arXiv subjects

Claudio Bonati

Publications and source records attributed to Claudio Bonati.

At least 91 records · Page 5Linked to original sources

arXiv2018

Curvature of the pseudocritical line in QCD: Taylor expansion matches analytic continuation

We determine the curvature of the pseudo-critical line of $N_f = 2 + 1$ QCD with physical quark masses via Taylor expansion in the quark chemical potentials. We adopt a discretization based on stout improved staggered fermions and the tree level Symanzik gauge action; the location of the pseudocritical temperature is based on chiral symmetry restoration. Simulations are performed on lattices with different temporal extent ($N_t = 6, 8, 10$), leading to a continuum extrapolated curvature $κ= 0.0145(25)$, which is in very good agreement with the continuum extrapolation obtained via analytic continuation and the same discretization, $κ= 0.0135(20)$. This result eliminates the possible tension emerging when comparing analytic continuation with earlier results obtained via Taylor expansion.

hep-lat↗

arXiv2018

Portable multi-node LQCD Monte Carlo simulations using OpenACC

This paper describes a state-of-the-art parallel Lattice QCD Monte Carlo code for staggered fermions, purposely designed to be portable across different computer architectures, including GPUs and commodity CPUs. Portability is achieved using the OpenACC parallel programming model, used to develop a code that can be compiled for several processor architectures. The paper focuses on parallelization on multiple computing nodes using OpenACC to manage parallelism within the node, and OpenMPI to manage parallelism among the nodes. We first discuss the available strategies to be adopted to maximize performances, we then describe selected relevant details of the code, and finally measure the level of performance and scaling-performance that we are able to achieve. The work focuses mainly on GPUs, which offer a significantly high level of performances for this application, but also compares with results measured on other processors.

hep-lat↗

arXiv2017

Gauge-invariant screening masses and static quark free energies in $N_f = 2+1$ QCD at non-zero baryon density

We discuss the extension of gauge-invariant electric and magnetic screening masses in the Quark-Gluon Plasma to the case of a finite baryon density, defining them in terms of a matrix of Polyakov loop correlators. We present lattice results for $N_f=2+1$ QCD with physical quark masses, obtained using the imaginary chemical potential approach, which indicate that the screening masses increase as a function of $μ_B$. A separate analysis is carried out for the theoretically interesting case $μ_B/T=3 i π$, where charge conjugation is not explicitly broken and the usual definition of the screening masses can be used for temperatures below the Roberge-Weiss transition. Finally, we investigate the dependence of the static quark free energy on the baryon chemical potential, showing that it is a decreasing function of $μ_B$ which displays a peculiar behavior as the pseudocritical transition temperature at $μ_B=0$ is approached.

hep-lat↗

arXiv2017

Influence of magnetic fields on the color screening masses

We present some recent results obtained in the study of the color magnetic and electric screening masses in the QCD plasma. In particular, we discuss how the masses get modified by strong external fields which are expected to be created in physical situations such as heavy-ion collisions.

hep-lat↗

arXiv2017

Flux tubes in $N_f=2+1$ QCD with external magnetic fields

We study the behavior of the confining flux tube in $N_f=2+1$ QCD at the physical point, discretized with the stout smearing improved staggered quark action and the tree level Symanzik gauge action. We discuss how it depends on a uniform external magnetic field, showing how it displays anisotropies with respect to the magnetic field direction. Moreover, we compare the observed anisotropy pattern with that of the static quark-antiquark potential we obtained in our previous works.

hep-lat↗

arXiv2017

The topological properties of QCD at high temperature: problems and perspectives

Lattice computations are the only first principle method capable of quantitatively assessing the topological properties of QCD at high temperature, however the numerical determination of the topological properties of QCD, especially in the high temperature phase, is a notoriously difficult problem. We will discuss the difficulties encountered in such a computation and some strategies that have been proposed to avoid (or at least to alleviate) them.

hep-lat↗

arXiv2017

Screening masses in strong external magnetic fields

We present results for the (color)magnetic and (color)electric screening masses of the Quark-Gluon Plasma in the presence of an external magnetic field. The screening masses are extracted from the correlators of Polyakov loops, determined by lattice QCD simulations at the physical point. We explore temperatures in the range $200\,\mathrm{MeV}\lesssim T\lesssim 330\,\mathrm{MeV}$ and magnetic field intensities up to $|e|B \sim 1.3\,\mathrm{GeV}^2$. We find that both screening masses are increasing functions of the magnetic field and that the dependence on $B$ becomes weaker for larger temperatures. In the case of the magnetic screening mass a slight anisotropy is also observable.

hep-lat↗

arXiv2017

Effects of magnetic fields on quark-antiquark interactions

We discuss some recent results obtained in the study of strong quark-antiquark interactions in the presence of intense external magnetic fields by means of lattice QCD simulations. We confirm previous findings and show that both at zero and finite temperature the external field induces anisotropies in the static quark potential. An in-depth study suggests that the effects are essentially due to the variation of the string tension whose angular dependence can be nicely parametrized by the first allowed term in a Fourier expansion. In the confined phase at high temperature, we observe that the suppression of the string tension is enhanced as the strength of the external field increases. Our results support the idea that the loss of confining properties is the dominant effect related to the decrease of $T_c$ as a function of $B$.

hep-lat↗

arXiv2017

Theta dependence in the large N limit

Studies of the large $N$ behaviour of the topological properties of gauge theories typically focused on the large $N$ scaling of the topological susceptibility. A much more difficult task is the study of the behaviour of higher cumulants of the topological charge in the large $N$ limit, which up to now remained elusive. We will present first results confirming the expected large $N$ scaling of the coefficient commonly denoted by $b_2$, related to the kurtosis of the topological charge.

hep-lat↗

arXiv2017

Design and optimization of a portable LQCD Monte Carlo code using OpenACC

The present panorama of HPC architectures is extremely heterogeneous, ranging from traditional multi-core CPU processors, supporting a wide class of applications but delivering moderate computing performance, to many-core GPUs, exploiting aggressive data-parallelism and delivering higher performances for streaming computing applications. In this scenario, code portability (and performance portability) become necessary for easy maintainability of applications; this is very relevant in scientific computing where code changes are very frequent, making it tedious and prone to error to keep different code versions aligned. In this work we present the design and optimization of a state-of-the-art production-level LQCD Monte Carlo application, using the directive-based OpenACC programming model. OpenACC abstracts parallel programming to a descriptive level, relieving programmers from specifying how codes should be mapped onto the target architecture. We describe the implementation of a code fully written in OpenACC, and show that we are able to target several different architectures, including state-of-the-art traditional CPUs and GPUs, with the same code. We also measure performance, evaluating the computing efficiency of our OpenACC code on several architectures, comparing with GPU-specific implementations and showing that a good level of performance-portability can be reached.

hep-lat↗

arXiv2016

Modeling of strain-induced Pockels effect in Silicon

We propose a theoretical model to describe the strain-induced linear electro-optic (Pockels) effect in centro-symmetric crystals. The general formulation is presented and the specific case of the strained silicon is investigated in detail because of its attractive properties for integrated optics. The outcome of this analysis is a linear relation between the second order susceptibility tensor and the strain gradient tensor, depending generically on fifteen coefficients. The proposed model greatly simplifies the description of the electro-optic effect in strained silicon waveguides, providing a powerful and effective tool for design and optimization of optical devices.

physics.optics↗

arXiv2016

Recent progress on QCD inputs for axion phenomenology

The properties of the QCD axion are strictly related to the dependence of strong interactions on the topological parameter theta. We present a determination of the topological properties of QCD for temperatures up to around 600 MeV, obtained by lattice QCD simulations with 2+1 flavors and physical quark masses. Numerical results for the topological susceptibility, when compared to instanton gas computations, differ both in size and in the temperature dependence. We discuss the implications of such findings for axion phenomenology, also in comparison to similar studies in the literature, and the prospects for future investigations.

hep-lat↗

arXiv2016

$θ$ dependence of 4D $SU(N)$ gauge theories in the large-$N$ limit

We study the large-$N$ scaling behavior of the $θ$ dependence of the ground-state energy density $E(θ)$ of four-dimensional (4D) $SU(N)$ gauge theories and two-dimensional (2D) $CP^{N-1}$ models, where $θ$ is the parameter associated with the Lagrangian topological term. We consider its $θ$ expansion around $θ=0$, $E(θ)-E(0) = {1\over 2}χ\,θ^2 ( 1 + b_2 θ^2 + b_4θ^4 +\cdots)$ where $χ$ is the topological susceptibility and $b_{2n}$ are dimensionless coefficients. We focus on the first few coefficients $b_{2n}$, which parametrize the deviation from a simple Gaussian distribution of the topological charge at $θ=0$. We present a numerical analysis of Monte Carlo simulations of 4D $SU(N)$ lattice gauge theories for $N=3,\,4,\,6$ in the presence of an imaginary $θ$ term. The results provide a robust evidence of the large-$N$ behavior predicted by standard large-$N$ scaling arguments, i.e. $b_{2n}= O(N^{-2n})$. In particular, we obtain $b_2=\bar{b}_2/N^2 + O(1/N^4)$ with $\bar{b}_2=-0.23(3)$. We also show that the large-$N$ scaling scenario applies to 2D $CP^{N-1}$ models as well, by an analytic computation of the leading large-$N$ dependence.

hep-lat↗

arXiv2016

Across the deconfinement

The deconfinement transition at vanishing chemical potential can be reliably studied by lattice simulations and its general features are by now well known. On the contrary, what happens at finite density is still largely unknown and we will review the results obtained in the last year regarding the dependence, for small density, of the (pseudo)critical temperature on the baryonic chemical potential.

hep-lat↗

arXiv2016

Magnetic field effects on the static quark potential at zero and finite temperature

We investigate the static $Q\bar{Q}$ potential at zero and finite temperature in the presence of a constant and uniform external magnetic field $\vec{B}$, for several values of the lattice spacing and for different orientations with respect to $\vec{B}$. As a byproduct, we provide continuum limit extrapolated results for the string tension, the Coulomb coupling and the Sommer parameter at $T = 0$ and $B = 0$. We confirm the presence in the continuum of a $B$-induced anisotropy, regarding essentially the string tension, for which it is of the order of 15\% at $|e| B \sim 1~{\rm GeV}^2$ and would suggest, if extrapolated to larger fields, a vanishing string tension along the magnetic field for $|e| B \gtrsim 4$ GeV$^2$. The angular dependence for $|e| B \lesssim 1$ GeV$^2$ can be nicely parametrized by the first allowed term in an angular Fourier expansion, corresponding to a quadrupole deformation. Finally, for $T \neq 0$, the main effect of the magnetic field is a general suppression of the string tension, leading to a precocious loss of the confining properties: this happens even before the appearance of inverse magnetic catalysis in the chiral condensate, supporting the idea that the influence of the magnetic field on the confining properties is the leading effect originating the decrease of $T_c$ as a function of $B$.

hep-lat↗

arXiv2016

Axion phenomenology and $θ$-dependence from $N_f = 2+1$ lattice QCD

We investigate the topological properties of $N_f = 2+1$ QCD with physical quark masses, both at zero and finite temperature. We adopt stout improved staggered fermions and explore a range of lattice spacings $a \sim 0.05 - 0.12$ fm. At zero temperature we estimate both finite size and finite cut-off effects, comparing our continuum extrapolated results for the topological susceptibility $χ$ with predictions from chiral perturbation theory. At finite temperature, we explore a region going from $T_c$ up to around $4\, T_c$, where we provide continuum extrapolated results for the topological susceptibility and for the fourth moment of the topological charge distribution. While the latter converges to the dilute instanton gas prediction the former differs strongly both in the size and in the temperature dependence. This results in a shift of the axion dark matter window of almost one order of magnitude with respect to the instanton computation.

hep-lat↗

arXiv2016

Roberge-Weiss endpoint at the physical point of $N_f = 2+1$ QCD

We study the phase diagram of $N_f = 2+1$ QCD in the $T - μ_B$ plane and investigate the critical point corresponding to the onset of the Roberge-Weiss transition, which is found for imaginary values of $μ_B$. We make use of stout improved staggered fermions and of the tree level Symanzik gauge action, and explore four different sets of lattice spacings, corresponding to $N_t = 4,6,8,10$, and different spatial sizes, in order to assess the universality class of the critical point. The continuum extrapolated value of the endpoint temperature is found to be $T_{\rm RW} = 208(5)$ MeV, i.e. $T_{\rm RW}/T_c \sim 1.34(7)$, where $T_c$ is the chiral pseudocritical temperature at zero chemical potential, while our finite size scaling analysis, performed on $N_t = 4$ and $N_t = 6$ lattices, provides evidence for a critical point in the $3d$ Ising universality class.

hep-lat↗

arXiv2016

$θ$ dependence in $SU(3)$ Yang-Mills theory from analytic continuation

We investigate the topological properties of the $SU(3)$ pure gauge theory by performing numerical simulations at imaginary values of the $θ$ parameter. By monitoring the dependence of various cumulants of the topological charge distribution on the imaginary part of $θ$ and exploiting analytic continuation, we determine the free energy density up to the sixth order order in $θ$, $f(θ,T) = f(0,T) + {1\over 2} χ(T) θ^2 (1 + b_2(T) θ^2 + b_4(T) θ^4 + O(θ^6))$. That permits us to achieve determinations with improved accuracy, in particular for the higher order terms, with control over the continuum and the infinite volume extrapolations. We obtain $b_2=-0.0216(15)$ and $|b_4|\lesssim 4\times 10^{-4}$.

hep-lat↗