546 research outputs found
The Weakly Coupled Gross-Neveu Model with Wilson Fermions
The nature of the phase transition in the lattice Gross-Neveu model with
Wilson fermions is investigated using a new analytical technique. This involves
a new type of weak coupling expansion which focuses on the partition function
zeroes of the model. Its application to the single flavour Gross-Neveu model
yields a phase diagram whose structure is consistent with that predicted from a
saddle point approach. The existence of an Aoki phase is confirmed and its
width in the weakly coupled region is determined. Parity, rather than chiral
symmetry breaking naturally emerges as the driving mechanism for the phase
transition.Comment: 15 pages including 1 figur
A numerical reinvestigation of the Aoki phase with N_f=2 Wilson fermions at zero temperature
We report on a numerical reinvestigation of the Aoki phase in lattice QCD
with two flavors of Wilson fermions where the parity-flavor symmetry is
spontaneously broken. For this purpose an explicitly symmetry-breaking source
term was added to the fermion action.
The order parameter was computed with
the Hybrid Monte Carlo algorithm at several values of on
lattices of sizes to and extrapolated to . The existence of a
parity-flavor breaking phase can be confirmed at and 4.3, while we
do not find parity-flavor breaking at and 5.0.Comment: 8 pages, 5 figures, Revised version as to be published in Phys.Rev.
Fermion-scalar interactions with domain wall fermions
Domain wall fermions are defined on a lattice with an extra direction the
size of which controls the chiral properties of the theory. When gauge fields
are coupled to domain wall fermions the extra direction is treated as an
internal flavor space. Here it is found that this is not the case for scalar
fields. Instead, the interaction takes place only along the link that connects
the boundaries of the extra direction. This reveals a richness in the way
different spin particles are coupled to domain wall fermions. As an
application, 4-Fermi models are studied using large N techniques and the
results are supported by numerical simulations with N=2. It is found that the
chiral properties of domain wall fermions in these models are good across a
large range of couplings and that a phase with parity-flavor broken symmetry
can develop for negative bare masses if the number of sites along the extra
direction is finite.Comment: LaTeX, 17 pages, 8 eps figures; comment regarding the width of Aoki
phase added in sec. 3; references adde
Comparative Study of full QCD Hadron Spectrum and Static Quark Potential with Improved Actions
We investigate effects of action improvement on the light hadron spectrum and
the static quark potential in two-flavor QCD for GeV and
. We compare a renormalization group improved action with
the plaquette action for gluons, and the SW-clover action with the Wilson
action for quarks. We find a significant improvement in the hadron spectrum by
improving the quark action, while the gluon improvement is crucial for a
rotationally invariant static potential. We also explore the region of light
quark masses corresponding to on a 2.7 fm lattice using
the improved gauge and quark action. A flattening of the potential is not
observed up to 2 fm.Comment: LaTeX, 35 pages, 22 eps figures, uses revtex and eps
Surface Instability in Windblown Sand
We investigate the formation of ripples on the surface of windblown sand
based on the one-dimensional model of Nishimori and Ouchi [Phys. Rev. Lett. 71,
197 (1993)], which contains the processes of saltation and grain relaxation. We
carry out a nonlinear analysis to determine the propagation speed of the
restabilized ripple patterns, and the amplitudes and phases of their first,
second, and third harmonics. The agreement between the theory and our numerical
simulations is excellent near the onset of instability. We also determine the
Eckhaus boundary, outside which the steady ripple patterns are unstable.Comment: 23 pages, 8 figure
Convective Motion in a Vibrated Granular Layer
Experimental results are presented for a vertically shaken granular layer. In
the range of accelerations explored, the layer develops a convective motion in
the form of one or more rolls. The velocity of the grains near the wall has
been measured. It grows linearly with the acceleration, then the growing rate
slows down. A rescaling with the amplitude of the wall velocity and the height
of the granular layer makes all data collapse in a single curve. This can
provide insights on the mechanism driving the motion.Comment: 10 pages, 5 figures submitted to Phys. Rev. Let
Fictitious Magnetic Resonance by Quasi-Electrostatic Field
We propose a new kind of spin manipulation method using a {\it fictitious}
magnetic field generated by a quasi-electrostatic field. The method can be
applicable to every atom with electron spins and has distinct advantages of
small photon scattering rate and local addressability. By using a
laser as a quasi-electrostatic field, we have experimentally demonstrated the
proposed method by observing the Rabi-oscillation of the ground state hyperfine
spin F=1 of the cold atoms and the Bose-Einstein condensate.Comment: 5 pages, 5 figure
Parallel tempering in full QCD with Wilson fermions
We study the performance of QCD simulations with dynamical Wilson fermions by
combining the Hybrid Monte Carlo algorithm with parallel tempering on
and lattices. In order to compare tempered with standard simulations,
covariance matrices between sub-ensembles have to be formulated and evaluated
using the general properties of autocorrelations of the parallel tempering
algorithm. We find that rendering the hopping parameter dynamical does
not lead to an essential improvement. We point out possible reasons for this
observation and discuss more suitable ways of applying parallel tempering to
QCD.Comment: 16 pages, 3 figure
Chirally improving Wilson fermions - I. O(a) improvement
We show that it is possible to improve the chiral behaviour and the approach
to the continuum limit of correlation functions in lattice QCD with Wilson
fermions by taking arithmetic averages of correlators computed in theories
regularized with Wilson terms of opposite sign. Improved hadronic masses and
matrix elements can be obtained by similarly averaging the corresponding
physical quantities separately computed within the two regularizations. To deal
with the problems related to the spectrum of the Wilson--Dirac operator, which
are particularly worrisome when Wilson and mass terms are such as to give
contributions of opposite sign to the real part of the eigenvalues, we propose
to use twisted-mass lattice QCD for the actual computation of the quantities
taking part to the averages. The choice for the twisting angle is
particularly interesting, as O() improved estimates of physical quantities
can be obtained even without averaging data from lattice formulations with
opposite Wilson terms. In all cases little or no extra computing power is
necessary, compared to simulations with standard Wilson fermions or
twisted-mass lattice QCD.Comment: 71 pages, Latex, Keywords: Lattice, Improvement, Chirality. Version
v2: mistake corrected in transformation properties under \omega -> -\omega,
sect. 5.3.1 (see also sect. 6.1). Minor corrections in App. D and argument
clarified in App. F. Version v3: minor modifications in sect. 2 (pag. 8-10:
on the odd r-parity of M_crit(r)), in sect. 3.1.3 and 5.4.1 (few sentences
about cutoff effects at small quark mass) and in sect. 3.2 (details of
discussion below eq. 3.17); updated/added some reference
Bifurcations of a driven granular system under gravity
Molecular dynamics study on the granular bifurcation in a simple model is
presented. The model consists of hard disks, which undergo inelastic
collisions; the system is under the uniform external gravity and is driven by
the heat bath. The competition between the two effects, namely, the
gravitational force and the heat bath, is carefully studied. We found that the
system shows three phases, namely, the condensed phase, locally fluidized
phase, and granular turbulent phase, upon increasing the external control
parameter. We conclude that the transition from the condensed phase to the
locally fluidized phase is distinguished by the existence of fluidized holes,
and the transition from the locally fluidized phase to the granular turbulent
phase is understood by the destabilization transition of the fluidized holes
due to mutual interference.Comment: 35 pages, 17 figures, to be published in PR
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