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Maria Paola Lombardo

Publications and source records attributed to Maria Paola Lombardo.

46 records · Page 3Linked to original sources
arXiv2010

The bulk transition of many-flavour QCD and the search for a UVFP at strong coupling

We explore the nature of the bulk transition observed at strong coupling in the SU(3) gauge theory with Nf=12 fermions in the fundamental representation. The transition separates a weak coupling chirally symmetric phase from a strong coupling chirally broken phase and is compatible with the scenario where conformality is restored by increasing the flavour content of a non abelian gauge theory. We explore the intriguing possibility that the observed bulk transition is associated with the occurrence of an ultraviolet fixed point (UVFP) at strong coupling, where a new theory emerges in the continuum.

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arXiv2010

Study of finite temperature QCD with 2+1 flavors via Taylor expansion and imaginary chemical potential

We study QCD with 2+1 flavors at nonzero temperature and nonzero chemical potential. We present preliminary results obtained from lattice calculations performed with an improved staggered fermions action (p4-action) on lattice with temporal extent N_t = 4 on a line of constant physics with the strange quark mass adjusted to its physical value and a pion mass of about 220 MeV. We compute at imaginary chemical potential and compare with Taylor expansion results. We focus our study on a range of temperatures 0.94 < T/T_c < 1.08.

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arXiv2010

Thermal transition temperature from twisted mass QCD

We present the current status of lattice simulations with N_f=2 maximally twisted mass Wilson fermions at finite temperature. In particular, the determination of the thermal transition temperature is discussed.

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arXiv2009

Traces of a fixed point: Unravelling the phase diagram at large Nf

With a sufficiently high number of fundamental fermionic flavours present, Yang-Mills theory develops an infrared fixed point and becomes (quasi-)conformal in nature. The range of flavour numbers for which this occurs defines the conformal window, the lower limit of which has yet to be determined. We studied the phase diagram of SU(3) Yang-Mills theory with twelve flavours of staggered fermions. Here we present evidence for the occurrence of a bulk transition towards a chirally broken phase and the existence of a Coulomb phase on the symmetric side of this transition, using results from the measurements of the chiral condensate and spectrum, leading to the determination of a positive sign of the beta function. Assuming the validity of the Appelquist-Miransky-Yamawaki scenario, this implies the existence of a conformal window that comprises the theory under investigation.

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arXiv2008

The physics of eight flavours

When the flavour content of QCD is increased sufficiently, the theory develops a non-trivial infra red fixed point. Thus, for a number of flavours above a certain critical value, but not yet so high that asymptotic freedom is lost, QCD becomes a conformal field theory. The location of the lower limit of this conformal window has not yet been unequivocally determined. Using an improved lattice action, and exploiting modern algorithms allowing for larger lattices and lower quark masses, we have shown that the theory of QCD with eight flavours breaks chiral symmetry in the continuum. We present proof that the accompanying transition is thermal in nature and as a consequence, the conformal window of QCD can only start afterwards, corroborating recent analytical studies at the expense of older results.

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arXiv2008

Glueballs and the superfluid phase of Two-Color QCD

We present the first results on scalar glueballs in cold, dense matter using lattice simulations of two color QCD. The simulations are carried out on a $6^3 \times 12$ lattice and use a standard hybrid molecular dynamics algorithm for staggered fermions for two values of quark mass. The glueball correlators are evaluated via a multi-step smearing procedure. The amplitude of the glueball correlator peaks in correspondence with the zero temperature chiral transition, $μ_c = m_π/2$, and the propagators change in a significant way in the superfluid phase, while the Polyakov loop is mearly insensitive to the transition. Standard analysis suggest that lowest mass in the $0^{++}$ gluonic channel decreases in the superfluid phase, but these observations need to be confirmed on larger and more elongated lattices These results indicate that a nonzero density induces nontrivial modifications of the gluonic medium.

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arXiv2008

The physics of eight flavours

We study Quantum Chromodynamics with eight flavours by use of lattice simulations and present evidence that the theory still breaks chiral symmetry in the zero temperature, continuum limit. This confirms that the lower end of the conformal window of QCD lies above Nf = 8.

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arXiv2007

The strongly interacting Quark Gluon Plasma, and the critical behaviour of QCD at imaginary chemical potential

We explore the highly non-perturbative hot region of the QCD phase diagram close to Tc by use of an imaginary chemical potential mu which avoids the sign problem. The number density and the quark number susceptibility are consistent with a critical behaviour associated with the transition line in the negative mu^2 half-plane. We compare the analytic continuation of these results with various phenomenological models, none of which provides a satisfactory description of data, a failure on which we make some comments. These results complement and extend the information obtained via the analysis of the susceptibilities evaluated at zero mu, yielding a simple description of the candidate strongly interacting QGP phase. As a byproduct of our analysis we investigate the Polyakov loop and its hermitian conjugate. Our data offer a vivid evidence of the importance of the complex nature of the functional integral measure, which results in L (mu) ne \bar L(mu) for a real chemical potential.

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arXiv2004

QCD thermodynamics from an imaginary mu_B: results on the four flavor lattice model

We study four flavor QCD at nonzero temperature and density by analytic continuation from an imaginary chemical potential. The explored region is T = 0.95 T_c < T < 3.5 T_c, and the baryochemical potentials range from 0 to approx. 500 MeV. Observables include the number density, the order parameter for chiral symmetry, and the pressure, which is calculated via an integral method at fixed temperature and quark mass. The simulations are carried out on a 16^3 X 4 lattice, and the mass dependence of the results is estimated by exploiting the Maxwell relations. In the hadronic region we confirm that the results are consistent with a simple resonance hadron gas model, and we estimate the critical density by combining the results for the number density with those for the critical line. In the hot phase, above the endpoint of the Roberge-Weiss transition T_E approx 1.1 T_c the results are consistent with a free lattice model with a fixed effective number of flavor slightly different from four. We confirm that confinement and chiral symmetry are coincident by a further analysis of the critical line, and we discuss the interrelation between thermodynamics and critical behavior. We comment on the strength and weakness of the method, and propose further developments.

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