arXiv subjects

R. Di Palma

Publications and source records attributed to R. Di Palma.

7 recordsLinked to original sources

arXiv2026

Charmonium radiative transitions to dileptons from lattice QCD: The case of $h_c \to η_c \ell^+\ell^-$ and $χ_{c1} \to J/ψ\,\ell^+\ell^-$

We present a lattice QCD study of dilepton production in charmonium transitions, specifically focusing on the $1^{+-} \to 0^{-+}$ and $1^{++} \to 1^{--}$ processes: $h_c \to η_c \ell^+ \ell^-$ and $χ_{c1} \to J/ψ\ell^+ \ell^-$, where $\ell = e, μ$. The relevant hadronic matrix elements are computed using gauge field configurations generated by the Extended Twisted Mass Collaboration with $N_f = 2+1+1$ dynamical Wilson--Clover twisted-mass fermions at four lattice spacings. Simulations are performed at physical dynamical $u$, $d$, $s$, and $c$ quark masses, except for the coarsest lattice, where the lightest sea quark mass corresponds to a slightly heavier pion mass. A controlled continuum extrapolation is carried out. In the continuum limit for the $h_c$ decays, we obtain $Γ(h_c \to η_c e^+ e^-) = 5.45(19)~\mathrm{keV}$, and $Γ(h_c \to η_c μ^+ μ^-) = 0.635(22)~\mathrm{keV}$. For the $χ_{c1}$ decays, we find: $Γ(χ_{c1} \to J/ψe^+ e^-)= 2.869(90)~\mathrm{keV}$, and $Γ(χ_{c1} \to J/ψμ^+ μ^-) = 0.1993(72)~\mathrm{keV}$. Our results for the $χ_{c1}$ decays show good compatibility with experimental data. However, our prediction for the $h_c \to η_c e^+ e^- $ decay rate is approximately $3σ$ larger than the BESIII result. We also present predictions for the differential decay widths as functions of the dilepton invariant mass, $q^2$, and for angular observables sensitive to longitudinal transition form factors, which are inaccessible in radiative decays with real photon emission. These results constitute the first fully dynamical lattice QCD predictions for dilepton decay rates in $h_c$ and $χ_{c1}$ charmonium transitions, including their differential distributions and angular observables. They provide benchmark predictions for future experimental studies.

hep-lat↗

arXiv2026

Rare kaon decays $K^- \to \ell^- \barν_\ell \ell'^{+} \ell'^{-}$: Standard Model predictions from lattice QCD

Weak decays of charged kaons with an additional lepton-antilepton pair, $K^- \to \ell^- \barν_\ell \ell'^{+} \ell'^{-}$ ($K_{\ell2\ell'}$), are suppressed at order $O(G_{F}^{2}α_{\rm em}^{2})$ in the Standard Model (SM) and provide sensitive probes of its flavour structure, as well as independent determinations of the Cabibbo angle $|V_{us}|$. In this Letter we present the SM predictions for all four channels with $\ell,\ell' =e,μ$, based on the first complete lattice QCD calculation of the structure-dependent form factors reported in a companion paper [1]. Using the PDG value [2] $|V_{us}|^{\rm PDG}=0.22431(85)$, we obtain branching fractions with controlled uncertainties and precisions ranging from $2\%$ to $7\%$, depending on the channel. For the three modes with published measurements, our results agree with experiment. For the $K_{\mu2μ}$ mode, for which no published experimental result is available, we compare our prediction with the preliminary NA62 result, finding agreement at the $1.4σ$ level. Conversely, the measured decay rates can be used together with our results to extract $|V_{us}|$ from these modes. A weighted average over the two most precise channels, $K_{\mu2e}$ and $K_{\mu2μ}$, yields $|V_{us}|=0.2283(42)$, corresponding to a $1.8\%$ determination. These results pave the way for using $K_{\ell2\ell'}$ decays as precision probes of the SM.

hep-ph↗

arXiv2026

Complete lattice QCD calculation of $K^{-}\to \ell^{-}\barν_{\ell}\ell^{'+}\ell^{'-}$ form factors

We present the first complete lattice QCD calculation of the four structure-dependent form factors governing the rare charged kaon decay $K^- \to \ell^- \barν_\ell \ell'^+ \ell'^-$, with fully controlled statistical and systematic uncertainties. Our calculation is based on gauge ensembles generated by the Extended Twisted Mass Collaboration (ETMC) with $N_f = 2+1+1$ flavors of Wilson-clover twisted-mass fermions. Simulations are performed directly at the physical values of the light and strange quark masses, and include an estimate of the quark-disconnected contributions in which the virtual photon couples to sea quarks. All four form factors are determined across the kinematical region probed by experiments. The Spectral Function Reconstruction (SFR) method of Ref. [1] is employed to overcome the analytic continuation problem for dilepton invariant masses above the two-pion threshold. Finite-volume effects are investigated using ensembles with spatial extents $L\simeq [3.8,7.6]~\mathrm{fm}$, while the continuum limit is obtained from three lattice spacings in the range $a\in[0.057, 0.08]~\mathrm{fm}$. Our results for the form factors enable the evaluation of decay rates and differential observables for all four channels, $K^- \to e^- \barν_e e^+ e^-$, $K^- \to e^- \barν_e μ^+ μ^-$, $K^- \to μ^- \barν_μe^+ e^-$, and $K^- \to μ^- \barν_μμ^+ μ^-$, thereby providing first-principles Standard Model predictions against which existing and upcoming measurements can be directly compared. A detailed phenomenological analysis of the decay rates and associated observables is presented in a companion paper [2].

hep-lat↗

arXiv2025

Kaon radiative leptonic decay rates from lattice QCD simulations at the physical point

We present a lattice QCD calculation of the radiative leptonic decay rates of the kaon, improving upon our previous work, arXiv:2006.05358. Our analysis uses gauge ensembles generated by the Extended Twisted Mass Collaboration (ETMC) with $N_{f} = 2 + 1 + 1$ flavors of Wilson-clover twisted mass fermions. For the first time, we go beyond the electroquenched approximation by including quark-disconnected contributions. Several key improvements have been implemented: (i) the simulations are now performed directly at physical light- and strange-quark masses, (ii) finite-size effects are carefully investigated using lattices with spatial extents ranging from $L \simeq 3.8\,\mathrm{fm}$ to $L \simeq 7.7\,\mathrm{fm}$, and (iii) the continuum extrapolation is based on three lattice spacings in the range $a \in [0.08, 0.058]\,\mathrm{fm}$. As a result of the high-precision determination of the relevant correlation functions, we reduce the uncertainties on both the axial and vector form factors by nearly a factor of two compared to our previous analysis. When compared to experimental measurements in the electron channel ($K^- \to e^- \barν_e γ$), our results show a tension -- at the level of $2.6$ standard deviations -- with respect to KLOE data. On the other hand, they are compatible with measurements from the E36 Collaboration at J-PARC. In the muonic decay channel ($K^- \to μ^- \barν_μγ$), we confirm the tensions, already observed in our previous study, between lattice QCD predictions and ISTRA+ and OKA data, which are both primarily sensitive to the value of the negative-helicity form factor $F^{-}$.

hep-lat↗

arXiv2025

Lattice QCD study of the $χ_{c1}\to J/ψ\, γ$ decay

We present the results of our lattice QCD computation of the electric ($E_{1}$) and magnetic $(M_{2})$ form factors relevant to the $χ_{c1}\to J/ψ\,γ$ decay by using the gauge field configurations produced by the Extended Twisted Mass Collaboration with $N_{f}=2+1+1$ dynamical Wilson-Clover twisted mass fermions at four different lattice spacings with physical dynamical $u$ , $d$, $s$ and $c$ quark masses (except for the coarsest lattice for which the lightest sea quark corresponds to a pion with $m_π\simeq 175~\mathrm{MeV}$). In the continuum limit, we obtain $Γ( χ_{c1}\to J/ψ γ) = 0.3265(79)~\mathrm{MeV}$, which agrees to $(1÷2) σ$ with the experimental results and disagrees with a previous (unquenched) lattice QCD calculation. Our result for the magnetic quadrupole fractional transition amplitude, $a_{2} =M_{2}/\sqrt{E_{1}^{2}+M_{2}^{2}} = -0.0666(22)$, is in agreement with the experiment and represents an improvement by a factor of about $30$ with respect to the only existing (quenched) lattice QCD result.

hep-lat↗

arXiv2025

Global analysis of charm mixing parameters and determination of the CKM angle $γ$

We present an updated global analysis of beauty decays sensitive to the angle $γ$ of the Cabibbo-Kobayashi-Maskawa matrix and of $D$-meson mixing data in the framework of approximate universality, in which CP violation in $D-\overline{D}$ mixing is described in terms of two universal weak phases corresponding to dispersive and absorptive contributions. We extract the fundamental theoretical parameters determining absorptive and dispersive contributions to $D$ meson mixing and CP violation, together with the angle $γ$. The results for the charm mixing parameters are $x_{12} \simeq x = (0.401 \pm 0.043)\%$ and $y_{12} \simeq y = (0.610 \pm 0.017)\%$, while the two CP-violating phases are given by $ϕ_2^M = (0.13 \pm 0.70)^{\circ}$ and $ϕ_2^Γ = (2.1 \pm 1.6)^{\circ}$. The angle $γ$ is found to be $γ= (65.7 \pm 2.5)^{\circ}$, in excellent agreement with the indirect determination from the Unitarity Triangle analysis.

hep-ph↗

arXiv2025

Lattice QCD determination of the radiative decay rates $h_{c}\to η_{c}\, γ$ and $h_{b}\to η_{b}\, γ$

We present the results of our lattice QCD computation of the hadronic matrix elements relevant to the $h_{c}\to η_{c}γ$ and $h_{b}\to η_{b}γ$ decays by using the gauge configurations produced by the Extended Twisted Mass Collaboration with $N_{f}=2+1+1$ dynamical Wilson-Clover twisted mass fermions at five different lattice spacings with physical dynamical $u$ , $d$, $s$ and $c$ quark masses (except for the the coarsest lattice for which the lightest sea quark corresponds to a pion with $m_π\simeq 175~\mathrm{MeV}$). While the hadronic matrix element for $h_{c}\to η_{c}γ$ is obtained directly, the one relevant to $h_{b}\toη_{b}γ$ is reached by working with heavy quark masses $m^{(n)}_{H} = λ^{n-1} m_{c}$, with $λ\sim 1.24$ and $n=1,2, \ldots ,6$, and then extrapolated to $m_{b}$ by several judicious ansätze. In the continuum limit we obtain $Γ( h_{c}\to η_{c} γ) = 0.604(24)~\mathrm{MeV}$, which is by a factor of $2.3$ more accurate than the previous lattice estimates, and in good agreement with the experimental measurement. In the $b$-quark case we obtain $Γ( h_{b}\to η_{b} γ) =46.0(4.8)~\mathrm{keV}$.

hep-lat↗