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D. Toublan

Publications and source records attributed to D. Toublan.

At least 19 recordsLinked to original sources
arXiv2006

Heavy-quark contribution to the proton's magnetic moment

We study the contribution to the proton's magnetic moment from a heavy quark sea in quantum chromodynamics. The heavy quark is integrated out perturbatively to obtain an effective dimension-6 magnetic moment operator composed of three gluon fields. The leading contribution to the matrix element in the proton comes from a quadratically divergent term associated with a light-quark tensor operator. With an approximate knowledge of the proton's tensor charge, we conclude that a heavy sea-quark contribution to the proton's magnetic moment is positive in the asymptotic limit. We comment on the implication of this result for the physical strange quark.

hep-ph↗
arXiv2006

Microscopic eigenvalue correlations in QCD with imaginary isospin chemical potential

We consider the chiral limit of QCD subjected to an imaginary isospin chemical potential. In the epsilon-regime of the theory we can perform precise analytical calculations based on the zero-momentum Goldstone modes in the low-energy effective theory. We present results for the spectral correlation functions of the associated Dirac operators.

hep-th↗
arXiv2006

Extracting $F_π$ from small lattices: unquenched results

We calculate the response of the microscopic Dirac spectrum to an imaginary isospin chemical potential for QCD with two dynamical flavors in the chiral limit. This extends our previous calculation from the quenched to the unquenched theory. The resulting spectral correlation function in the $ε$-regime provides here, too, a new and efficient way to measure $F_π$ on the lattice. We test the method in a hybrid Monte Carlo simulation of the theory with two staggered quarks.

hep-lat↗
arXiv2005

Light Quark Condensates at Nonzero Chemical Potentials

We show that the quark condensates for the two light up and down flavors can have significantly different values in the hadronic phase at nonzero temperature, baryon and isospin chemical potentials. We quantify this difference using a simple model.

hep-ph↗
arXiv2005

QCD Phase Transitions in the 1/Nc Expansion

We study the QCD phase diagram at nonzero baryon and isospin chemical potentials using the 1/Nc expansion. We find that there are two phase transitions between the hadronic phase and the quark gluon plasma phase. We discuss the consequences of this result for the universality class of the critical endpoint at nonzero baryon and zero isospin chemical potential.

hep-th↗
arXiv2005

Quark-Antiquark Condensates in the Hadronic Phase

We use a hadron resonance gas model to calculate the quark-antiquark condensates for light (up and down) and strange quark flavors at finite temperatures and chemical potentials. At zero chemical potentials, we find that at the temperature where the light quark-antiquark condensates entirely vanish the strange quark-antiquark condensate still keeps a relatively large fraction of its value in the vacuum. This is in agreement with results obtained in lattice simulations and in chiral perturbation theory at finite temperature and zero chemical potentials. Furthermore, we find that this effect slowly disappears at larger baryon chemical potential. These results might have significant consequences for our understanding of QCD at finite temperatures and chemical potentials. Concretely, our results imply that there might be a domain of temperatures where chiral symmetry is restored for light quarks, but still broken for strange quark that persists at small chemical potentials. This might have practical consequences for heavy ion collision experiments.

hep-ph↗
arXiv2005

Confinement-Deconfinement Phase Transition at Nonzero Chemical Potential

We present arguments suggesting that large size overlapping instantons are the driving mechanism of the confinement-deconfinement phase transition at nonzero chemical potential mu. The arguments are based on the picture that instantons at very large chemical potential in the weak coupling regime are localized configurations with finite size rho~1/mu. At the same time, the same instantons at smaller chemical potential in the strong coupling regime are well represented by the so-called instanton-quarks with fractional topological charge 1/Nc. We estimate the critical chemical potential mu_c(T) where this phase transition takes place as a function of temperature in the domain where our approach is justified. In this picture, the long standing problem of the "accidental" coincidence of the chiral and deconfinement phase transitions at nonzero temperature (observed in lattice simulations) is naturally resolved. We also derive results at nonzero isospin chemical potential mu_I where direct lattice calculations are possible, and our predictions can be explicitly tested.

hep-ph↗
arXiv2004

The QCD phase diagram at nonzero baryon, isospin and strangeness chemical potentials: Results from a hadron resonance gas model

We use a hadron resonance gas model to study the QCD phase diagram at nonzero temperature, baryon, isospin and strangeness chemical potentials. We determine the temperature of the transition from the hadronic phase to the quark gluon plasma phase using two different methods. We find that the critical temperatures derived in both methods are in very good agreement. We find that the critical surface has a small curvature. We also find that the critical temperature's dependence on the baryon chemical potential at zero isospin chemical potential is almost identical to its dependence on the isospin chemical potential at vanishing baryon chemical potential. This result, which holds when the chemical potentials are small, supports recent lattice simulation studies. Finally, we find that at a given baryon chemical potential, the critical temperature is lowered as either the isospin or the strangeness chemical potential are increased. Therefore, in order to lower the critical temperature, it might be useful to use different isotopes in heavy ion collision experiments.

hep-ph↗
arXiv2004

The QCD Phase Diagram at Non-zero Baryon and Isospin Chemical Potentials

In heavy ion collision experiments as well as in neutron stars, both baryon and isospin chemical potentials are different from zero. In particular, the regime of small isospin chemical potential is phenomenologically important. Using a random matrix model, we find that the phase diagram at non-zero temperature and baryon chemical potential is greatly altered by an arbitrarily small isospin chemical potential: There are two first order phase transitions at low temperature, two critical endpoints, and two crossovers at high temperature. As a consequence, in the region of the phase diagram explored by RHIC experiments, there are two crossovers that separate the hadronic phase from the quark-gluon plasma phase at high temperature.

hep-lat↗
arXiv2004

Diquark and Pion Condensation in Random Matrix Models for two-color QCD

We introduce a random matrix model with the symmetries of QCD with two colors at nonzero isospin and baryon chemical potentials and temperature. We analyze its phase diagram and find phases with condensation of pion and diquark states in addition to the phases with spontaneously broken chiral symmetries. In the limit of small chemical potentials and quark masses, we reproduce the mean field results obtained from chiral Lagrangians. As in the case of QCD with three colors, the presence of two chemical potentials breaks the flavor symmetry and leads to phases that are characterized by different behaviors of the chiral condensates for each flavor. In particular, the phase diagram we obtain is similar to QCD with three colors and three flavors of quarks of equal masses at zero baryon chemical potential and nonzero isospin and strange chemical potentials. A tricritical point of the superfluid transitions found in lattice calculations and from an analysis in terms of chiral Lagrangians does not appear in the random matrix model. Remarkably, at fixed isospin chemical potential, for the regions outside of the superfluid phases, the phase diagram in the temperature - baryon chemical potential plane for two colors and three colors are qualitatively the same.

hep-ph↗
arXiv2003

Finite Density Lattice Gauge Theories with Positive Fermion Determinants

We perform simulations of (3-colour) QCD with 2 quark flavours at a finite chemical potential $μ_I$ for isospin($I_3$), and of 2-colour QCD at a finite chemical potential $μ$ for quark number. At zero temperature, QCD at finite $μ_I$ has a mean-field phase transition at $μ_I=m_π$ to a superfluid state with a charged pion condensate which spontaneously breaks $I_3$. We study the finite temperature transition as a function of $μ_I$. For $μ_I < m_π$, where this is closely related to the transition at finite $μ$, this appears to be a crossover independent of quark mass, with no sign of the proposed critical endpoint. For $μ_I > m_π$ this becomes a true phase transition where the pion condensate evaporates. For $μ_I$ just above $m_π$ the transition seems to be second order, while for larger $μ_I$ it appears to become first order. At zero temperature, 2-colour QCD also possesses a superfluid state with a diquark condensate. We study its spectrum of Goldstone and pseudo-Goldstone bosons associated with chiral and quark-number symmetry breaking.

hep-lat↗
arXiv2003

Exact constraints on the QCD Phase Diagram

We derive exact constraints relating QCD at nonzero baryon chemical potential and temperature to QCD at nonzero isospin chemical potential and temperature, a theory which can be simulated by conventional methods. These results challenge the consistency of dynamical models of superconductivity and superfluidity in QCD.

hep-ph↗
arXiv2003

The pseudo-Goldstone spectrum of 2-colour QCD at finite density

We examine the spectrum of 2-colour lattice QCD with 4 continuum flavours at a finite chemical potential ($μ$) for quark-number, on a $12^3 \times 24$ lattice. First we present evidence that the system undergoes a transition to a state with a diquark condensate, which spontaneously breaks quark number at $μ=m_π/2$, and that this transition is mean field in nature. We then examine the 3 states that would be Goldstone bosons at $μ=0$ for zero Dirac and Majorana quark masses. The predictions of chiral effective Lagrangians give a good description of the behaviour of these masses for $μ< m_π/2$. Except for the heaviest of these states, these predictions diverge from our measurements, once $μ$ is significantly greater than $m_π/2$. However, the qualitative behaviour of these masses, indicates that the physics is very similar to that predicted by these effective Lagrangians, and there is some indication that at least part of these discrepancies is due to saturation, a lattice artifact.

hep-lat↗
arXiv2003

Isospin Chemical Potential and the QCD Phase Diagram at Nonzero Temperature and Baryon Chemical Potential

We use the Nambu--Jona-Lasinio model to study the effects of the isospin chemical potential on the QCD phase diagram at nonzero temperature and baryon chemical potential. We find that the phase diagram is qualitatively altered by a small isospin chemical potential. There are two first order phase transitions that end in two critical endpoints, and there are two crossovers at low baryon chemical potential. These results have important consequences for systems where both baryon and isospin chemical potentials are nonzero, such as heavy ion collision experiments. Our results are in complete agreement with those recently obtained in a Random Matrix Model.

hep-ph↗
arXiv2003

The QCD Phase Diagram at Nonzero Temperature, Baryon and Isospin Chemical Potentials in Random Matrix Theory

We introduce a random matrix model with the symmetries of QCD at finite temperature and chemical potentials for baryon number and isospin. We analyze the phase diagram of this model in the chemical potential plane for different temperatures and quark masses. We find a rich phase structure with five different phases separated by both first and second order lines. The phases are characterized by the pion condensate and the chiral condensate for each of the flavors. In agreement with lattice simulations, we find that in the phase with zero pion condensate the critical temperature depends in the same way on the baryon number chemical potential and on the isospin chemical potential. At nonzero quark mass, we remarkably find that the critical end point at nonzero temperature and baryon chemical potential is split in two by an arbitrarily small isospin chemical potential. As a consequence, there are two crossovers that separate the hadronic phase from the quark-gluon plasma phase at high temperature. Detailed analytical results are obtained at zero temperature and in the chiral limit.

hep-ph↗
arXiv2002

Dirac Spectra and Real QCD at Nonzero Chemical Potential

We show that QCD Dirac spectra well below Lambda_{QCD}, both at zero and at nonzero chemical potential, can be obtained from a chiral Lagrangian. At nonzero chemical potential Goldstone bosons with nonzero baryon number condense beyond a critical value. Such superfluid phase transition is likely to occur in any system with a chemical potential with the quantum numbers of the Goldstone bosons. We discuss the phase diagram for one such system, QCD with two colors, and show the existence of a tricritical point in an effective potential approach.

hep-th↗
arXiv2002

Thermodynamics of chiral symmetry at low densities

The phase diagram of two-color QCD as a function of temperature and baryon chemical potential is considered. Using a low-energy chiral Lagrangian based on the symmetries of the microscopic theory, we determine, at the one-loop level, the temperature dependence of the critical chemical potential for diquark condensation and the temperature dependence of the diquark condensate and baryon density. The prediction for the temperature dependence of the critical chemical potential is consistent with the one obtained for a dilute Bose gas. The associated phase transition is shown to be of second order for low temperatures and first order at higher temperatures. The tricritical point at which the second order phase transition ends is determined. The results are carried over to QCD with quarks in the adjoint representation and to ordinary QCD at a non-zero chemical potential for isospin.

hep-ph↗