamma$ and $\eta^{\prime}\to\eta\gamma\gamma$ to signatures of a leptophobic $B$ boson in the MeV-GeV mass range is analyzed in this work. By adding an explicit $B$-boson resonance exchange, $\eta\to B\gamma\to\pi^{0}\gamma\gamma$, to the Standard Model contributions from vector and scalar meson exchanges, and employing experimental data for the associated branching ratios, it allows us to improve the current constraints on the $B$-boson mass $m_{B}$ and coupling to Standard Model particles $\alpha_{B}$. From these constraints and the analysis of the available experimental $\gamma\gamma$ invariant mass distribution, we show that a $B$-boson signature in the resonant mass range $m_{\pi^{0}}\lesssim m_{B}\lesssim m_{\eta}$ is strongly suppressed and would be very difficult to experimentally identify, assuming that the leptophobic $B$ boson only decays to Standard Model particles. In contrast, the limits outside this mass window are less stringent and the corresponding $t$- and $u$-channel signatures may still be observable in the data, as it occurs with the nonresonant Standard Model $\rho$, $\omega$ and $\phi$ meson exchanges. In addition, we make use of experimental data from the $\eta^{\prime}\to\pi^{0}\gamma\gamma$ and $\eta^{\prime}\to\eta\gamma\gamma$ decays to explore larger $B$-boson masses. Our results are relevant for the $B$-boson search programs at existing and forthcoming light-meson facilities, such as KLOE(-II) and Jefferson Lab Eta Factory experiments.

hep-ph↗

arXiv2022

New-physics signatures via $C\!P$ violation in $\eta^{(\prime)}\to\pi^0\mu^+\mu^-$ and $\eta^\prime\to\eta\mu^+\mu^-$ decays

In this work we investigate the prospect of observing new-physics signatures via $C\!P$ violation in $\eta^{(\prime)}\to\pi^0\mu^+\mu^-$ and $\eta^\prime\to\eta\mu^+\mu^-$ decays at the REDTOP experiment. We make use of the SMEFT to parametrise the new-physics $C\!P$-violating effects and find that the projected REDTOP statistics are not competitive with respect to nEDM experiments. This reasserts the $\eta\to\mu^+\mu^-$ process as the most promising channel to find $C\!P$-violation at this experimental facility.

hep-ph↗

arXiv2021

On Broad Kaluza-Klein Gluons

In theories with a warped extra dimension, composite fermions, as e.g. the right-handed top quark, can be very strongly coupled to Kaluza-Klein (KK) fields. In particular, the KK gluons in the presence of such composite fields become very broad resonances, thus remarkably modifying their experimental signatures. We have computed the pole mass and the pole width of the KK gluon, triggered by its interaction with quarks, as well as the prediction for proton-proton cross-sections using the full propagator and compared it with that obtained from the usual Breit-Wigner approximation. We compare both approaches, along with the existing experimental data from ATLAS and CMS, for the $t\bar t$, $t\bar t W$, $t\bar t Z$, $t\bar t H$, and $t\bar t t\bar t$ channels. We have found differences between the two approaches of up to about 100%, highlighting that the effect of broad resonances can be dramatic on present, and mainly future, experimental searches. The channel $t\bar t t\bar t$ is particularly promising because the size of the cross-section signal is of the same order of magnitude as the Standard Model prediction, and future experimental analyses in this channel, especially for broad resonances, can shed light on the nature of possible physics beyond the Standard Model.

hep-ph↗

arXiv2020

A theoretical analysis of the doubly radiative decays $η^{(\prime)}\toπ^0γγ$ and $η^\prime\toηγγ$

The scalar and vector meson exchange contributions to the doubly radiative decays $η^{(\prime)}\toπ^0γγ$ and $η^\prime\toηγγ$ are analysed within the Linear Sigma Model and Vector Meson Dominance frameworks, respectively. Predictions for the diphoton invariant mass spectra and the associated integrated branching ratios are given and compared with current available experimental data. Whilst a satisfactory description of the shape of the $η\toπ^{0}γγ$ and $η^\prime\toπ^{0}γγ$ decay spectra is obtained, thus supporting the validity of the approach, the corresponding branching ratios cannot be reproduced simultaneously. A first theoretical prediction for the recently measured $η^\prime\toηγγ$ by the BESIII collaboration is also presented.

hep-ph↗

arXiv2020

A theoretical analysis of the semileptonic decays $\eta^{(\prime)}\to\pi^0l^+l^-$ and $\eta^\prime\to\eta l^+l^-$

A complete theoretical analysis of the $C$-conserving semileptonic decays $\eta^{(\prime)}\to\pi^0l^+l^-$ and $\eta^\prime\to\eta l^+l^-$ ($l=e$ or $\mu$) is carried out within the framework of the Vector Meson Dominance (VMD) model. An existing phenomenological model is used to parametrise the VMD coupling constants and the associated numerical values are obtained from an optimisation fit to $V\to P\gamma$ and $P\to V\gamma$ radiative decays ($V=\rho^0$, $\omega$, $\phi$ and $P=\pi^0$, $\eta$, $\eta^{\prime}$). The decay widths and dilepton energy spectra for the two $\eta\to\pi^0l^+l^-$ processes obtained using this approach are compared and found to be in good agreement with other results available in the published literature. Theoretical predictions for the four $\eta^{\prime}\to\pi^0l^+l^-$ and $\eta^\prime\to\eta l^+l^-$ decay widths and dilepton energy spectra are calculated and presented for the first time in this work.

hep-ph↗

arXiv2020

$π^0$-$η$-$η^{\prime}$ mixing from $V\!\rightarrow\!Pγ$ and $P\!\rightarrow\!Vγ$ decays

An enhanced phenomenological model that includes isospin-symmetry breaking is presented in this letter. The model is then used in a number of statistical fits to the most recent experimental data for the radiative transitions $V\!Pγ$ ($V=ρ$, $K^*$, $ω$, $ϕ$ and $P=π$, $K$, $η$, $η^{\prime}$) and estimations for the mixing angles amongst the three pseudoscalar states with vanishing third-component of isospin are obtained. The quality of the performed fits is good, e.g. $χ^2_{\textrm{min}}/\textrm{d.o.f} = 1.9$. The current experimental uncertainties allow for isospin symmetry violations with a confidence level of approximately $2.5σ$.

hep-ph↗