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Massimo D'Elia

Publications and source records attributed to Massimo D'Elia.

At least 109 records · Page 6Linked to original sources
arXiv2015

Heavy quarkonia in strong magnetic fields

We investigate the influence of a homogeneus and constant strong external magnetic field on the heavy-meson spectrum. Quarkonium states $c\bar{c}$ and $b\bar{b}$ are described within a non-relativistic framework and by means of a suitable potential model based on the Cornell parametrization. In particular, in this work we propose a model which takes into account the possible anisotropies emerging at the level of the static quark-antiquark potential, as observed in recent lattice studies. The investigation is perfomed both with and without taking into account the anisotropy of the static potential, in order to better clarify its effects

hep-ph↗
arXiv2015

Curvature of the chiral pseudo-critical line in QCD: continuum extrapolated results

We determine the curvature of the pseudo-critical line of strong interactions by means of numerical simulations at imaginary chemical potentials. We consider $N_f=2+1$ stout improved staggered fermions with physical quark masses and the tree level Symanzik gauge action, and explore four different sets of lattice spacings, corresponding to $N_t = 6,8,10,12$, in order to extrapolate results to the continuum limit. Our final estimate is $κ= 0.0135(20)$.

hep-lat↗
arXiv2015

Lattice QCD with purely imaginary sources at zero and non-zero temperature

We discuss various aspects and recent progress concerning lattice QCD studies in the presence of external sources. We focus, in particular, on issues regarding QCD with non-zero imaginary chemical potentials or with a $θ$-term, and on the properties of strongly interacting matter in the presence of electromagnetic background fields.

hep-lat↗
arXiv2015

Curvature of the chiral pseudo-critical line in QCD

We determine the curvature of the pseudo-critical line of strong interactions by means of numerical simulations at imaginary chemical potentials. We consider $N_f=2+1$ stout improved staggered fermions with physical quark masses and the tree level Symanzik gauge action, and explore two different sets of lattice spacings, corresponding to temporal extensions $N_t = 6$ and $N_t = 8$. Both the renormalized chiral condensate and the renormalized chiral susceptibility are used to locate the transition. The determinations obtained from the two quantities are in good agreement, a preliminary continuum extrapolation yields $κ= 0.013(2)(1)$. We also investigate the impact of a non-zero strange quark chemical potential and compare our results to previous determinations in the literature, discussing the possible sources of systematic errors affecting the various procedures.

hep-lat↗
arXiv2014

The chiral phase transition in two-flavor QCD from imaginary chemical potential

We investigate the order of the finite temperature chiral symmetry restoration transition for QCD with two massless fermions, by using a novel method, based on simulating imaginary values of the quark chemical potential $μ=iμ_i,μ_i\in\mathbb{R}$. Our method exploits the fact that, for low enough quark mass $m$ and large enough chemical potential $μ_i$, the chiral transition is decidedly first order, then turning into crossover at a critical mass $m_c(μ)$. It is thus possible to determine the critical line in the $m - μ^2$ plane, which can be safely extrapolated to the chiral limit by taking advantage of the known tricritical indices governing its shape. We test this method with standard staggered fermions and the result of our simulations is that $m_c(μ=0)$ is positive, so that the phase transition at zero density is definitely first order in the chiral limit, on our coarse $N_t=4$ lattices with $a\simeq 0.3\,\mathrm{fm}$.

hep-lat↗
arXiv2014

Universal scaling effects of a temperature gradient at first-order transitions

We study the effects of smooth inhomogeneities at first-order transitions. We show that a temperature gradient at a thermally-driven first-order transition gives rise to nontrivial universal scaling behaviors with respect to the length scale of the variation of the local temperature T(x). We propose a scaling ansatz to describe the crossover region at the surface where T(x)=Tc, where the typical discontinuities of a first-order transition are smoothed out. The predictions of this scaling theory are checked, and get strongly supported, by numerical results for the 2D Potts models, for a sufficiently large number of q-states to have first-order transitions. Comparing with analogous results at the 2D Ising transition, we note that the scaling behaviors induced by a smooth inhomogeneity appear quite similar in first-order and continuous transitions.

cond-mat.stat-mech↗
arXiv2014

Anisotropy of the quark-antiquark potential in a magnetic field

We investigate the static $\overline{Q}Q$-potential for $N_f = 2+1$ QCD at the physical point in the presence of a constant and uniform external magnetic field. The potential is found to be anisotropic and steeper in the directions transverse to the magnetic field than in the longitudinal one. In particular, when compared to the standard case with zero background field, the string tension increases (decreases) in the transverse (longitudinal) direction, while the absolute value of the Coulomb coupling and the Sommer parameter show an opposite behavior.

hep-lat↗
arXiv2014

Magnetic susceptibility and equation of state of N_f = 2+1 QCD with physical quark masses

We determine the free energy of strongly interacting matter as a function of an applied constant and uniform magnetic field. We consider N_f = 2+1 QCD with physical quark masses, discretized on a lattice by stout improved staggered fermions and a tree level improved Symanzik pure gauge action, and explore three different lattice spacings. For magnetic fields of the order of those produced in non-central heavy ion collisions (eB ~ 0.1 GeV^2) strongly interacting matter behaves like a medium with a linear response, and is paramagnetic both above and below the deconfinement transition, with a susceptibility which steeply rises in the deconfined phase. We compute the equation of state, showing that the relative increase in the pressure due to the magnetic field gets larger around the transition, and of the order of 10 % for eB ~ 0.1 GeV^2.

hep-lat↗
arXiv2014

Comparison of the gradient flow with cooling in $SU(3)$ pure gauge theory

The gradient (Wilson) flow has been introduced recently in order to provide a solid theoretical framework for the smoothing of ultraviolet noise in lattice gauge configurations. It is interesting to ask how it compares with other, more heuristic and numerically cheaper smoothing techniques, such as standard cooling. In this study we perform such a comparison, focusing on observables related to topology. We show that, already for moderately small lattice spacings, standard cooling and the gradient flow lead to equivalent results, both for average quantities and configuration by configuration.

hep-lat↗
arXiv2013

The magnetic susceptibility in QCD

Recently much work has been devoted to the study of QCD coupled to a background magnetic field. Strongly interacting matter acts as a magnetic medium and it is natural to study the properties of this medium, in particular to understand if it behaves like a diamagnetic or a paramagnetic material. A serious difficulty in studying these properties by means of LQCD simulations is the quantization of the magnetic field in a toroidal geometry. We will expose a method to overcome this difficulty and we will present data obtained for the $N_f=2$ theory that show that the QCD medium is paramagnetic.

hep-lat↗
arXiv2013

Effective $θ$ term by CP-odd electromagnetic background fields

We discuss our study of QCD in the presence of CP-odd electromagnetic (e.m.) background fields. We investigate the propagation of the CP-odd term from the e.m. sector to the strong sector, inducing an effective $θ$ term. We discuss the method we have used in our lattice QCD simulations, and the results of our analysis, which are relevant to the determination of the effective pseudoscalar QED-QCD interactions. We also explore how these CP-odd e.m. background fields influence the number of the Dirac zero modes in our configurations.

hep-lat↗
arXiv2013

The Maximal Abelian Gauge in SU(N) Gauge Theories and Thermal Monopoles for N = 3

We discuss and propose a proper extension of the Abelian projection based on the Maximal Abelian Gauge to SU(N) gauge theories. Based, on that, we investigate the properties of thermal Abelian monopoles in the deconfined phase of the SU(3) pure gauge theory. Such properties are very similar to those already found for SU(2), confirming the relevance of the magnetic component close to Tc and the possible condensation of thermal monopoles as the deconfinement temperature is crossed from above. Moreover, we study the correlation functions among monopoles related to different U(1) subgroups, which show interesting features and reveal the presence of non-trivial interactions.

hep-lat↗
arXiv2013

Phase diagram of Yang-Mills theories in the presence of a theta term

We study the phase diagram of non-Abelian pure gauge theories in the presence of a topological theta term. The dependence of the deconfinement temperature on theta is determined on the lattice both by analytic continuation and by reweighting, obtaining consistent results. The general structure of the diagram is discussed on the basis of large-N considerations and of the possible analogies and dualities existing with the phase diagram of QCD in presence of an imaginary baryon chemical potential.

hep-lat↗
arXiv2013

The chiral phase transition for two-flavour QCD at imaginary and zero chemical potential

The chiral symmetry of QCD with two massless quark flavours gets restored in a non-analytic chiral phase transition at finite temperature and zero density. Whether this is a first-order or a second-order transition has not yet been determined unambiguously, due to the difficulties of simulating light quarks. We investigate the nature of the chiral transition as a function of quark mass and imaginary chemical potential, using staggered fermions on N_t=4 lattices. At sufficiently large imaginary chemical potential, a clear signal for a first-order transition is obtained for small masses, which weakens with decreasing imaginary chemical potential. The second-order critical line m_c(mu_i), which marks the boundary between first-order and crossover behaviour, extrapolates to a finite m_c(mu_i=0) with known critical exponents. This implies a definitely first-order transition in the chiral limit on relatively coarse, N_t=4 lattices.

hep-lat↗
arXiv2013

Theta dependence of 4D SU(N) gauge theories at finite temperature

We report a study of the dependence of 4D SU(N) gauge theories on the topological theta term at finite temperature, and in particular in the large-N limit. We show that the theta dependence drastically changes across the deconfinement transition. The low-temperature phase is characterized by a large-N scaling with theta/N as relevant variable, while in the high-temperature phase the free energy is essentially determined by the dilute instanton-gas approximation, with a simple theta dependence of the free-energy density proportional to 1-Cos(theta).

hep-lat↗
arXiv2013

Phase diagram of the 4D U(1) model at finite temperature

We explore the phase diagram of the 4D compact U(1) gauge theory at finite temperature as a function of the gauge coupling and of the compactified Euclidean time dimension L_t. We show that the strong-to-weak coupling transition, which is first order at T=0 (L_t=\infty), becomes second order for high temperatures, i.e. for small values of L_t, with a tricritical temporal size \bar{L_t} located between 5 and 6. The critical behavior around the tricritical point explains and reconciles previous contradictory evidences found in the literature.

hep-lat↗
arXiv2013

Change of theta dependence in 4D SU(N) gauge theories across the deconfinement transition

We investigate the dependence of four-dimensional SU(N) gauge theories on the topological theta term at finite temperature and, in particular, across the deconfinement transition. For this purpose, we exploit the lattice formulation of the theory and present numerical results for the expansion of the free energy up to O(theta^6), for N=3 and N=6. Our numerical analysis shows that the theta dependence of 4D SU(N) gauge theory experiences a drastic change across the deconfinement transition: the low-temperature phase is characterized by a large-N scaling with theta/N as relevant variable, while in the high-temperature phase the scaling variable is just theta and the free energy is essentially determined by the instanton-gas approximation. The crossover between the two different behaviours gets sharper with increasing N, suggesting that the instanton-gas regime sets in just above Tc at large N.

hep-lat↗