10,809 research outputs found

### Effective Gap Equation for the Inhomogeneous LOFF Superconductive Phase

We present an approximate gap equation for different crystalline structures
of the LOFF phase of high density QCD at T=0. This equation is derived by using
an effective condensate term obtained by averaging the inhomogeneous condensate
over distances of the order of the crystal lattice size. The approximation is
expected to work better far off any second order phase transition. As a
function of the difference of the chemical potentials of the up and down
quarks, $\delta\mu$, we get that the octahedron is energetically favored from
$\delta\mu=\Delta_0/\sqrt 2$ to $0.95\Delta_0$, where $\Delta_0$ is the gap for
the homogeneous phase, while in the range $0.95\Delta_0-1.32\Delta_0$ the face
centered cube prevails. At $\delta\mu=1.32\Delta_0$ a first order phase
transition to the normal phase occurs.Comment: 11 pages, 5 figure

### Do Instantons and Strings Cluster when the Number of Colors is Large?

We consider the $N_c\to\infty$ limit of QCD using a toy model in which
instantons exchange color-singlet scalar fields which do not self-interact. Our
main observation is that collective attraction leads the formation of large
clusters containing $O(N_c)$ nonperturbative objects. We further show that this
clustering of instantons is limited due to a non-trivial repulsion inherent in
the ADHM multi-instanton solution. As a result the vacuum is very different
from that at low $N_c$, notably being more inhomogeneous, in ways which will
affect chiral symmetry breaking of light quarks. We also briefly discuss a
similar phenomenon for color strings in baryons made of medium-mass
(charm-like) quarks.Comment: 10 pages, 1 figure, uses epsf.st

### Chiral Loops and Ghost States in the Quenched Scalar Propagator

The scalar, isovector meson propagator is analyzed in quenched QCD, using the
MQA pole-shifting ansatz to study the chiral limit. In addition to the expected
short-range exponential falloff characteristic of a heavy scalar meson, the
propagator also exhibits a longer-range, negative metric contribution which
becomes pronounced for smaller quark masses. We show that this is a quenched
chiral loop effect associated with the anomalous structure of the $\eta '$
propagator in quenched QCD. Both the time dependence and the quark mass
dependence of this effect are well-described by a chiral loop diagram
corresponding to an $\eta '- \pi$ intermediate state, which is light and
effectively of negative norm in the quenched approximation. The relevant
parameters of the effective Lagrangian describing the scalar sector of the
quenched theory are determined.Comment: 29 pages, 10 figures, Late

### Revolution and journalism higher education in the Middle East/North Africa region

2012 Summer.Includes bibliographical references.The disruptions brought by the Arab Spring revolutions in the Middle East/North Africa (MENA) region in 2010-2011 created a series of personal and professional challenges for those involved in higher education in journalism in the region. This research uses narrative inquiry to examine the impact revolution had on a group of educators in the MENA region. The work reveals the impact of transportation woes, diminished student engagement, and personal considerations on a group of individuals involved in higher education in the region. The participants' work in journalism education receives additional consideration due to the restrictions on journalists prior to the Arab Spring and the uncertain space for journalists in the post-revolutionary environment

### The Ginzburg-Landau Free Energy Functional of Color Superconductivity at Weak Coupling

We derive the Ginzburg-Landau free energy functional of color
superconductivity in terms of the thermal diagrams of QCD in its perturbative
region. The zero mode of the quadratic term coefficient yields the same
transition temperature, including the pre-exponential factor, as the one
obtained previously from the Fredholm determinant of the two quark scattering
amplitude. All coefficients of the free energy can be made identical to those
of a BCS model by setting the Fermi velocity of the latter equal to the speed
of light. We also calculate the induced symmetric color condensate near $T_c$
and find that it scales as the cubic power of the dominant antisymmetric color
component. We show that in the presence of an inhomogeneity and a nonzero gauge
potential, while the color-flavor locked condensate dominates in the bulk, the
unlocked condensate, the octet, emerges as a result of a simultaneous
color-flavor rotation in the core region of a vortex filament or at the
junction of super and normal phases.Comment: 32 pages, Plain Tex, 3 figure

### Neutrino processes in the $K^0$ condensed phase of color flavor locked quark matter

We study weak interactions involving Goldstone bosons in the neutral kaon
condensed phase of color flavor locked quark matter. We calculate the rates for
the dominant processes that contribute to the neutrino mean free p ath and to
neutrino production. A light $K^+$ state, with a mass $\tilde{m}_{K^+} \propto
(\Delta/\mu) (\Delta/m_s)(m_d-m_u)$, where $\mu$ and $\Delta$ are the quark
chemical potential and superconducting gap respectively, is shown to play an
important role. We identify unique characteristics of weak interaction rates in
this novel phase and discuss how they might influence neutrino emission in core
collapse supernova and neutron stars.Comment: 21 pages, 4 figure

### Short distance current correlators: Comparing lattice simulations to the instanton liquid

Point to point correlators of currents are computed in quenched QCD using a
chiral lattice fermion action, the overlap action. I compare correlators made
of exact quark propagators with correlators restricted to low (less than 500
MeV) eigenvalue eigenmodes of the Dirac operator. In many cases they show
qualitative resemblence (typically at small values of the quark mass and
distances larger than 0.4 fm) and they differ qualitatively at larger quark
masses or at very short distance. Lattice results are in qualitative agreement
(and in the difference of vector and axial vector channels, quantitative
agreement) with the expectations of instanton liquid models. The scalar channel
shows the effects of a quenched finite volume zero mode artifact, a negative
correlator.Comment: 18 pages, Revtex, 11 postscript figures. Some changes. Version to
appear in Phys. Rev.

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