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M. P. Lombardo

Publications and source records attributed to M. P. Lombardo.

At least 37 records · Page 2Linked to original sources
arXiv2014

One,Two,Zero: Scales of Strong Interactions

We discuss our results on QCD with a number of fundamental fermions ranging from zero to sixteen. These theories exhibit a wide array of fascinating phenomena which have been under close scrutiny, especially in recent years, first and foremost is the approach to conformality. To keep this review focused, we have chosen scale generation, or lack thereof as a guiding theme, however the discussion will be set in the general framework of the analysis of the phases and phase transitions of strong interactions at zero and nonzero temperature.

hep-lat↗
arXiv2014

On the particle spectrum and the conformal window

We study the SU(3) gauge theory with twelve flavours of fermions in the fundamental representation as a prototype of non-Abelian gauge theories inside the conformal window. Guided by the pattern of underlying symmetries, chiral and conformal, we analyze the two-point functions theoretically and on the lattice, and determine the finite size scaling and the infinite volume fermion mass dependence of the would-be hadron masses. We show that the spectrum in the Coulomb phase of the system can be described in the context of a universal scaling analysis and we provide the nonperturbative determination of the fermion mass anomalous dimension gamma*=0.235(46) at the infrared fixed point. We comment on the agreement with the four-loop perturbative prediction for this quantity and we provide a unified description of all existing lattice results for the spectrum of this system, them being in the Coulomb phase or the asymptotically free phase. Our results corroborate the view that the fixed point we are studying is not associated to a physical singularity along the bare coupling line and estimates of physical observables can be attempted on either side of the fixed point. Finally, we observe the restoration of the U(1) axial symmetry in the two-point functions.

hep-lat↗
arXiv2014

Quark-Gluon Plasma: from lattice simulations to experimental results

Theoretical studies of quarkonia can elucidate some of the important properties of the quark--gluon plasma, the state of matter realised when the temperature exceeds 150 MeV, currently probed by heavy-ion collisions experiments at BNL and the LHC. We report on our results of lattice studies of bottomonia for temperatures in the range 100 MeV < T < 450 MeV, introducing and discussing the methodologies we have applied. Of particular interest is the analysis of the spectral functions, where Bayesian methods borrowed and adapted from nuclear and condensed matter physics have proven very successful.

hep-lat↗
arXiv2013

Melting of P wave bottomonium states in the quark-gluon plasma from lattice NRQCD

We study the fate of P wave bottomonium states in the quark-gluon plasma, using a spectral function analysis of euclidean lattice correlators. The correlators are obtained from lattice QCD simulations with two light quark flavours on highly anisotropic lattices, treating the bottom quark nonrelativistically. We find clear indications of melting immediately after the deconfinement transition.

hep-lat↗
arXiv2013

High Temperature QCD

I review recent results on QCD at high temperature on a lattice. Steady progress with staggered fermions and Wilson type fermions allow a quantitative description of hot QCD whose accuracy in many cases parallels that of zero temperature studies. Simulations with chiral quarks are coming of age, and togheter with theoretical developments trigger interesting developments in the analysis of the critical region. Issues related with the universality class of the chiral transition and the fate of the axial symmetry are discussed in the light of new numerical and analytical results. Transport coefficients and analysis of bottomonium spectra compare well with results of heavy ion collisions at RHIC and LHC. Model field theories, lattice simulations and high temperature systematic expansions help building a coherent picture of the high temperature phase of QCD. The (strongly coupled) Quark Gluon Plasma is heavily investigated, and asserts its role as an inspiring theoretical laboratory.

hep-lat↗
arXiv2013

Phases of many flavors QCD : Lattice results

This note is based on our recent results on QCD with varying number of flavors of fundamental fermions. Topics include unusual, strong dynamics in the preconformal, confining phase, the physics of the conformal window and the role of ab-initio lattice simulations in establishing our current knowledge of the phases of many flavor QCD

hep-lat↗
arXiv2013

Making the most of Taylor expansion and imaginary chemical potential

We present preliminary results for the curvature of the pseudocritical line and susceptibilities in Nf = 2 + 1 flavor QCD. The computations are carried out on lattice sizes of 16x16x16x4, at matching parameters of early work of the Bielefeld group. Emphasis is placed on the control of systematic errors, by cross-validating results obtained by use of the Taylor expansion and measurements at imaginary chemical potential. To this end, we generalize the magnetic equation of state to the analysis of the number density, and we extend it to imaginary values of the chemical potential.

hep-lat↗
arXiv2012

Bottomonium from lattice QCD as a probe of the Quark-Gluon Plasma

We study the temperature dependence of bottomonium for temperatures in the range 0.4 Tc < T < 2.1 Tc, using non-relativistic dynamics for the bottom quark and full relativistic lattice QCD simulations for Nf=2 light flavors. We consider the behaviour of the correlators in Euclidean space, we analyze the associated spectral functions and we study the dependence on the momentum. Our results are amenable to a successful comparison with effective field theories. They help build a coherent picture of the behaviour of bottomonium in the plasma, consistent which the current LHC results.

hep-lat↗
arXiv2012

The thermal QCD transition with two flavours of twisted mass fermions

We investigate the thermal QCD transition with two flavors of maximally twisted mass fermions for a set of pion masses, 300 MeV \textless $m_π$ \textless 500 MeV, and lattice spacings $a$ \textless 0.09 fm. We determine the pseudo-critical temperatures and discuss their extrapolation to the chiral limit using scaling forms for different universality classes, as well as the scaling form for the magnetic equation of state. For all pion masses considered we find resonable consistency with O(4) scaling plus leading corrections. However, a true distinction between the O(4) scenario and a first order scenario in the chiral limit requires lighter pions than are currently in use in simulations of Wilson fermions.

hep-lat↗
arXiv2010

Chiral symmetry of QCD with twelve light flavors

We study QCD with twelve light flavors at intermediate values of the bare lattice coupling. We contrast the results for the order parameter with different theoretical models motivated by the physics of the Goldstone phase and of the symmetric phase, and we perform a model independent analysis of the meson spectrum inspired by universal properties of chiral symmetry. Our analysis favors chiral symmetry restoration.

hep-lat↗
arXiv2010

How the Quark Number fluctuates in QCD at small chemical potential

We discuss the distribution of the quark number over the gauge fields for QCD at nonzero quark chemical potential. As the quark number operator is non-hermitian, the distribution is over the complex plane. Moreover, because of the fermion determinant, the distribution is not real and positive. The computation is carried out within leading order chiral perturbation theory and gives direct insight into the delicate cancellations that take place in contributions to the total baryon number.

hep-lat↗
arXiv2010

Bottomonium above deconfinement in lattice nonrelativistic QCD

We study the temperature dependence of bottomonium for temperatures in the range $0.4 T_c < T < 2.1 T_c$, using nonrelativistic dynamics for the bottom quark and full relativistic lattice QCD simulations for $N_f=2$ light flavors on a highly anisotropic lattice. We find that the $Υ$ is insensitive to the temperature in this range, while the $χ_b$ propagators show a crossover from the exponential decay characterizing the hadronic phase to a power-law behaviour consistent with nearly-free dynamics at $T \simeq 2 T_c$.

hep-lat↗
arXiv2010

Evidence for a conformal phase in SU(N) gauge theories

We discuss the existence of a conformal phase in SU(N) gauge theories in four dimensions. In this lattice study we explore the model in the bare parameter space, varying the lattice coupling and bare mass. Simulations are carried out with three colors and twelve flavors of dynamical staggered fermions in the fundamental representation. The analysis of the chiral order parameter and the mass spectrum of the theory indicates the restoration of chiral symmetry at zero temperature and the presence of a Coulomb-like phase, depicting a scenario compatible with the existence of an infrared stable fixed point at nonzero coupling. Our analysis supports the conclusion that the onset of the conformal window for QCD-like theories is smaller than Nf=12, before the loss of asymptotic freedom at sixteen and a half flavors. We discuss open questions and future directions.

hep-ph↗
arXiv2009

Lattice QCD and dense quark matter

This talk summarizes recent progress in lattice QCD for dense quark matter. The emphasis is on the insights obtained from analytical results derived within chiral perturbation theory.

hep-lat↗
arXiv2009

Fluctuations, correlations and the sign problem in QCD

We study the distribution of the phase angle and the magnitude of the fermion determinant as well as its correlation with the chiral condensate and the baryon number for QCD at non-zero quark chemical potential. Results are derived to one-loop order in Chiral Perturbation Theory (ChPT), as well as by analytical and numerical calculations in QCD in one Euclidean dimension. We find a qualitative change of the distribution of the phase of the fermion determinant when the quark mass enters the spectrum of the Dirac operator: it changes from a periodicized Gaussian distribution to a periodicized Lorentzian distribution. We also explore the possibility that some observables remain weakly correlated with the phase of the fermion determinant even though the sign problem is severe. We discuss the practical implications of our findings on lattice simulations of QCD at non-zero baryon chemical potential.

hep-lat↗
arXiv2009

Phase structure of thermal lattice QCD with N_f=2 twisted mass Wilson fermions

We present numerical results for the phase diagram of lattice QCD at finite temperature in the formulation with twisted mass Wilson fermions and a tree-level Symanzik-improved gauge action. Our simulations are performed on lattices with temporal extent N_τ=8, and lattice coupling βranging from strong coupling to the scaling domain. Covering a wide range in the space spanned by the lattice coupling βand the hopping and twisted mass parameters κand μ, respectively, we obtain a comprehensive picture of the rich phase structure of the lattice theory. In particular, we verify the existence of an Aoki phase in the strong coupling region and the realisation of the Sharpe-Singleton scenario at intermediate couplings. In the weak coupling region we identify the phase boundary for the physical finite temperature phase transition/crossover. Its shape in the three-dimensional parameter space is consistent with Creutz's conjecture of a cone-shaped thermal transition surface.

hep-lat↗
arXiv2009

The Fluctuations of the Quark Number and of the Chiral Condensate

The distributions of the quark number and chiral condensate over the gauge fields are computed for QCD in Euclidean space at nonzero quark chemical potential. As both operators are non-hermitian the distributions are in the complex plane. Moreover, because of the sign problem, the distributions are not real and positive. The computations are carried out within leading order chiral perturbation theory and give a direct insight into the delicate cancellations that take place in contributions to the total baryon number and the chiral condensate.

hep-lat↗