arXiv subjects

Mattia Bruno

Publications and source records attributed to Mattia Bruno.

At least 19 recordsLinked to original sources

arXiv2026

Isospin-breaking effects in inclusive hadronic $\tau$ data for the muon $(g-2)$ from first principles

The knowledge of isospin-breaking effects in hadronic $\tau$ decays is required for a high-precision determination of the Hadronic-Vacuum-Polarization contribution to $(g-2)_\mu$ from experimental $\tau$ data. In this work we present a strategy for their calculation in a fully inclusive setup from first-principles Lattice QCD+QED simulations. We separate radiative corrections in three infrared safe classes, which we study individually. We provide analytic expressions for their effects in the initial state and propose a strategy for final-state corrections directly in Euclidean space. We also examine the non-factorizable contributions and highlight the challenges associated with their analytic continuation from Euclidean to Minkowski space. By studying short-distance corrections in the context of momentum schemes, we provide a prescription for the renormalization of the individual terms at first order in the ispospin-breaking parameters.

hep-lat↗

arXiv2026

Finite-volume effects on smeared spectral densities

Using two distinct approaches, we derive a universal expression for the leading finite-volume effects of the smeared vector-vector spectral density (proportional to the smeared hadronic $R$-ratio) in a periodic cubic spatial volume of side length $L$. First, building on the results of previous work for finite-volume effects on Euclidean two-point functions, we show that the $L$ dependence is exponentially suppressed for a certain class of smearing kernels, and that the leading effects can be expressed universally in terms of the pion form factor. The same representation is then derived starting from the Lellouch-L\"uscher-Meyer expression for the spectral decomposition of the correlator. The results may prove useful for controlling the $L \to \infty$ extrapolation of smeared spectral densities, in particular by defining a scaling regime in which the finite-volume effects are dominated by the leading terms in a large $L$ expansion and thus can be reliably estimated. To illustrate this point, we also present numerical estimates based on various kernels and models of particle interactions. Despite focusing on the vector channel, our derivation defines a general framework applicable to other cases as well.

hep-lat↗

arXiv2026

Towards the time-like pion form factor beyond the elastic regime using domain-wall QCD

In this work, we investigate the time-like pion form factor from lattice QCD in the isosymmetric limit, a quantity that plays an important role in understanding hadron physics with substantial phenomenological applications. This observable can be calculated in the elastic region using the finite-volume approach, up to the first (four-particle) open channel. With the goal of accessing the exclusive two-pion form factor in the inelastic region, starting from a three-point correlator involving the vector current and two (temporally-displaced) pion interpolating operators, we examine the associated underlying spectral density and calculate the form factor using a formalism based on the LSZ reduction. A preliminary analysis on one ensemble generated by the RBC/UKQCD collaboration using domain-wall fermions is presented.

hep-lat↗

arXiv2026

Isospin breaking corrections to the hadronic vacuum polarization with stochastic coordinate sampling

In the recent Muon g-2 Theory Initiative white paper update, the hadronic vacuum polarization (HVP) contribution -- which dominates the theoretical uncertainty -- is evaluated as an average of different lattice QCD calculations. Since lattice simulations are mostly carried out in isospin symmetric QCD, corrections due to the mass difference of the up and down quarks and the coupling to photons have to be accounted for. These isospin breaking effects are of order 1\% and can be treated as corrections to the result for the HVP contribution in isospin symmetric QCD. In the current estimate of the HVP contribution, these effects are a large source of uncertainty due to the extensive computational cost to compute all occurring Wick contractions and degrading signal-to-noise behaviour especially for quark disconnected diagrams. We present the current status of the calculation of isospin breaking corrections in the HVP contribution for the RBC/UKQCD collaborations. We use a dataset of propagators computed using stochastic coordinate sampling (SCS) to construct all necessary Wick contractions for the electromagnetic and strong isospin breaking effects. We employ different versions of QED on the lattice, such as QED$_L$, QED$_r$ and QED$_\infty$ to improve our estimate of finite-volume uncertainties.

hep-lat↗

arXiv2025

Bounding statistical errors in lattice field theory simulations

Simulations of strongly interacting lattice field theories are typically performed using Markov chain Monte Carlo algorithms. Therefore estimators of statistical errors must incorporate the effect of autocorrelations by integrating the corresponding autocorrelation function. Since in practical calculations its integral is truncated to a finite window, in this work we propose a stopping criterion based on upper and lower bounds of the autocorrelation function. We examine its application to both traditional Monte Carlo analysis and the recently introduced master-field approach. By leveraging both bounds, we introduce an automatic windowing procedure which we test on numerical simulations of a few simplified toy models.

hep-lat↗

arXiv2025

Spectral densities from Euclidean-time lattice correlation functions

In quantum field theories, spectral densities are directly related to relevant physical observables. In Lattice QCD, their non-perturbative extraction from first principles requires the Inverse Laplace transform of Euclidean-time correlation functions, a notorious ill-posed problem. Here we review our recent proposal [1,2] for a new strategy to perform this inversion both in the continuum and on the lattice, also suitable for smeared spectral densities, both in the continuum and in the discrete cases.

hep-lat↗

arXiv2024

Lattice QCD calculation of the $\pi^0$-pole contribution to the hadronic light-by-light scattering in the anomalous magnetic moment of the muon

We develop a method to compute the pion transition form factor directly at arbitrary space-like photon momenta and use it to determine the $\pi^0$-pole contribution to the hadronic light-by-light scattering in the anomalous magnetic moment of the muon. The calculation is performed using eight gauge ensembles generated with 2+1 flavor domain wall fermions, incorporating multiple pion masses, lattice spacings, and volumes. By introducing a pion structure function and performing a Gegenbauer expansion, we demonstrate that about 98\% of the $\pi^0$-pole contribution can be extracted in a model-independent manner, thereby ensuring that systematic effects are well controlled. After applying finite-volume corrections, as well as performing chiral and continuum extrapolations, we obtain the final result for the $\pi^0$-pole contribution to the hadronic light-by-light scattering in the muon's anomalous magnetic moment, $a_{\mu}^{\pi^0\mathrm{-pole}}=61.2(1.7)\times 10^{-11}$, and the $\pi^0$ decay width, $\Gamma_{\pi^0\to \gamma\gamma}=7.60(27)$ eV.

hep-lat↗

arXiv2024

Thermal evolution of dark matter and gravitational-wave production in the early universe from a symplectic glueball model

The hypothesis that dark matter could be a bound state of a strongly coupled non-Abelian gauge theory is theoretically appealing and has a variety of interesting phenomenological implications. In particular, an interpretation of dark matter as the lightest glueball state in the spectrum of a dark Yang-Mills theory, possibly coupled to the visible sector only through gravitational interactions, has been discussed quite extensively in the literature, but most of previous work has been focused on dark SU(N) gauge theories. In this article, we consider an alternative model, based on a symplectic gauge group, which has a first-order confinement/deconfinement phase transition at a finite critical temperature. We first determine the equation of state of this theory, focusing on temperatures close to the transition, and evaluating the associated latent heat. Then we discuss the evolution of this dark-matter model in the early universe, commenting on the mechanisms by which it could indirectly interact with the visible sector, on the spectrum of gravitational waves it could produce, and on the relic abundances it would lead to. Our discussion includes an extensive review of relevant literature, a number of comments on similarities and differences between our model and dark SU(N) gauge theories, as well as some possible future extensions of the present study.

hep-ph↗

arXiv2024

Spectral densities from Euclidean lattice correlators via the Mellin transform

Spectral densities connect correlation functions computed in quantum field theory to observables measured in experiments. For strongly-interacting theories, their non-perturbative determinations from lattice simulations are therefore of primary importance. They entail the inverse Laplace transform of correlation functions calculated in Euclidean time. By making use of the Mellin transform, we derive explicit analytic formulae to define spectral densities from the time dependence of correlation functions, both in the continuum and on the lattice. The generalization to smeared spectral densities turns out to be straightforward. The formulae obtained here within the context of lattice field theory can be easily applied or extended to other areas of research.

hep-lat↗

arXiv2024

Exponential optimization of adiabatic quantum-state preparation

The preparation of a given quantum state on a quantum computing register is a typically demanding operation, requiring a number of elementary gates that scales exponentially with the size of the problem. Using the adiabatic theorem for state preparation, whose error decreases exponentially as a function of the preparation time, we derive an explicit analytic expression for the dependence of the characteristic time on the Hamiltonian used in the adiabatic evolution. Exploiting this knowledge, we then design a preconditioning term that modifies the adiabatic preparation, thus reducing its characteristic time and hence giving an exponential advantage in state preparation. We prove the efficiency of our method with extensive numerical experiments on prototypical spin-models, which gives a promising strategy to perform quantum simulations of manybody models via Trotter evolution on near-term quantum processors.

quant-ph↗

arXiv2024

On the prediction of spectral densities from Lattice QCD

Hadronic spectral densities play a pivotal role in particle physics, a prime example being the R-ratio defined from electron-positron scattering into hadrons. To predict them from first principles using Lattice QCD, we face a numerically ill-posed inverse problem, due to the Euclidean signature adopted in practical simulations. Here we present a recent numerical analysis of the vector isovector spectral density extracted using the multi-level algorithm (recently extended also to the case of dynamical fermions) and discuss its implications.

hep-lat↗

arXiv2023

Exploiting stochastic locality in lattice QCD: hadronic observables and their uncertainties

Because of the mass gap, lattice QCD simulations exhibit stochastic locality: distant regions of the lattice fluctuate independently. There is a long history of exploiting this to increase statistics by obtaining multiple spatially-separated samples from each gauge field; in the extreme case, we arrive at the master-field approach in which a single gauge field is used. Here we develop techniques for studying hadronic observables using position-space correlators, which are more localized, and compare with the standard time-momentum representation. We also adapt methods for estimating the variance of an observable from autocorrelated Monte Carlo samples to the case of correlated spatially-separated samples.

hep-lat↗

arXiv2023

Phase Transitions in Particle Physics -- Results and Perspectives from Lattice Quantum Chromo-Dynamics

Phase transitions in a non-perturbative regime can be studied by ab initio Lattice Field Theory methods. The status and future research directions for LFT investigations of Quantum Chromo-Dynamics under extreme conditions are reviewed, including properties of hadrons and of the hypothesized QCD axion as inferred from QCD topology in different phases. We discuss phase transitions in strong interactions in an extended parameter space, and the possibility of model building for Dark Matter and Electro-Weak Symmetry Breaking. Methodological challenges are addressed as well, including new developments in Artificial Intelligence geared towards the identification of different phases and transitions.

hep-lat↗

arXiv2023

Isospin 0 and 2 two-pion scattering at physical pion mass using all-to-all propagators with periodic boundary conditions in lattice QCD

A study of two-pion scattering for the isospin channels, $I=0$ and $I=2$, using lattice QCD is presented. Möbius domain wall fermions on top of the Iwasaki-DSDR gauge action for gluons with periodic boundary conditions are used for the lattice computations which are carried out on two ensembles of gauge field configurations generated by the RBC and UKQCD collaborations with physical masses, inverse lattice spacings of 1.023 and 1.378 GeV, and spatial extents of $L=4.63$ and 4.58 fm, respectively. The all-to-all propagator method is employed to compute a matrix of correlation functions of two-pion operators. The generalized eigenvalue problem (GEVP) is solved for a matrix of correlation functions to extract phase shifts with multiple states, two pions with a non-zero relative momentum as well as two pions at rest. Our results for phase shifts for both $I=0$ and $I=2$ channels are consistent with and the Roy Equation and chiral perturbation theory, though at this preliminary stage our errors for $I=0$ are large. An important outcome of this work is that we are successful in extracting two-pion excited states, which are useful for studying $K\toππ$ decay, on physical-mass ensembles using GEVP.

hep-lat↗

arXiv2023

Isospin 0 and 2 two-pion scattering at physical pion mass using distillation with periodic boundary conditions in lattice QCD

The two pion channel in Lattice QCD has long been a primary testing ground for studying multiparticle scattering in finite volume QCD. With the development of sophisticated techniques such as distillation, it is possible to carefully study two-pion scattering in order to constrain associated low-energy constants. In this work, correlation functions with multiparticle interpolating operators are constructed to compute pion scattering phase shifts and scattering lengths in the isospin 0 and 2 channels with both sea and valence quarks at physical mass. Contamination from vacuum and thermal contributions are explicitly quantified with dedicated calculations and the results obtained after subtracting these nuisance terms are compared with the traditional correlator time series subtraction method. Two physical point ensembles with different lattice actions are used, and our finest ensemble gives results for scattering lengths and phase shifts consistent with phenomenology to within the reported statistical uncertainty.

hep-lat↗

arXiv2023

On fits to correlated and auto-correlated data

Observables in particle physics and specifically in lattice QCD calculations are often extracted from fits. Standard $χ^2$ tests require a reliable determination of the covariance matrix and its inverse from correlated and auto-correlated data, a challenging task often leading to close-to-singular estimates. These motivate modifications of the definition of $χ^2$ such as uncorrelated fits. We show how the goodness-of-fit measured by their p-value can still be estimated robustly for a broad class of such fits.

hep-lat↗

arXiv2023

Hadronic observables from master-field simulations

Substantial progress has been made recently in the generation of master-field ensembles. This has to be paired with efficient techniques to compute observables on gauge field configurations with a large volume. Here we present the results of the computation of hadronic observables, including hadron masses and meson decay constants, on large-volume and master-field ensembles with physical volumes of up to $(18\,\mathrm{fm})^4$ and $m_πL$ up to $25$, simulated using $N_{\mathrm{f}}=2+1$ stabilized Wilson fermions. We obtain sub-percent determinations from single gauge configurations with the combined use of position-space techniques, volume averages and master-field error estimation.

hep-lat↗

arXiv2022

Translating topological benefits in very cold lattice simulations

Master-field simulations offer an approach to lattice QCD in which calculations are performed on a small number of large-volume gauge-field configurations. The latter is advantageous for simulations in which the global topological charge is frozen due to a very fine lattice spacing, as the effect of this on observables is suppressed by the spacetime volume. Here we make use of the recently developed Stabilised Wilson Fermions to investigate a variation of this approach in which only the temporal direction ($T$) is taken larger than in traditional calculations. As compared to a hyper-cubic lattice geometry, this has the advantage that finite-$L$ effects can be useful, e.g. for multi-hadron observables, while compared to open boundary conditions, time-translation invariance is not lost. In this proof-of-concept contribution, we study the idea of using very cold (i.e. long-$T$) lattices to topologically "defrost" observables at fine lattice spacing. We identify the scalar-scalar meson two-point correlation function as a useful probe and present first results from $N_f=3$ ensembles with time extents up to $T=2304$ and a lattice spacing of $a=0.055\,\rm{fm}$.

hep-lat↗