68 research outputs found
Instanton-Induced Interactions in Finite Density QCD
We consider the finite density, zero-temperature behaviour of quark matter in
the instanton picture. Since the instanton-induced interactions are attractive
in both and channels, a competition ensues between phases of
matter with condensation in either or both. It results in chiral symmetry
restoration due to the onset of diquark condensation, a `colour
superconductor', at finite density.Comment: 4 pages, 5 figures, uses espcrc1.sty. To appear in Proceedings of
Quark Matter 99 (10-14 May 1999, Torino, Italy) and PANIC 99 (10-16 June
1999, Uppsala, Sweden
Self-consistent parametrization of the two-flavor isotropic color-superconducting ground state
Lack of Lorentz invariance of QCD at finite quark chemical potential in
general implies the need of Lorentz non-invariant condensates for the
self-consistent description of the color-superconducting ground state.
Moreover, the spontaneous breakdown of color SU(3) in this state naturally
leads to the existence of SU(3) non-invariant non-superconducting expectation
values. We illustrate these observations by analyzing the properties of an
effective 2-flavor Nambu-Jona-Lasinio type Lagrangian and discuss the
possibility of color-superconducting states with effectively gapless fermionic
excitations. It turns out that the effect of condensates so far neglected can
yield new interesting phenomena.Comment: 16 pages, 3 figure
The Nonperturbative Color Meissner Effect in a Two-Flavor Color Superconductor
Color superconductivity in QCD breaks the SU(3) color gauge group down to
SU(2), inducing masses in five of the eight gluons. This is a dynamical Higgs
effect, in which the diquark condensate acts as the vacuum expectation value of
a composite scalar field. In order to analyze this effect at low quark density,
when gaps are large and generated nonperturbatively, we use instanton-induced
quark interactions augmented with gauge-invariant interactions between quarks
and perturbative gluons. The five gluon masses are found from the static limit
of the relevant polarization operators, in which transversality is maintained
via the Nambu-Goldstone modes of broken color symmetry. Working in the
microscopic theory we calculate these masses to one-loop order and estimate
their density dependence. Finally, we speculate that the Meissner effect may
postpone the onset of color superconductivity to higher matter density than
estimated previously.Comment: 27 pages, 7 figures, uses epsf.sty; typos corrected in Eqs. (38
Photon and dilepton emission rates from high density quark matter
We compute the rates of real and virtual photon (dilepton) emission from
dense QCD matter in the color-flavor locked (CFL) phase, focusing on results at
moderate densities (3-5 times the nuclear saturation density) and temperatures
MeV. We pursue two approaches to evaluate the electromagnetic
(e.m.) response of the CFL ground state: (i) a direct evaluation of the photon
self energy using quark particle/-hole degrees of freedom, and (ii) a Hidden
Local Symmetry (HLS) framework based on generalized mesonic excitations where
the meson is introduced as a gauge boson of a local SU(3) color-flavor
group. The coupling to generalized two-pion states induces a finite
width and allows to address the issue of vector meson dominance (VMD) in the
CFL phase. We compare the calculated emissivities (dilepton rates) to those
arising from standard hadronic approaches including in-medium effects. For
rather large superconducting gaps (several tens of MeV at moderate densities),
as suggested by both perturbative and nonperturbative estimates, the dilepton
rates from CFL quark matter turn out to be very similar to those obtained in
hadronic many-body calculations, especially for invariant masses above
GeV. A similar observation holds for (real) photon production.Comment: 18 pages, 12 figure
Debye screening and Meissner effect in a two-flavor color superconductor
I compute the gluon self-energy in a color superconductor with two flavors of
massless quarks, where condensation of Cooper pairs breaks SU(3)_c to SU(2)_c.
At zero temperature, there is neither Debye screening nor a Meissner effect for
the three gluons of the unbroken SU(2)_c subgroup. The remaining five gluons
attain an electric as well as a magnetic mass. For temperatures approaching the
critical temperature for the onset of color superconductivity, or for gluon
momenta much larger than the color-superconducting gap, the self-energy assumes
the form given by the standard hard-dense loop approximation. The gluon
self-energy determines the coefficient of the kinetic term in the effective
low-energy theory for the condensate fields.Comment: 29 pages, RevTe
Color-Neutral Superconducting Quark Matter
We investigate the consequences of enforcing local color neutrality on the
color superconducting phases of quark matter by utilizing the
Nambu-Jona-Lasinio model supplemented by diquark and the t'Hooft six-fermion
interactions. In neutrino free matter at zero temperature, color neutrality
guarantees that the number densities of u, d, and s quarks in the
Color-Flavor-Locked (CFL) phase will be equal even with physical current quark
masses. Electric charge neutrality follows as a consequence and without the
presence of electrons. In contrast, electric charge neutrality in the less
symmetric 2-flavor superconducting (2SC) phase with ud pairing requires more
electrons than the normal quark phase. The free energy density cost of
enforcing color and electric charge neutrality in the CFL phase is lower than
that in the 2SC phase, which favors the formation of the CFL phase. With
increasing temperature and neutrino content, an unlocking transition occurs
from the CFL phase to the 2SC phase with the order of the transition depending
on the temperature, the quark and lepton number chemical potentials. The
astrophysical implications of this rich structure in the phase diagram,
including estimates of the effects from Goldstone bosons in the CFL phase, are
discussed.Comment: 20 pages, 4 figures; version to appear in Phys. Rev.
Massive quark propagator and competition between chiral and diquark condensate
The Green-function approach has been extended to the moderate baryon density
region in the framework of an extended Nambu--Jona-Lasinio model, and the
thermodynamic potential with both chiral and diquark condensates has been
evaluated by using the massive quark propagator. The phase structure along the
chemical potential direction has been investigated and the strong competition
between the chiral and diquark condensate has been analyzed by investigating
the influence of the diquark condensate on the sharp Fermi surface. The
influence of the diquark condensate on the quark properties has been
investigated, even though the quarks in the color breaking phase are very
different from that in the chiral breaking phase, the difference between quarks
in different colors is very small.Comment: Revtex, 34 pages, 7 figures, section V revised, accepted by PR
Longitudinal gluons and Nambu-Goldstone bosons in a two-flavor color superconductor
In a two-flavor color superconductor, the SU(3)_c gauge symmetry is
spontaneously broken by diquark condensation. The Nambu-Goldstone excitations
of the diquark condensate mix with the gluons associated with the broken
generators of the original gauge group. It is shown how one can decouple these
modes with a particular choice of 't Hooft gauge. We then explicitly compute
the spectral density for transverse and longitudinal gluons of adjoint color 8.
The Nambu-Goldstone excitations give rise to a singularity in the real part of
the longitudinal gluon self-energy. This leads to a vanishing gluon spectral
density for energies and momenta located on the dispersion branch of the
Nambu-Goldstone excitations.Comment: 16 pages, 4 figures, minor revisions to text, one ref. adde
Spectral Function of Quarks in Quark Matter
We investigate the spectral function of light quarks in infinite quark matter
using a simple, albeit self-consistent model. The interactions between the
quarks are described by the SU(2) Nambu--Jona-Lasinio model. Currently mean
field effects are neglected and all calculations are performed in the chirally
restored phase at zero temperature. Relations between correlation functions and
collision rates are used to calculate the spectral function in an iterative
process.Comment: final version, published in PRC; 15 pages, RevTeX
Fluctuation modes in color-superconductors
We investigate fluctuation effects of a gap parameter in
color-superconductors. The fluctuation modes in the super phase are described
by two scalar fields of diquarks. One of them is a Nambu-Goldstone boson and
the other is a diquark boson whose mass is about twice of the gap energy (an
extended quasi-supersymmetry). In the normal phase the fluctuation becomes a
precursory (soft) mode whose amplitude increases near the critical temperature.Comment: 6 page
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