186 research outputs found
Phase Structure of Lattice QCD at Finite Density with Dynamical Fermions
We compare the chemical potential associated with the onset of non-zero
baryon number density on and lattices at and ma=0.01.
We provide evidence for tunnelling. We determine a critical chemical
potential of which is unexpectedly low. We also determine
the dependence of the onset of the observed phase transition on the quark mass.
The physically misleading result of the quenched theory is shown to persist
despite the inclusion of the complex fermion determinant.Comment: 3 pages, Latex, 5 postscript figures, Talk presented at
LATTICE96(finite temperature
Lattice Gauge Theory Simulations at Nonzero Chemical Potential in the Chiral Limit
We present a method of simulating lattice QCD at nonzero chemical potential
in the chiral limit. By adding a weak four-fermi interaction to the standard
staggered fermion SU(3) QCD action, we produce an algorithm in which the limit
of massless fermions is well-behaved and physical. Using configurations at zero
chemical potential, and an exact fugacity expansion of the fermion determinant,
we can simulate QCD at nonzero chemical potential and evade the notorious
problem of the complex action. Small lattice simulations give physical results:
At strong gauge coupling the critical chemical potential \mu_c agrees with
theoretical expectations and at weak gauge coupling \mu_c is nonzero in the low
temperature confined phase of QCD and jumps to zero in the high temperature
quark-gluon plasma phase. In all these simulations the quarks are exactly
massless and there is a Goldstone pion.Comment: contains .tex file of text and three figures as .epsi file
Shape Space Methods for Quantum Cosmological Triangleland
With toy modelling of conceptual aspects of quantum cosmology and the problem
of time in quantum gravity in mind, I study the classical and quantum dynamics
of the pure-shape (i.e. scale-free) triangle formed by 3 particles in 2-d. I do
so by importing techniques to the triangle model from the corresponding 4
particles in 1-d model, using the fact that both have 2-spheres for shape
spaces, though the latter has a trivial realization whilst the former has a
more involved Hopf (or Dragt) type realization. I furthermore interpret the
ensuing Dragt-type coordinates as shape quantities: a measure of
anisoscelesness, the ellipticity of the base and apex's moments of inertia, and
a quantity proportional to the area of the triangle. I promote these quantities
at the quantum level to operators whose expectation and spread are then useful
in understanding the quantum states of the system. Additionally, I tessellate
the 2-sphere by its physical interpretation as the shape space of triangles,
and then use this as a back-cloth from which to read off the interpretation of
dynamical trajectories, potentials and wavefunctions. I include applications to
timeless approaches to the problem of time and to the role of uniform states in
quantum cosmological modelling.Comment: A shorter version, as per the first stage in the refereeing process,
and containing some new reference
Remarks on the issue of time and complex numbers in canonical quantum gravity
We develop the idea that, as a result of the arbitrariness of the factor
ordering in Wheeler-DeWitt equation, gauge phases can not, in general, being
completely removed from the wave functional in quantum gravity. The latter may
be conveniently described by means of a remnant complex term in WDW equation
depending of the factor ordering. Taking this equation for granted we can
obtain WKB complex solutions and, therefore, we should be able to derive a
semiclassical time parameter for the Schroedinger equation corresponding to
matter fields in a given classical curved space.Comment: Typewritten using RevTex, to appear in Phys. Rev.
Finite Density Fat QCD
Lattice formulation of Finite Baryon Density QCD is problematic from computer
simulation point of view; it is well known that for light quark masses the
reconstructed partition function fails to be positive in a wide region of
parameter space. For large bare quark masses, instead, it is possible to obtain
more sensible results; problems are still present but restricted to a small
region. We present evidence for a saturation transition independent from the
gauge coupling and for a transition line that, starting from the
temperature critical point at , moves towards smaller with
increasing as expected from simplified phenomenological arguments.Comment: 14 pages, 10 figure
Quenched QCD at finite density: and
We report on our ongoing effort to understand quenched lattice QCD at finite
baryon number density. The quenched theory is sensitive to the baryon mass both
at strong coupling and in the scaling region. However, we find that the
quenched model is pathological for at , in
agreement with past Lanczos analyses of the Dirac operator.Comment: Contribution to Lat94, 3 pages, tar-compressed uuencoded ps fil
QCD with chiral 4-fermion interactions (QCD)
Lattice QCD with staggered quarks is augmented by the addition of a chiral
4-fermion interaction. The Dirac operator is now non-singular at ,
decreasing the computing requirements for light quark simulations by at least
an order of magnitude. We present preliminary results from simulations at
finite and zero temperatures for , with and without gauge fields.Comment: 3 pages. uuencoded, gzipped, tared LateX with 2 encapsulated
postscript figures. Uses epscrc2.sty. Talk presented at LATTICE96(chirality
in qcd). Title changed; minor changes at beginning and end of paper and
reference
Arrow of time in a recollapsing quantum universe
We show that the Wheeler-DeWitt equation with a consistent boundary condition
is only compatible with an arrow of time that formally reverses in a
recollapsing universe. Consistency of these opposite arrows is facilitated by
quantum effects in the region of the classical turning point. Since
gravitational time dilation diverges at horizons, collapsing matter must then
start re-expanding ``anticausally" (controlled by the reversed arrow) before
horizons or singularities can form. We also discuss the meaning of the
time-asymmetric expression used in the definition of ``consistent histories".
We finally emphasize that there is no mass inflation nor any information loss
paradox in this scenario.Comment: Many conceptual clarifications include
The Four-Fermi Model in Three Dimensions at Non-Zero Density and Temperature
The Four Fermi model with discrete chiral symmetry is studied in three
dimensions at non-zero chemical potential and temperature using the Hybrid
Monte Carlo algorithm. The number of fermion flavors is chosen large
to compare with analytic results. A first order chiral symmetry restoring
transition is found at zero temperature with a critical chemical potential
in good agreement with the large calculations. The critical index
of the correlation length is measured in good agreement with analytic
calculations. The two dimensional phase diagram (chemical potential vs.
temperature) is mapped out quantitatively. Finite size effects on relatively
small lattices and non-zero fermion mass effects are seen to smooth out the
chiral transition dramatically.Comment: 21 pages, sorry, no figure
The QCD thermal phase transition in the presence of a small chemical potential
We propose a new method to investigate the thermal properties of QCD with a
small quark chemical potential . Derivatives of the phase transition point
with respect to are computed at for 2 flavors of p-4 improved
staggered fermions with on a lattice. The resulting
Taylor expansion is well behaved for the small values of relevant for RHIC phenomenology, and predicts a critical curve
in reasonable agreement with estimates obtained using exact
reweighting. In addition, we contrast the case of isoscalar and isovector
chemical potentials, quantify the effect of on the equation of
state, and comment on the complex phase of the fermion determinant in QCD with
.Comment: 26 pages, 25 figures, minor modificatio
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