20,477 research outputs found

### Improved lattice QCD with quarks: the 2 dimensional case

QCD in two dimensions is investigated using the improved fermionic lattice
Hamiltonian proposed by Luo, Chen, Xu, and Jiang. We show that the improved
theory leads to a significant reduction of the finite lattice spacing errors.
The quark condensate and the mass of lightest quark and anti-quark bound state
in the strong coupling phase (different from t'Hooft phase) are computed. We
find agreement between our results and the analytical ones in the continuum.Comment: LaTeX file (including text + 10 figures

### Phase Structure of Compact $QED_3$ with Massless Fermions

In the framework of (2+1)-dimensional compact lattice QED with light
fermions, we investigate the phase diagram in the $(\beta, N)$ plane. The
approximations involved are related to an expansion of the effective fermionic
action as a power series of the flavor number $N$. We also develop a new
mechanism for understanding the $N-$critical phenomenon in the full theory. Our
results for the specific heat indicate that only one phase does exist. We give
strong evidences that this qualitative result should not be changed with the
inclusion of higher order terms in the $N$ expansion.Comment: 10 pages and two figures; DFTUZ 92.2

### Exotic mesons from quantum chromodynamics with improved gluon and quark actions on the anisotropic lattice

Hybrid (exotic) mesons, which are important predictions of quantum
chromodynamics (QCD), are states of quarks and anti-quarks bound by excited
gluons. First principle lattice study of such states would help us understand
the role of ``dynamical'' color in low energy QCD and provide valuable
information for experimental search for these new particles. In this paper, we
apply both improved gluon and quark actions to the hybrid mesons, which might
be much more efficient than the previous works in reducing lattice spacing
error and finite volume effect. Quenched simulations were done at $\beta=2.6$
and on a $\xi=3$ anisotropic $12^3\times36$ lattice using our PC cluster. We
obtain $2013 \pm 26 \pm 71$ MeV for the mass of the $1^{-+}$ hybrid meson
${\bar q}qg$ in the light quark sector, and $4369 \pm 37 \pm 99$Mev in the
charm quark sector; the mass splitting between the $1^{-+}$ hybrid meson ${\bar
c}c g$ in the charm quark sector and the spin averaged S-wave charmonium mass
is estimated to be $1302 \pm 37 \pm 99$ MeV. As a byproduct, we obtain $1438
\pm 32 \pm 57$ MeV for the mass of a P-wave $1^{++}$ ${\bar u}u$ or ${\bar d}d$
meson and $1499 \pm 28 \pm 65$ MeV for the mass of a P-wave $1^{++}$ ${\bar
s}s$ meson, which are comparable to their experimental value 1426 MeV for the
$f_1(1420)$ meson. The first error is statistical, and the second one is
systematical. The mixing of the hybrid meson with a four quark state is also
discussed.Comment: 12 pages, 3 figures. Published versio

### Thermodynamical quantities of lattice full QCD from an efficient method

I extend to QCD an efficient method for lattice gauge theory with dynamical
fermions. Once the eigenvalues of the Dirac operator and the density of states
of pure gluonic configurations at a set of plaquette energies (proportional to
the gauge action) are computed, thermodynamical quantities deriving from the
partition function can be obtained for arbitrary flavor number, quark masses
and wide range of coupling constants, without additional computational cost.
Results for the chiral condensate and gauge action are presented on the $10^4$
lattice at flavor number $N_f=0$, 1, 2, 3, 4 and many quark masses and coupling
constants. New results in the chiral limit for the gauge action and its
correlation with the chiral condensate, which are useful for analyzing the QCD
chiral phase structure, are also provided.Comment: Latex, 11 figures, version accepted for publicatio

### Bound States and Critical Behavior of the Yukawa Potential

We investigate the bound states of the Yukawa potential $V(r)=-\lambda
\exp(-\alpha r)/ r$, using different algorithms: solving the Schr\"odinger
equation numerically and our Monte Carlo Hamiltonian approach. There is a
critical $\alpha=\alpha_C$, above which no bound state exists. We study the
relation between $\alpha_C$ and $\lambda$ for various angular momentum quantum
number $l$, and find in atomic units, $\alpha_{C}(l)= \lambda [A_{1} \exp(-l/
B_{1})+ A_{2} \exp(-l/ B_{2})]$, with $A_1=1.020(18)$, $B_1=0.443(14)$,
$A_2=0.170(17)$, and $B_2=2.490(180)$.Comment: 15 pages, 12 figures, 5 tables. Version to appear in Sciences in
China

### Comment on "General nonlocality in quantum fields"

In this paper, we first incorporate the weak interaction into the theory of
General Nonlocality by finding a appropriate metric for it. Accordingly, we
suggest the theoretical frame of General Nonlocality as the candidate theory of
unifying three microscope interactions in low energy limit. In this unifying
scenario, the essential role of photon field is stressed.Comment: Only partial content published in the following reference. The part
asserting the fermion mass problem now proved to be wrong, though remains in
the versio

### Comment on "Time-Dependent Density-Matrix Renormalization Group: A Systematic Method for the Study of Quantum Many-Body Out-of- Equilibrium Systems"

In a recent Letter [Phys. Rev. Lett. 88, 256403(2002), cond-mat/0109158]
Cazalilla and Marston proposed a time-dependent density- matrix renormalization
group (TdDMRG) algorithm for the accurate evaluation of out-of-equilibrium
properties of quantum many-body systems. For a point contact junction between
two Luttinger liquids, a current oscillation develops after initial transient
in the insulating regime. Here we would like to point out that (a) the observed
oscillation is an artifact of the method; (b) the TdDMRG can be significantly
improved by incorporating the non-equilibrium evolution of the goundstate into
the density matrix.Comment: 1 page, 2 figure

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