arXiv subjects

Roberto Frezzotti

Publications and source records attributed to Roberto Frezzotti.

At least 19 recordsLinked to original sources

arXiv2026

Inclusive $\bar B_s\mapsto X_{\bar sc} \ell \bar \nu$ decays from lattice QCD: computational strategy and a first physical result

We present a strategy to compute the inclusive decay rate for the process $\bar B_s \mapsto X_{\bar sc} \ell \bar{\nu}$ from first principles in lattice QCD. The physical decay rate is obtained from the interpolation of non-perturbative lattice data, obtained at lighter than physical heavy meson masses ($M_{\bar B_s}^\mathrm{max}=4.3$GeV), with the Operator Product Expansion predictions, which become exact in the limit of infinitely heavy quarks. We also present a new method for the computation of the required lattice four-point correlators, which represents a considerable improvement over the state-of-the-art on the subject. We show the effectiveness of the strategy by performing the calculation on a subset of the available $n_f=2+1+1$ physical-point Extended Twisted Mass Collaboration (ETMC) gauge ensembles. Our current determination of the inclusive decay rate has a 7% total error, that is dominated by uncertainties due to the relatively limited configuration ensembles considered herein, and can be significantly reduced in the near future.

hep-lat↗

arXiv2026

Performance of Low Mode Averaging on Twisted-Mass Fermion Ensembles at the physical pion mass point

We study the performance of low-mode averaging (LMA) on twisted-mass fermion ensembles at near-physical quark masses, assessing both its theoretical framework and practical cost-effectiveness in modern lattice QCD. In particular, we present a numerical study of light-quark meson and baryon observables. For mesons, we analyse two-point functions, including the vector-vector correlator relevant for the hadronic vacuum polarisation contribution to the muon anomalous magnetic moment, comparing two implementations of LMA: an exact approach based on explicit low modes and an approximate, high-statistics variant using multigrid techniques. For baryons, we restrict to the exact approach and study both two- and three-point functions, quantifying the resulting noise and cost reductions at large Euclidean times. In addition, we compute the eigenvalue density of the massless Wilson operator and determine the renormalised chiral condensate via the Banks-Casher relation, obtaining $\sqrt[3]{\Sigma_{\mathrm{R}}}=269.5(4.5)~\mathrm{MeV}$ for $N_f{=}2{+}1{+}1$ isospin-symmetric QCD at a scale $2~\mathrm{GeV}$ in the $\overline{\mathrm{MS}}$ scheme, with an uncertainty dominated by the chiral extrapolation. Additionally, from the pion-mass dependence of $\Sigma_{\mathrm{R}}$, we extract the scale-independent low-energy constant $\bar{h}_1=5.2(1.1)$.

hep-lat↗

arXiv2026

First-principle evaluation of inclusive hadronic $\tau$ decays in QCD+QED

We present a strategy to extend lattice calculations of inclusive hadronic $\tau$ decays from isosymmetric QCD to QCD+QED. The inclusive decay rate can be related to suitable Euclidean correlation functions, allowing for a first-principles evaluation of electromagnetic and isospin-breaking effects. Within the RM123 framework, radiative corrections are decomposed into leptonic, factorizable and non-factorizable contributions. We report preliminary results for the leptonic and factorizable terms in the electro-quenched approximation and discuss the remaining steps towards a complete calculation. This programme aims at a first-principles determination of inclusive $\tau$ decay rates with direct implications for the extraction of the CKM matrix element $|V_{us}|$.

hep-lat↗

arXiv2026

The smeared $R$-ratio in isoQCD from first-principles lattice simulations

The $R$-ratio is a phenomenological observable of great relevance, both in itself and in applications such as the dispersive approach to the muon anomalous magnetic moment. It can be investigated from first-principles with controlled statistical and systematic errors in lattice QCD by introducing an arbitrary smearing kernel and employing spectral reconstruction techniques, such as the well-known Hansen-Lupo-Tantalo method. Improving upon a first study published in 2023, we show preliminary results using the correlation functions produced by ETMC in $N_f = 2+1+1$ lattice simulations at four lattice spacings, different volumes and with higher statistics w.r.t. our previous study. The new correlators, thanks to the implementation of the Low Mode Average technique, allow the determination of the $R$-ratio smeared with Gaussian kernels of widths down to $\sigma \sim 200$ with phenomenologically relevant precision.

hep-lat↗

arXiv2026

An update on the HVP contribution to $g_{\mu}{-}2$ in isoQCD from ETMC

We present an update on the determination of the leading-order hadronic vacuum polarisation contribution to the muon anomalous magnetic moment in isospin-symmetric QCD by the Extended Twisted Mass Collaboration. The calculation is based on five $N_f = 2+1+1$ gauge ensembles generated with Wilson-clover twisted-mass quarks at maximal-twist and near-physical pion masses, spanning four lattice spacings and two volumes. For the dominant quark-connected contributions, we employ two distinct valence-quark regularisations and present results for both the isovector and isoscalar components.

hep-lat↗

arXiv2025

Lattice determination of the QCD low-energy constant $\ell_{\scriptscriptstyle{7}}$

We provide a non-perturbative determination of the scheme- and scale-independent low-energy constant $\ell_{\scriptscriptstyle{7}}$, appearing in the QCD effective chiral Lagrangian at next-to-leading order, by means of lattice QCD simulations with $N_{\scriptscriptstyle{\rm f}}=2+1$ quark flavors. We adopt staggered fermions and extract $\ell_{\scriptscriptstyle{7}}$ from the pion mass splitting by suitably generalizing the method introduced in [Phys. Rev. D 104 (2021) 074513] for the Wilson discretization. Adopting 12 gauge ensembles with 3 different values of the pion mass, and 4 different values of the lattice spacing, we are able to achieve controlled extrapolations towards the continuum, infinite volume, and chiral limits. Our final result $\ell_{\scriptscriptstyle{7}} \,\times \, 10^3 = 2.79(58)_{\scriptscriptstyle{\rm stat}}(19)_{\scriptscriptstyle{\rm syst}} = 2.79(61)_{\scriptscriptstyle{\rm tot}}$ agrees with and substantially improves on previous determinations.

hep-lat↗

arXiv2025

Inclusive semileptonic decays of the $D_s$ meson: A first-principles lattice QCD calculation

We present the results of a first-principles theoretical study of the inclusive semileptonic decays of the $D_s$ meson. We performed a state-of-the-art lattice QCD calculation using the gauge ensembles produced by the Extended Twisted Mass Collaboration (ETMC) with dynamical light, strange and charm quarks with physical masses and employed the so-called Hansen-Lupo-Tantalo (HLT) method to extract the decay rate and the first two lepton-energy moments from the relevant Euclidean correlators. We have carefully taken into account all sources of systematic errors, including the ones associated with the continuum and infinite-volume extrapolations and with the HLT spectral reconstruction method. We obtained results in very good agreement with the currently available experimental determinations and with a total accuracy at the few-percent level, of the same order of magnitude of the experimental error. Our total error is dominated by the lattice QCD simulations statistical uncertainties and is certainly improvable. From the results presented and thoroughly discussed in this paper we conclude that it is nowadays possible to study heavy mesons inclusive semileptonic decays on the lattice at a phenomenologically relevant level of accuracy. The phenomenological implications of our physical results are the subject of a companion letter [1].

hep-lat↗

arXiv2025

Inclusive semileptonic decays of the $D_{s}$ meson: Lattice QCD confronts experiments

We present the results of a first-principles theoretical study of the inclusive semileptonic decays of the $D_{s}$ meson. We performed a state-of-the-art lattice QCD calculation by taking into account all sources of systematic errors. A detailed discussion of our lattice calculation, demonstrating that inclusive semileptonic decays can nowadays be studied on the lattice at a phenomenologically relevant level of accuracy, is the subject of a companion paper [1]. Here we focus on the phenomenological implications of our results. Using the current best estimates of the relevant Cabibbo-Kobayashi-Maskawa (CKM) matrix elements, our theoretical predictions for the decay rate and for the first two lepton-energy moments are in very good agreement with the corresponding experimental measurements. We also argue that, while the inclusive $D_{s}$ channel is not yet competitive with the exclusive channels in the $|V_{cs}|$ determination, the situation can be significantly improved in the near future.

hep-lat↗

arXiv2025

Isovector axial and pseudoscalar form factors from twisted mass lattice QCD at the physical point

We present the isovector axial, induced pseudoscalar, and pseudoscalar form factors of the nucleon using three twisted-mass fermion ensembles with degenerate up- and down-, strange-, and charm-quarks with masses tuned to their physical values (physical point). The three ensembles have lattice spacing $a$=0.08, 0.068, and 0.057 fm and approximately equal physical volume allowing for the continuum limit to be taken at the physical point. Excited-state contributions to the matrix elements are evaluated using several sink-source separations from 0.5 fm to 1.5 fm and multistate fits. We check the partially conserved axial-vector current (PCAC) hypothesis and the pion pole dominance (PPD) and show that in the continuum limit both relations are satisfied. We provide results at the continuum limit for the isovector nucleon axial charge, axial radius, pion-nucleon coupling constant, and for the induced pseudoscalar form factor at the muon capture point.

hep-lat↗

arXiv2025

Smeared $R$-ratio in isospin symmetric QCD with Low Mode Averaging

Low Mode Average (LMA) is a technique to improve the quality of the signal-to-noise ratio in the long time separation of Euclidean correlation functions. We report on its beneficial impact in computing the vector-vector light connected two-point correlation functions and derived physical quantities in the mixed action lattice setup adopted by ETM collaboration. We focus on preliminary results of the computation within isospin symmetric QCD (isoQCD) of the $R$-ratio smeared with Gaussian kernels of widths down to $\sigma\sim250$ MeV, which is enough to appreciate the $\rho$ resonance around 770 MeV, using the Hansen-Lupo-Tantatlo (HLT) spectral-density reconstruction method.

hep-lat↗

arXiv2025

Valence leading isospin breaking contributions to $a_{\mu}^{\mathrm{HVP-LO}}$

By employing the RM123 approach to QCD+QED, we computed the valence quark-connected isospin-breaking corrections to the light, strange and charm contributions at leading order in $\alpha_{\mathrm{em}}$ and $\left(\mu_d-\mu_u\right) / \Lambda_{\mathrm{QCD}}$. Here we report the preliminary results on two different volumes ($L \sim 3.8$ fm and $L \sim 5.1$ fm) and a fixed lattice spacing (corresponding to $a_{\text {isoQCD }} \sim 0.07951(4)$ fm), obtained in the framework of the ongoing computation by ETMC of the leading-order hadronic vacuum polarization (HVP) contribution to the muon anomalous magnetic moment $a_\mu^{\mathrm{HVP}}$ in QCD+QED.

hep-lat↗

arXiv2024

Nucleon axial and pseudoscalar form factors using twisted-mass fermion ensembles at the physical point

We compute the nucleon axial and pseudoscalar form factors using three $N_f=$2+1+1 twisted mass fermion ensembles with all quark masses tuned to approximately their physical values. The values of the lattice spacings of these three physical point ensembles are 0.080 fm, 0.068 fm, and 0.057 fm, and spatial sizes 5.1 fm, 5.44 fm, and 5.47 fm, respectively, yielding $m_πL$>3.6. Convergence to the ground state matrix elements is assessed using multi-state fits. We study the momentum dependence of the three form factors and check the partially conserved axial-vector current (PCAC) hypothesis and the pion pole dominance (PPD). We show that in the continuum limit, the PCAC and PPD relations are satisfied. We also show that the Goldberger-Treimann relation is approximately fulfilled and determine the Goldberger-Treiman discrepancy. We find for the nucleon axial charge $g_A$=1.245(28)(14), for the axial radius $\langle r^2_A \rangle$=0.339(48)(06) fm$^2$, for the pion-nucleon coupling constant $g_{πNN} \equiv \lim_{Q^2 \rightarrow -m_π^2} G_{πNN}(Q^2)$=13.25(67)(69) and for $G_P(0.88m_μ^2)\equiv g_P^*$=8.99(39)(49).

hep-lat↗

arXiv2023

Inclusive hadronic decay rate of the $τ$ lepton from lattice QCD

Inclusive hadronic decays of the $τ$ lepton are very interesting from the phenomenological point of view since they give access to the CKM matrix elements $V_{ud}$ and $V_{us}$. In this paper, for the first time, by employing the HLT method for hadronic smeared spectral densities we compute on the lattice the inclusive decay rate of the processes $τ\to X_{ud}\, ν_τ$, where $X_{ud}$ is a generic hadronic state with $\bar{u} d$ flavor quantum numbers. Our computation, which avoids any recourse to OPE and/or perturbative approximations, is carried out in isospin symmetric $N_{f}=2+1+1$ lattice QCD at physical quark masses, using ensembles produced by the ETMC at three lattice spacings and two volumes. All uncertainties, except for isospin breaking effects, are taken into account and a result with a subpercent error is obtained for $|V_{ud}|$, which is nicely consistent with the current world average. These findings validate our approach and also motivate the inclusion of isospin breaking corrections and its extension to the inclusive decay $τ\to X_{us}\, ν_τ$, paving the way towards a high-precision first principles determination of $|V_{us}|$ and $|V_{ud}|$ from inclusive $τ$ decay.

hep-lat↗

arXiv2023

Probing the energy-smeared R-ratio on the lattice

We present a first-principles lattice QCD investigation of the $R$-ratio between the $e^+e^-$ cross-section into hadrons and that into muons. By using the method of Ref.[1], that allows to extract smeared spectral densities from Euclidean correlators, we compute the $R$-ratio convoluted with Gaussian smearing kernels of widths of about $600$ MeV and central energies from $220$ MeV up to $2.5$ GeV. Our theoretical results are compared with the corresponding quantities obtained by smearing the KNT19 compilation [2] of $R$-ratio experimental measurements with the same kernels and, by centring the Gaussians in the region around the $ρ$-resonance peak, a tension of about three standard deviations is observed. From the phenomenological perspective, we have not included yet in our calculation QED and strong isospin-breaking corrections and this might affect the observed tension. From the methodological perspective, our calculation demonstrates that it is possible to study the $R$-ratio in Gaussian energy bins on the lattice at the level of accuracy required in order to perform precision tests of the Standard Model.

hep-lat↗

arXiv2023

Direct lattice calculation of inclusive hadronic decay rates of the $τ$ lepton

The inclusive hadronic decay-rates of the $τ$ lepton are particularly interesting from the phenomenological point of view since they give access to the CKM matrix elements $V_{ud}$ and $V_{us}$. In this talk, we discuss how a recent method for the extraction of smeared spectral densities from Euclidean lattice correlators can be used to obtain a direct lattice determination of inclusive hadronic $τ$ decay rates. We also present preliminary numerical results obtained by applying this method to correlators measured on two gauge ensembles produced by the ETMC with $N_f=2+1+1$ dynamical flavours at physical pion masses, lattice spacing $a\simeq 0.08$ fm and volumes $L\simeq 5.1$ fm and $L\simeq 7.6$ fm.

hep-lat↗

arXiv2022

Lattice calculation of the R-ratio smeared with Gaussian kernel

The ratio $R(E)$ of the cross-sections for $e^+e^-\to$ hadrons and $e^+e^-\to μ^+μ^-$ is a valuable energy-dependent probe of the hadronic sector of the Standard Model. Moreover, the experimental measurements of $R(E)$ are the inputs of the dispersive calculations of the leading hadronic vacuum polarization contribution to the muon $g-2$ and these are in significant tension with direct lattice calculations and with the muon $g-2$ experiment. In this talk we discuss the results of our first-principles lattice study of $R(E)$. By using a recently proposed method for extracting smeared spectral densities from Euclidean lattice correlators, we have calculated $R(E)$ convoluted with Gaussian kernels of different widths $σ$ and central energies up to $2.5$ GeV. Our theoretical results have been compared with the KNT19 [1] compilation of experimental results smeared with the same Gaussian kernels and a tension (about three standard deviations) has been observed for $σ\sim 600$ MeV and central energies around the $ρ$-resonance peak.

hep-lat↗

arXiv2022

Short \& intermediate distance HVP contributions to muon g-2: SM (lattice) prediction versus $e^+e^-$ annihilation data

We present new lattice results of the ETM Collaboration, obtained from extensive simulations of lattice QCD with dynamical up, down, strange and charm quarks at physical mass values, different volumes and lattice spacings, concerning the SM prediction for the so-called intermediate window (W) and short-distance (SD) contributions to the leading order hadronic vacuum polarization (LO-HVP) term of the muon anomalous magnetic moment, $a_μ$. Results for $a_μ^{\rm LO-HVP,W}$ and $a_μ^{\rm LO-HVP,SD}$, besides representing a step forward to a complete lattice computation of $a_μ^{\rm LO-HVP}$ and a useful benchmark among lattice groups, are compared here with their dispersive counterparts based on experimental data for $e^+e^-$ into hadrons. The comparison confirms the tension in $a_μ^{\rm LO-HVP,W}$, already noted in 2020 by the BMW Collaboration, while showing no tension in $a_μ^{\rm LO-HVP,SD}$.

hep-ph↗

arXiv2022

Disconnected contribution to the LO HVP term of muon g-2 from ETMC

We present a lattice determination of the disconnected contributions to the leading-order hadronic vacuum polarization (HVP) to the muon anomalous magnetic moment in the so-called short and intermediate time-distance windows. We employ gauge ensembles produced by the Extended Twisted Mass Collaboration (ETMC) with $N_f = 2 + 1 + 1$ flavours of Wilson twisted-mass clover-improved quarks with masses approximately tuned to their physical value. We take the continuum limit employing three lattice spacings at about 0.08, 0.07 and 0.06 fm.

hep-lat↗