arXiv subjects

Antonin Portelli

Publications and source records attributed to Antonin Portelli.

At least 19 recordsLinked to original sources

arXiv2026

$D \to (K π)_{\mathbf{27}}$ at the SU(3)-flavour-symmetric point I: Methodology and strong phase determination

We present part one of an SU(3)-flavour-symmetric lattice QCD calculation of the amplitude for a $D$-meson decaying to a $Kπ$ final state in the 27-dimensional irreducible representation of the flavour symmetry group, denoted $(Kπ)_{\mathbf{27}}$. The Wilson--clover gauge ensembles used in this work, generated by the OpenLat collaboration, are tuned such that $M_π= M_K \approx 410\,\mathrm{MeV}$. Using the distillation framework, we construct a matrix of Euclidean correlation functions from pairs of single-hadron operators projected to definite spatial momentum. Solving a generalised eigenvalue problem yields the finite-volume energy spectrum that is used to determine the scattering phase shift from threshold up to $4 M_π\approx 1640 \,\mathrm{MeV}$, which sits below but plausibly within reach of $M_D^{\rm SU(3)} \simeq 1900\,\mathrm{MeV}$. The calculation is performed across three lattice spacings, and we apply two strategies in which the continuum limit is taken at different stages of the computation: (i) on the extracted scattering parameters and (ii) on the finite-volume energies at fixed physical volume before extracting the scattering parameters. We find consistent results across these methods for the scattering phase shift as a function of the centre-of-mass energy, $δ_{\mathbf{27}}(E_{\sf cm})$. Taking a scattering-length-only parametrisation, we infer a value for the strong phase of the weak decay, $δ_{\mathbf{27}}(M_D^{\rm SU(3)})=-38.4(2.4)^\circ$. We further describe the methodology for using the same operator basis to compute three-point correlation functions to extract $\langle (Kπ)_{\mathbf{27}}| H_W| D\rangle$, for the tree-level effective weak Hamiltonian $H_W$, and for relating such finite-volume matrix elements to the full decay amplitude. The complete analysis leading to the latter will be presented in a forthcoming manuscript.

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arXiv2026

Perturbative quantum electrodynamics with generalized domain wall fermions

In this paper we derive the expansion of the generalized domain-wall fermion Dirac operator including electromagnetic corrections up to $\mathcal{O}(e^2)$, which are relevant for lattice computations of radiative corrections to hadronic processes with chiral fermions. In the generalized formulation of the domain-wall fermionic QCD+QED action, physical quark fields are related to the corresponding five-dimensional fields in a way which depends on the (QCD+QED) gauge links, generating extra contact terms when expanding correlation functions with respect to the electric charge. We re-derive the known first-order correction using a background-field approach and, at second order, obtain new local operator insertions (seagull vertices) required for gauge covariant calculations.

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arXiv2026

Efficiently unquenching QCD+QED at O($α$)

We outline a strategy to efficiently include the electromagnetic interactions of the sea quarks in QCD+QED. When computing iso-spin breaking corrections to hadronic quantities at leading order in the electromagnetic coupling, the sea-quark charges result in quark-line disconnected diagrams which are challenging to compute precisely. An analysis of the variance of stochastic estimators for the relevant traces of quark propagators helps us to improve the situation for certain flavour combinations and space-time decompositions. We present preliminary numerical results for the variances of the corresponding contributions using an ensemble of $N_\mathrm{f}=2+1$ domain-wall fermions generated by the RBC/UKQCD collaboration.

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arXiv2026

Real radiative decays of heavy pseudoscalar mesons

We report our ongoing lattice QCD study of radiative leptonic decays of the charged pseudoscalar mesons $D$, $D_s$, $B$, and $B_c \to \ell ν_\ell γ$. We carry out our analysis on a single JLQCD ensemble with lattice spacing $a=0.044~\text{fm}$. This work is a step towards a complete QCD+QED lattice calculation of these modes, aimed at reducing theoretical uncertainties in the extraction of $|V_{cd}|$ and $|V_{cs}|$ and providing first-principles estimates of the corresponding form factors in the $B$ sector.

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arXiv2026

$K π$ scattering as a step towards $B \to K^* \ell^+ \ell^-$ from Lattice QCD

Rare $b\to s\ell^+\ell^-$ decays provide some of the most sensitive tests of the Standard Model and require precise and systematically improvable hadronic input from lattice QCD. For the phenomenologically important channel $B\to K^*\ell^+\ell^-$ this entails a first-principles treatment of a resonant $Kπ$ final state together with controlled heavy-quark dynamics. We present the status of a new exploratory lattice calculation that combines a variational determination of finite-volume $Kπ$ states with the $1+J\to2$ finite-volume formalism to access the relevant matrix elements. The computation is carried out on an RBC/UKQCD domain-wall fermion ensemble with $a^{-1} \approx 2.7\,\mathrm{GeV}$ and employs a dual heavy-quark strategy, using both a relativistic heavy-quark action tuned to the physical $b$ mass and domain-wall heavy masses extrapolating from charm. All correlation functions are computed using (stochastic) distillation, providing a versatile setup that supports a broad range of heavy-to-light transitions into resonant final states. We show first two-point results for the $K^*\leftrightarrow Kπ$ system and discuss the accessible kinematic region, which allows for a controlled study at high $q^2$. The outlook for extending the calculation to lower $q^2$ and for incorporating effects from charmonium resonances is outlined.

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arXiv2025

Application of Laplace filters to the analysis of lattice time correlators

The analysis of lattice simulation correlation function data is notoriously hindered by the ill-conditioning of the Euclidean time covariance matrix. Additionally, the isolation of a single physical state in such functions is generally affected by systematic contamination from unwanted states. In this paper, we present a new methodology based on regulated Laplace filters and demonstrate that it can be used to address both issues using state-of-the-art simulation data. Regulated Laplace filters are invertible high-pass filters that suppress local correlations in the data, and we show that they can reduce the condition number of covariance matrices by several orders of magnitude. Furthermore, Laplace filters can annihilate functions that decay exponentially with time, which can be used to alter the spectrum of a lattice correlation function. We show that this property can be exploited to significantly reduce excited-state contamination in the determination of matrix elements. The same property can also be used to constrain the spectral content of a correlation function and has the potential to form the basis of new methods to extract physical information from lattice data.

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arXiv2025

$\text{QED}_\text{r}$: a finite-volume QED action with redistributed spatial zero-momentum modes

We present a finite-volume QED action designed to improve the infinite-volume extrapolation of hadronic observables in precision lattice QCD+QED calculations. The action proposed in this work, which we call $\text{QED}_\text{r}$, can be seen as a particular case of the infrared-improved QED actions introduced by Davoudi et al. in 2019, and is specifically designed to remove kinematics-independent finite-volume corrections that appear at $\mathrm{O}(1/L^3)$ in the commonly used $\text{QED}_\text{L}$ formulation, where $L$ is the spatial extent of the physical volume. For a number of key observables, these effects depend on the internal structure of the hadrons and are difficult to evaluate non-perturbatively, making an analytical subtraction of the finite-volume effects impractical. We explicitly study the $\text{QED}_\text{r}$ electromagnetic finite-size effects on hadron masses and leptonic decay rates, relevant for Standard Model precision tests using the Cabibbo-Kobayashi-Maskawa matrix elements. In addition, we propose methods to remove the kinematics-dependent $\mathrm{O}(1/L^3)$ effects in leptonic decays. The removal of such contributions, shifting the leading contamination to $\mathrm{O}(1/L^4)$, will help to reduce the systematic uncertainties associated with finite-volume effects in future lattice QCD+QED calculations.

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arXiv2025

Exploratory calculation of the rare hyperon decay $Σ^+ \to p \ell^+ \ell^-$ from lattice QCD

The rare hyperon decay $Σ^+ \to p \ell^+ \ell^-$ is a flavour-changing neutral current process mediated by an $s \to d$ transition that occurs only at loop level within the Standard Model. Consequently, this decay is highly suppressed, making it a promising avenue for probing potential new physics. While phenomenological calculations have made important progress in predicting the decay amplitude, there remains a four-fold ambiguity in the relevant transition form factors that prevents a unique prediction for the branching fraction and angular observables. Fully resolving this ambiguity requires a first-principles Standard-Model calculation, and the recent observation of this process using LHCb Run 2 data reinforces the timeliness of such a calculation. In this work, we present the first lattice-QCD calculation of this decay, performed using a 2+1-flavour domain-wall fermion ensemble with a pion mass of 340 MeV. At a small baryon source-sink separation, we observe the emergence of a signal in the relevant baryonic four-point functions. This allows us to determine the positive-parity form factors for the rare hyperon decays from first-principles, albeit with large statistical and systematic uncertainties.

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arXiv2025

Physical-mass calculation of $ρ(770)$ and $K^*(892)$ resonance parameters via $ππ$ and $K π$ scattering amplitudes from lattice QCD

We present our study of the $ρ(770)$ and $K^*(892)$ resonances from lattice quantum chromodynamics (QCD) employing domain-wall fermions at physical quark masses. We determine the finite-volume energy spectrum in various momentum frames and obtain phase-shift parameterizations via the Lüscher formalism, and as a final step the complex resonance poles of the $ππ$ and $K π$ elastic scattering amplitudes via an analytical continuation of the models. By sampling a large number of representative sets of underlying energy-level fits, we also assign a systematic uncertainty to our final results. This is a significant extension to data-driven analysis methods that have been used in lattice QCD to date, due to the two-step nature of the formalism. Our final pole positions, $M+iΓ/2$, with all statistical and systematic errors exposed, are $M_{K^{*}} = 893(2)(8)(54)(2)~\mathrm{MeV}$ and $Γ_{K^{*}} = 51(2)(11)(3)(0)~\mathrm{MeV}$ for the $K^*(892)$ resonance and $M_ρ = 796(5)(15)(48)(2)~\mathrm{MeV}$ and $Γ_ρ = 192(10)(28)(12)(0)~\mathrm{MeV}$ for the $ρ(770)$ resonance. The four differently grouped sources of uncertainties are, in the order of occurrence: statistical, data-driven systematic, an estimation of systematic effects beyond our computation (dominated by the fact that we employ a single lattice spacing), and the error from the scale-setting uncertainty on our ensemble.

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arXiv2025

Light and strange vector resonances from lattice QCD at physical quark masses

We present the first ab initio calculation at physical quark masses of scattering amplitudes describing the lightest pseudoscalar mesons interacting via the strong force in the vector channel. Using lattice quantum chromodynamics, we postdict the defining parameters for two short-lived resonances, the $ρ(770)$ and $K^*(892)$, which manifest as complex energy poles in $ππ$ and $K π$ scattering amplitudes, respectively. The calculation proceeds by first computing the finite-volume energy spectrum of the two-hadron systems, and then determining the amplitudes from the energies using the Lüscher formalism. The error budget includes a data-driven systematic error, obtained by scanning possible fit ranges and fit models to extract the spectrum from Euclidean correlators, as well as the scattering amplitudes from the latter. The final results, obtained by analytically continuing multiple parameterizations into the complex energy plane, are $M_ρ= 796(5)(50)~\mathrm{MeV}$, $Γ_ρ= 192(10)(31)~\mathrm{MeV}$, $M_{K^*} = 893(2)(54)~\mathrm{MeV}$ and $Γ_{K^*} = 51(2)(11)~\mathrm{MeV}$, where the subscript indicates the resonance and $M$ and $Γ$ stand for the mass and width, respectively, and where the first bracket indicates the statistical and the second bracket the systematic uncertainty.

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arXiv2025

Split-even approach to the rare kaon decay $K \to π\ell^+ \ell^-$

In recent years the rare kaon decay has been computed directly at the physical point. However, this calculation is currently limited by stochastic noise stemming from a light and charm quark loop GIM subtraction. The split-even approach is an alternative estimator for such loop differences, and has shown a large variance reduction in certain quantities. We present an investigation into the use of the split-even estimator in the calculation of the rare kaon decay.

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arXiv2023

Structure-dependent electromagnetic finite-volume effects through order $1/L^3$

We consider electromagnetic finite-volume effects through order $1/L^3$ in different formulations of QED, where $L$ is the periodicity of the spatial volume. An inherent problem at this order is the appearance of structure-dependent quantities related to form factors and the analytical structure of the correlation functions. The non-local constraint of the widely used QED$_{\textrm{L}}$ regularization gives rise to structure-dependent effects that are difficult to evaluate analytically and can act as a precision bottleneck in lattice calculations. For this reason, we consider general volume expansions relevant for the mass spectrum as well as leptonic decay rates in QED$_{\textrm{C}}$, QED$_{\textrm{L}}$ and QED$_{\textrm{L}}^{\textrm{IR}}$, the latter being a class of non-local formulations generalising QED$_{\textrm{L}}$. One choice within this class is QED$_{\textrm{r}}$, first introduced at this conference, and we show that the effects of non-locality for the $1/L^3$ term in the expansion can be removed. We observe that there are still $1/L^3$ contributions unrelated to the (non-)locality of the studied QED formulations, but rather to collinear singularities in the physical amplitudes.

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arXiv2023

Prospects for a lattice calculation of the rare decay $Σ^+\to p\ell^+\ell^-$

We present a strategy for calculating the rare decay of a $Σ^+ (uus)$ baryon to a proton $(uud)$ and di-lepton pair using lattice QCD. To determine this observable one needs to numerically evaluate baryonic two-, three-, and four-point correlation functions related to the target process. In particular, the four-point function arises from the insertion of incoming and outgoing baryons, together with a weak Hamiltonian mediating the $s \to d$ transition and an electromagnetic current creating the outgoing leptons. As is described in previous work in other contexts, this four-point function has a highly non-trivial relation to the physical observable, due to nucleon and nucleon-pion intermediate states. These lead to growing Euclidean time dependence and, in the case of the nucleon-pion states, to power-like volume effects. We discuss how to treat these issues in the context of the $Σ^+\rightarrow p\ell^+\ell^-$ decay and, in particular, detail the relation between the finite-volume estimator and the physical, complex-valued amplitude. In doing so, we also make connections between various approaches in the literature.

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arXiv2022

Position-Space Renormalisation of the Energy-Momentum Tensor

There is increasing interest in the study of nonperturbative aspects of three-dimensional quantum field theories (QFT). They appear as holographic dual to theories of (strongly coupled) gravity. For instance, in Holographic Cosmology, the two-point function of the Energy-Momentum Tensor (EMT) of a particular class of three-dimensional QFTs can be mapped into the power spectrum of the Cosmic Microwave Background in the gravitational theory. However, the presence of divergent contact terms poses challenges in extracting a renormalised EMT two-point function on the lattice. Using a $ϕ^4$ theory of adjoint scalars valued in the $\mathfrak{su}(N)$ Lie Algebra as a proof-of-concept motivated by Holographic Cosmology, we apply a novel method for filtering out such contact terms by making use of infinitely differentiable "bump" functions which enforce a smooth window that excludes contributions at zero spatial separation. The process effectively removes the local contact terms and allows us to extract the continuum limit behaviour of the renormalised EMT two-point function.

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arXiv2022

Progress on the exploratory calculation of the rare Hyperon decay $Σ^+ \to p \ell^+ \ell^-$

The rare Hyperon decay $Σ^+ \to p \ell^+ \ell^-$ is an $s \to d$ flavour changing neutral current process, which is highly suppressed within the Standard Model, and is therefore sensitive to new physics. Due to recent improvements in experimental measurements of this decay, the Standard Model theory prediction must also be improved in order to identify any new physics in this channel. We present updates on our progress towards the first exploratory lattice calculation of the long-distance part of the form factors of this decay. This pilot calculation is performed on a 340 MeV pion mass ensemble using domain-wall fermions as part of the RBC-UKQCD collaboration.

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arXiv2022

Exploring distillation at the SU(3) flavour symmetric point

In these proceedings we present an exact distillation setup with stabilised Wilson fermions at the SU(3) flavour symmetric point utilising the flexibility of the Grid and Hadrons software libraries. This work is a stepping stone towards a non-perturbative investigation of hadronic D-decays, for which one needs to control the multi-hadron final states. As a first step we study two-to-two s-wave scattering of pseudoscalar mesons. In particular we examine the reliability of the extraction of finite-volume energies as a function of the number of eigenvectors of the gauge-covariant Laplacian entering our distillation setup.

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arXiv2022

Isospin-breaking corrections to light leptonic decays in lattice QCD+QED at the physical point

We report on the physical-point RBC/UKQCD calculation of the leading isospin-breaking corrections to light-meson leptonic decays. This is highly relevant for future precision tests in the flavour physics sector, in particular the first-row unitarity of the Cabibbo-Kobayashi-Maskawa matrix containing the elements $V_{us}$ and $V_{ud}$. The simulations were performed using Domain-Wall fermions for $2+1$ flavours, and with isospin-breaking effects included perturbatively in the path integral through order $α$ and $(m_u - m_d)/Λ_{\mathrm{QCD}}$. We use QED$_{\mathrm{L}}$ for the inclusion of electromagnetism, and discuss here the non-locality of this prescription which has significant impact on the infinite-volume extrapolation.

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arXiv2022

Towards $Kπ$ scattering with domain-wall fermions at the physical point using distillation

Resonances play an important role in Standard Model phenomenology. In particular, hadronic resonances feature in $B$ and $D$ decays, which can be central for New Physics searches. Lattice QCD simulations combined with the finite-volume method can nowadays be used to reliably study strongly coupled scattering processes such as $Kπ$ and thus the hadronic resonance $K^*$. In this work, we approach $Kπ$ scattering on a domain-wall $N_f = 2+1$ RBC-UKQCD ensemble at a physical pion mass. We use the distillation method within Grid and Hadrons software to compute sets of operator basis. That allows solving an eigenvalue problem to extract the low-energy finite-volume spectra, which are then translated into scattering information. We update the state of the calculation by reviewing the smearing process, outlining the variational analysis and concluding by showing preliminary data.

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