2,464 research outputs found
Relativistic BCS-BEC Crossover at Zero Temperature
We investigate the BCS-BEC crossover at zero temperature in the frame of a
relativistic model. The universality of the BCS-BEC crossover for
non-relativistic systems breaks down in relativistic case and the crossover can
be induced by changing the density. When the effective scattering length is
much less than the fermion Compton wavelength, we recover the non-relativistic
result if the gas is initially in non-relativistic state. At ultra-strong
coupling where the scattering length is of the order of the Compton wavelength,
a new BEC state appears. In this state the condensed bosons become nearly
massless and anti-fermions are excited. The behavior of the Goldstone mode and
the mixing between the amplitude and phase modes are significantly different in
different condensed regions.Comment: 8 pages, 3 figures. V2: typos corrected, a comment on mean field
theory adde
Nucleus-Nucleus Bremsstrahlung from Ultrarelativistic Collisions
The bremsstrahlung produced when heavy nuclei collide is estimated for
central collisions at the Relativistic Heavy Ion Collider. Soft photons can be
used to infer the rapidity distribution of the outgoing charge. An experimental
design is outlined.Comment: 12 pages, 7 figures, uses revte
Finite Temperature Quark Confinement
Confinement may be more easily demonstrated at finite temperature using the
Polyakov loop than at zero temperature using the Wilson loop. A natural
mechanism for confinement can arise via the coupling of the adjoint Polyakov
loop to F_{mu nu}^2. We demonstrate this mechanism with a one-loop calculation
of the effective potential for SU(2) gluons in a background field consisting of
a non-zero color magnetic field and a non-trivial Polyakov loop. The color
magnetic field drives the Polyakov loop to non-trivial behavior, and the
Polyakov loop can remove the well-known tachyonic mode associated with the
Saviddy vacuum. Minimizing the real part of the effective potential leads to
confinement, as determined by the Polyakov loop. Unfortunately, we cannot
arrange for simultaneous stability and confinement for this simple class of
field configurations. We show for a large class of abelian background fields
that at one loop tachyonic modes are necessary for confinement.Comment: 15 pages, 7 figures, RevTe
Bose-Einstein condensation in linear sigma model at Hartree and large N approximation
The BEC of charged pions is investigated in the framework of O(4) linear
sigma model. By using Cornwall-Jackiw-Tomboulis formalism, we have derived the
gap equations for the effective masses of the mesons at finite temperature and
finite isospin density. The BEC is discussed in chiral limit and non-chiral
limit at Hartree approximation and also at large N approximation.Comment: 11 pages, 9 figure
Maximum temperature for an Ideal Gas of Kac-Moody Fermions
A lagrangian for gauge fields coupled to fermions with the Kac-Moody group as
its gauge group yields, for the pure fermions sector, an ideal gas of Kac-Moody
fermions. The canonical partition function for the case is shown to
have a maximum temperature , where is the
coupling of the super charge operator to the fermions. This result is
similar to the case of strings but unlike strings the result is obtained from a
well-defined lagrangian.Comment: Needs subeqnarray.sty; To be published in Phys. Rev. D, Dec 15, 1995.
Some typographical errors have been corrected in the revised versio
Electromagnetic effects of neutrinos in an electron gas
We study the electromagnetic properties of a system that consists of an
electron background and a neutrino gas that may be moving or at rest, as a
whole, relative to the background. The photon self-energy for this system is
characterized by the usual transverse and longitudinal polarization functions,
and two additional ones which are the focus of our calculations, that give rise
to birefringence and anisotropic effects in the photon dispersion relations.
Expressions for them are obtained, which depend on the neutrino number
densities and involve momentum integrals over the electron distribution
functions, and are valid for any value of the photon momentum and general
conditions of the electron gas. Those expressions are evaluated explicitly for
several special cases and approximations which are generally useful in
astrophysical and cosmological settings. Besides studying the photon dispersion
relations, we consider the macroscopic electrodynamic equations for this
system, which involve the standard dielectric and permeability constants plus
two additional ones related to the photon self-energy functions. As an
illustration, the equations are used to discuss the evolution of a magnetic
field perturbation in such a medium. This particular phenomena has also been
considered in a recent work by Semikoz and Sokoloff as a mechanism for the
generation of large-scale magnetic fields in the Early Universe as a
consequence of the neutrino-plasma interactions, and allows us to establish
contact with a specific application in a well defined context, with a broader
scope and from a very different point of view.Comment: Revtex 20 page
Broken phase effective potential in the two-loop Phi-derivable approximation and nature of the phase transition in a scalar theory
We study the phase transition of a real scalar phi^4 theory in the two-loop
Phi-derivable approximation using the imaginary time formalism, extending our
previous (analytical) discussion of the Hartree approximation. We combine Fast
Fourier Transform algorithms and accelerated Matsubara sums in order to achieve
a high accuracy. Our results confirm and complete earlier ones obtained in the
real time formalism [1] but which were less accurate due to the integration in
Minkowski space and the discretization of the spectral density function. We
also provide a complete and explicit discussion of the renormalization of the
two-loop Phi-derivable approximation at finite temperature, both in the
symmetric and in the broken phase, which was already used in the real-time
approach, but never published. Our main result is that the two-loop
Phi-derivable approximation suffices to cure the problem of the Hartree
approximation regarding the order of the transition: the transition is of the
second order type, as expected on general grounds. The corresponding critical
exponents are, however, of the mean-field type. Using a "RG-improved" version
of the approximation, motivated by our renormalization procedure, we find that
the exponents are modified. In particular, the exponent delta, which relates
the field expectation value phi to an external field h, changes from 3 to 5,
getting then closer to its expected value 4.789, obtained from accurate
numerical estimates [2].Comment: 54 pages, 16 figure
Cold Quark Matter
We perform an O(alpha_s^2) perturbative calculation of the equation of state
of cold but dense QCD matter with two massless and one massive quark flavor,
finding that perturbation theory converges reasonably well for quark chemical
potentials above 1 GeV. Using a running coupling constant and strange quark
mass, and allowing for further non-perturbative effects, our results point to a
narrow range where absolutely stable strange quark matter may exist. Absent
stable strange quark matter, our findings suggest that quark matter in compact
star cores becomes confined to hadrons only slightly above the density of
atomic nuclei. Finally, we show that equations of state including quark matter
lead to hybrid star masses up to M~2M_solar, in agreement with current
observations. For strange stars, we find maximal masses of M~2.75M_solar and
conclude that confirmed observations of compact stars with M>2M_solar would
strongly favor the existence of stable strange quark matter.Comment: 51 pages, 11 figures, v2: minor modifications and additional
reference
Virial coefficients from 2+1 dimensional QED effective actions at finite temperature and density
From spinor and scalar 2+1 dimensional QED effective actions at finite
temperature and density in a constant magnetic field background, we calculate
the corresponding virial coefficients for particles in the lowest Landau level.
These coefficients depend on a parameter theta related to the time-component of
the gauge field, which plays an essential role for large gauge invariance. The
variation of the parameter theta might lead to an interpolation between
fermionic and bosonic virial coefficients, although these coefficients are
singular for theta=pi/2.Comment: 10 Latex pages, no figures. Version to appear in MPL
Possible Evidence of Disoriented Chiral Condensates from the Anomaly in Omega and AntiOmega Abundances at the SPS
No conventional picture of nucleus-nucleus collisions has yet been able to
explain the abundance of Omega and AntiOmega in central collisions between Pb
nuclei at 158 A GeV at the CERN SPS. We argue that such a deviation from
predictions of statistical thermal models and numerical simulations is evidence
that they are produced as topological defects in the form of skyrmions arising
from the formation of disoriented chiral condensates. The estimated domain size
falls in the right range to be consistent with the so far non-observation of
DCC from the distribution of neutral pions.Comment: paper presented at the ICPAQGP-2001, Jaipur, Indi
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