3,361 research outputs found

### Monopole Percolation and The Universality Class of the Chiral Transition in Four Flavor Noncompact Lattice QED

We simulate four flavor noncompact lattice QED using the Hybrid Monte Carlo
algorithm on $10^4$ and $16^4$ lattices. Measurements of the monopole
susceptibility and the percolation order parameter indicate a transition at
$\beta = {1/e^2} = .205(5)$ with critical behavior in the universality class of
four dimensional percolation. We present accurate chiral condensate
measurements and monitor finite size effects carefully. The chiral condensate
data supports the existence of a power-law transition at $\beta = .205$ in the
same universality class as the chiral transition in the two flavor model. The
resulting equation of state predicts the mass ratio $m_\pi^2/m_\sigma^2$ in
good agreement with spectrum calculations while the hypothesis of a
logarithmically improved mean field theory fails qualitatively.Comment: 17 pages, 10 figure

### Phase diagram of the three-dimensional NJL model

With the exception of confinement the three-dimensional Nambu-Jona-Lasinio
(NJL_3) model incorporates many of the essential properties of QCD. We discuss
the critical properties of the model at nonzero temperature T and/or nonzero
chemical potential $\mu$. We show that the universality class of the thermal
transition is that of the d=2 classical spin model with the same symmetry. We
provide evidence for the existence of a tricritical point in the
($\mu$,temperature) plane. We also discuss numerical results by Hands et al.
which showed that the system is critical for $\mu>\mu_c$ and the diquark
condensate is zero.Comment: 3 pages, To appear in the proceedings of Conference on Quarks and
Nuclear Physics (QNP 2002), Julich, Germany, 9-14 Jun 200

### Improving the Lattice QED Action

Strongly coupled QED is a model whose physics is dominated by short-ranged
effects. In order to assess which features of numerical simulations of the
chiral phase transition are universal and which are not, we have formulated a
quenched version of the model in which photon degrees of freedom are defined on
a lattice of spacing a, but fermions only on a lattice of spacing 2a. The
fermi-photon interaction is then obtained via a blocking procedure, whose
parameters allow a degree of control over the relative importance of short
wavelength modes. Results from a variety of models are presented; the critical
exponents delta and beta governing the transition appear to be independent of
the blocking, or even of whether a gauge-invariant action is used for the
photons.Comment: 3 pages LaTeX submission to Lat' 94 proceedings, 3 PostScript figures
incorporated using macro psfi

### Evidence for BCS Diquark Condensation in the 3+1d Lattice NJL Model

We present results of numerical simulations of the 3+1d Nambu - Jona-Lasinio
model with a non-zero baryon chemical potential mu, with particular emphasis on
the superfluid diquark condensate and associated susceptibilities. The results,
when extrapolated to the zero diquark source limit, are consistent with the
existence of a non-zero BCS condensate at high baryon density. The nature of
the infinite volume and zero temperature limits are discussed.Comment: 12 pages, 5 figure

### Can we study Quark Matter in the Quenched Approximation?

We study a quenched SU(2) lattice gauge theory in which, in an attempt to
distinguish between timelike and spacelike gauge fields, the gauge ensemble
{U_mu} is generated from a 3 dimensional gauge-Higgs model, the timelike link
variables being "reconstructed" from the Higgs fields. The resulting ensemble
is used to study quenched quark propagation with non-zero chemical potential
mu; in particular, the quark density, chiral and superfluid condensates, meson,
baryon and gauge-fixed quark propagators are all studied as functions of mu.
While it proves possible to alter the strength of the inter-quark interaction
by changing the parameters of the dimensionally reduced model, there is no
evidence for any region of parameter space where quarks exhibit deconfined
behaviour or thermodynamic observables scale as if there were a Fermi surface.Comment: 31 pages, 20 figures, enhanced discussion and analysis of finite
volume effects, version accepted for publication by Nucl. Phys.

### Supercurrent Flow in NJL_{2+1} at High Baryon Density

We present results of numerical simulations of the 2+1d Nambu -- Jona-Lasinio
model with non-zero baryon chemical potential mu and spatially-varying complex
diquark source strength j. By choosing arg(j) to vary smoothly through 2 pi
across the spatial extent of the lattice, a baryon number current is induced
which in the high density phase remains non-vanishing as |j|->0; we are hence
able to extract a quantity characteristic of a superfluid known as the helicity
modulus. We also study supercurrent flow at non-zero temperature and estimate
the critical temperature at which the normal phase is restored, which is
consistent with the conventional picture for thin-film superfluids in which the
transition is viewed in terms of vortex -- anti-vortex unbinding.Comment: 10 pages, 5 figure

### Hadron Spectrum in a Two-Colour Baryon-Rich Medium

The hadron spectrum of SU(2) lattice gauge theory with two flavours of Wilson
quark is studied on an 8^3x16 lattice using all-to-all propagators, with
particular emphasis on the dependence on quark chemical potential mu. As mu is
increased from zero the diquark states with non-zero baryon number B respond as
expected, while states with B=0 remain unaffected until the onset of non-zero
baryon density at mu=m_pi/2. Post onset the pi-meson mass increases in
accordance with chiral perturbation theory while the rho becomes lighter. In
the diquark sector a Goldstone state associated with a superfluid ground state
can be identified. A further consequence of superfluidity is an approximate
degeneracy between mesons and baryons with the same spacetime and isospin
quantum numbers. Finally we find tentative evidence for the binding of states
with kaon quantum numbers within the baryonic medium.Comment: 14 pages, 5 figure

### Magnetic Monopoles in non-compact QED - is there a Phase Transition?

The existence of the monopole condensation transition reported by
Kocic et al. in non-compact, quenched QED is tested. No phase transition is
found. This shows that divergence of the `monopole susceptibility' introduced
by Hands and Wensley is not a reliable indicator of second order phase
transitions. In view of these results
I discuss claims that the chiral phase transition seen in QED with fermions
is a lattice artefact driven by monopole condensation.Comment: Talk given at lattice '92, 4 pages, uses espcrc2.sty, 2 postscript
figures attached. (Replaced to add the figures.) Preprint FUB-HEP 22/9

### A BCS Gap on the Lattice

Monte Carlo simulations of the 3+1 dimensional NJL model are performed with
baryon chemical potential mu>0. For mu>Sigma_0, the constituent quark mass in
vacuum, chiral symmetry is restored and a diquark condensate forms. We
analyse the fermion propagator and find evidence for particle-hole mixing in
the vicinity of the Fermi surface and an energy gap Delta>0, both of which
provide evidence for superfluidity at high baryon density induced by a BCS
mechanism.
At (mu a)=0.8 the ratio between the BCS gap and the vacuum quark mass is
Delta/Sigma_0=0.15(2).Comment: 3 pages, 3 figures, Lattice2003(nonzero

### The Universlity Class of Monopole Condensation in Non-Compact, Quenched Lattice QED

Finite size scaling studies of monopole condensation in noncompact quenched
lattice $QED$ indicate an authentic second order phase transition lying in the
universality class of four dimensional percolation. Since the upper critical
dimension of percolation is six, the measured critical indices are far from
mean-field values. We propose a simple set of ratios as the exact critical
indices for this transition. The implication of these results for critical
points in Abelian gauge theories are discussed.Comment: ILL-(TH)-92-6, CERN-TH.6515/92, 10 pages, no figures available as PS
fil

- …