New articles on High Energy Physics - Lattice

[1] 2311.14806

The quenched glueball spectrum from smeared spectral densities

The standard approach to compute the glueball spectrum on the lattice relies on the evaluation of effective masses from two-point correlation functions of operators with the quantum numbers of the desired state. In this work, we propose an alternative procedure, based on the numerical computation of smeared spectral densities. Even though the extraction of the latter from lattice correlators is a notoriously ill-posed inverse problem, we show that a recently developed numerical method, based on the Backus-Gilbert regularization, provides a robust way to evaluate a smeared version of the spectral densities. Fitting the latter to a combination of Gaussians, we extract the masses of the lightest glueball and of its first excitation in the spectrum of the theory. While the preliminary results presented in this contribution are restricted to simulations at finite lattice spacing and finite volume, and for the purely gluonic sector of QCD, they represent the first step in a systematic investigation of glueballs using spectral-reconstruction methods.

[2] 2311.14825

Studying the phase diagram of the three-flavor Schwinger model in the presence of a chemical potential with measurement- and gate-based quantum computing

We propose an ansatz quantum circuit for the variational quantum eigensolver (VQE), suitable for exploring the phase structure of the multi-flavor Schwinger model in the presence of a chemical potential. Our ansatz is capable of incorporating relevant model symmetries via constrains on the parameters, and can be implemented on circuit-based as well as measurement-based quantum devices. We show via classical simulation of the VQE that our ansatz is able to capture the phase structure of the model, and can approximate the ground state to a high level of accuracy. Moreover, we perform proof-of-principle simulations on superconducting, gate-based quantum hardware. Our results show that our approach is suitable for current gate-based quantum devices, and can be readily implemented on measurement-based quantum devices once available.

[3] 2311.15046

Nonperturbative running of the tensor operator for $N_\rm{f}=3$ QCD from the chirally rotated Schrödinger Functional

We study the Renormalisation Group (RG) running of the non-singlet tensor operator, for $N_\mathrm{\scriptstyle f}=3$ QCD with Wilson fermions in a mixed action setup, with standard Schr\"odinger Functional (SF) boundary conditions for sea quarks and chirally rotated Schr\"odinger Functional ($\chi$SF) boundary conditions for valence quarks. Based on a recursive finite-size scaling technique we compute non-perturbatively the tensor step-scaling function for an energy range between a hadronic scale and an electroweak scale, above which perturbation theory may be safely applied. Our result is expressed as the RG-running factor $T^{\mathrm{RGI}}/[ T(\mu_{\mathrm{had}})]_{\scriptstyle \rm R}$, where the numerator is the scale independent (Renormalisation Group Invariant - RGI) tensor operator and the denominator is its renormalised counterpart at a hadronic scale $\mu_{\mathrm{had}} = 233(8)$~MeV in a given scheme. We determine the step-scaling function in four distinct renormalisation schemes. We also compute the renormalisation parameters of these schemes at $\mu_{\mathrm{had}}$ which, combined with the RG-running factor, gives the scheme-independent quantity $Z^{\mathrm{RGI}}_{\mathrm T}(g_0^2)$ in four schemes and for a range of bare gauge couplings in which large volume hadronic matrix element simulations are performed by the CLS consortium in $N_\mathrm{\scriptstyle f}=2+1$ QCD. All four results are compatible and also agree with a recent determination based on a unitary setup for Wilson quarks with Schr\"odinger Functional boundary conditions~arXiv:2309.04314 . This provides a strong universality test.

[4] 2311.15233

Sparse modeling approach to extract spectral functions with covariance of Euclidean-time correlators of lattice QCD

We present our sparse modeling study to extract spectral functions from Euclidean-time correlation functions. In this study covariance between different Euclidean times of the correlation function is taken into account, which was not done in previous studies. In order to check applicability of the method, we firstly test it with mock data which imitate possible charmonium spectral functions. Then, we extract spectral functions from correlation functions obtained from lattice QCD at finite temperature.

[5] 2311.15259

Speed of sound exceeding the conformal bound in dense 2-color QCD

We review recent works on the Monte Carlo simulations of dense two-color QCD (QC$_2$D) by focusing on the phase diagram, the equation of state, and the sound velocity at nonzero quark chemical potential. A possible upper bound of the sound velocity is known as the conformal bound, namely, $c_s^2/c^2 \leq 1/3$. The sound velocity is below the bound at least in the case of finite-temperature QCD. However, our recent work~\cite{Iida:2022hyy} shows the breaking of this bound in dense QC$_2$D. This phenomenon was previously unknown from any lattice calculations. We also discuss recent related works including lattice studies on QCD at nonzero isospin chemical potential, some effective model analyses, and an analysis based on recent neutron star observations. These works also suggest the breaking of the conformal bound.

[6] 2311.15522

New configuration set of HAL QCD collaboration

We give a brief report on the basic properties and cutoff scale of our new configuration set (HAL-Conf-2023). We generated 8,000 trajectories of the gauge configurations on $96^4$ lattices with the same lattice parameters as the PACS collaboration \cite{Ishikawa:2018jee, PACS:2019ofv}. The topological distribution, the PCAC masses, and decay constants for pseudo-scalar mesons are studied. As for the scale setting, we utilize the $\Omega$ baryon mass as a reference scale and carefully investigate the operator dependence of the correlation function. As a result, we obtain $a^{-1}=2338.8(1.5)^{+0.2}_{-3.3} \ [{\rm MeV}]$ as a lattice cutoff. Our hadron spectra in the physical unit reproduce well the experimental results.

[7] 2311.15796

Mass effects on the QCD $β$-function

In this study we present lattice results on the QCD $\beta$-function in the presence of quark masses. The $\beta$-function is calculated to three loops in perturbation theory and for improved lattice actions; it is extracted from the renormalization of the coupling constant $Z_g$. The background field method is used to compute $Z_g$, where it is simply related to the background gluon field renormalization constant $Z_A$. We focus on the quark mass effects in the background gluon propagator; the dependence of the QCD $\beta$-function on the number of colors $N_c$, the number of fermionic flavors $N_f$ and the quark masses, is shown explicitly. The perturbative results of the QCD $\beta$-function will be applied to the precise determination of the strong coupling constant, calculated by Monte Carlo simulations removing the mass effects from the nonperturbative Green's functions.

[8] 2311.15926

Testing the $\mathrm{SU}(2)$ lattice Hamiltonian built from $S_3$ partitionings

We test a possible digitization of $\mathrm{SU}(2)$ lattice gauge theories based on partitionings of the sphere $S_3$. In our construction the link operators are unitary and diagonal, with eigenvalues determined by the vertices of the partitioning. The canonical momenta are finite difference operators approximating the Lie derivatives on the manifold. In this formalism we implement the standard Wilson Hamiltonian. We show results for a 2-site Schwinger-type model in 1D and a single-plaquette system in 2D. Our calculations are performed on a classical computer, though in principle they can be implemented also on a quantum device.

[9] 2311.16074

A solution for infinite variance problem of fermionic observables

Fermionic Monte Carlo calculations with continuous auxiliary fields often encounter infinite variance problem from fermionic observables. This issue renders the estimation of observables unreliable, even with an infinite number of samples. In this work, we show that the infinite variance problem stems from the fermionic determinant. Also, we propose an approach to address this problem by employing a reweighting method that utilizes the distribution from an extra time-slice. Two strategies to compute the reweighting factor are explored: one involves truncating and analytically calculating the reweighting factor, while the other employs a secondary Monte Carlo estimation. With Hubbard model as a testbed, we demonstrate that utilizing the sub-Monte Carlo estimation, coupled with an unbiased estimator, offers a solution that effectively mitigates the infinite variance problem at a minimal additional cost.

[10] 2311.14791

The QCD vacuum as a disordered chromomagnetic condensate

An attempt is made to describe from first principles the large-scale structure of the confining vacuum in quantum chromodynamics. Starting from our previous variational studies of the SU(2) pure gauge theory in an external Abelian chromomagnetic field and extending the Feynman's qualitative analysis in (2+1)-dimensional SU(2) gauge theory, we show that the SU(3) vacuum in three-space and one-time dimensions behaves like a disordered chromomagnetic condensate. Color confinement is assured by the presence of a mass gap together with the absence of color long-range correlations. We offer a clear physical picture for the formation of the flux tube between static quark charges that allowed to determine the color structure and the transverse profile of the flux-tube chromoelectric field. The transverse profile of the flux-tube chromoelectric field turns out to be in reasonable agreement with lattice data. We, also, show that our quantum vacuum allows for both the color and ordinary Meissner effect. We find that for massless quarks the quantum vacuum could accommodate a finite non-zero density of fermion zero modes leading to the dynamical breaking of the chiral symmetry.

[11] 2311.14832

Pion and kaon electromagnetic and gravitational form factors

A unified set of predictions for pion and kaon elastic electromagnetic and gravitational form factors is obtained using a symmetry-preserving truncation of each relevant quantum field equation. A key part of the study is a description of salient aspects of the dressed graviton + quark vertices. The calculations reveal that each meson's mass radius is smaller than its charge radius, matching available empirical inferences; and meson core pressures are commensurate with those in neutron stars. The analysis described herein paves the way for a direct calculation of nucleon gravitational form factors.

[12] 2311.15258

Non-perturbative quantum Yang-Mills at finite temperature beyond lattice: Dyson-Schwinger approach

We perform an analysis of thermal SU(N) Yang-Mills theory in non-perturbative regimes in the framework of Dyson-Swinger approach with a non-trivial ground state involving Jacobi Elliptic functions. Basic thermodynamic observables such as energy density and pressure have been derived analytically and compared with well-known available lattice results. A good agreement has been found at low temperatures providing a viable scenario of a gas of massive glue states populating higher levels of the spectrum of the theory. At high-temperatures, we observe that a scenario without glue states but consistent with a massive scalar field shows an interesting agreement with lattice data. We discuss the possibility of the results derived in this analysis to open up a novel pathway for precision studies of phase transitions with false vacuum and cosmological relics which depend upon QCD equation of state in strongly-coupled gauge theories beyond lattice.

[13] 2311.15853

Structure of $cc \bar c \bar c$ tetraquarks and interpretation of LHC states

Motivated by recent experimental evidence for apparent $cc\bar c \bar c$ states at LHCb, CMS and ATLAS, we consider how the mass spectrum and decays of such states can be used to discriminate among their possible theoretical interpretations, with a particular focus on identifying whether quarks or diquarks are the most relevant degrees of freedom. Our preferred scenario is that $X(6600)$ and its apparent partner state $X(6400)$ are the tensor $(2^{++})$ and scalar $(0^{++})$ states of an S-wave multiplet of $cc\bar c \bar c$ states. Using tetraquark mass relations which are independent of (or only weakly dependent on) model parameters, we give predictions for the masses of additional partner states with axial and scalar quantum numbers. Additionally, we give predictions for relations among decay branching fractions to $J/\psi J/\psi$, $J/\psi \eta_c$, $\eta_c\eta_c$ and $D^{(*)} \bar{D}^{(*)}$ channels. The scenario we consider is consistent with existing experimental data on $J/\psi J/\psi$, and our predictions for partner states and their decays can be confronted with future experimental data, to discriminate between quark and diquark models.