19,190 research outputs found
Spontaneous Flavor and Parity Breaking with Wilson Fermions
We discuss the phase diagram of Wilson fermions in the -- plane for
two-flavor QCD. We argue that, as originally suggested by Aoki, there is a
phase in which flavor and parity are spontaneously broken. Recent numerical
results on the spectrum of the overlap Hamiltonian have been interpreted as
evidence against Aoki's conjecture. We show that they are in fact consistent
with the presence of a flavor-parity broken ``Aoki phase''. We also show how,
as the continuum limit is approached, one can study the lattice theory using
the continuum chiral Lagrangian supplemented by additional terms proportional
to powers of the lattice spacing. We find that there are two possible phase
structures at non-zero lattice spacing: (1) there is an Aoki phase of width
with two massless Goldstone pions; (2) there is no
symmetry breaking, and all three pions have an equal non-vanishing mass of
order . Present numerical evidence suggests that the former option is
realized for Wilson fermions. Our analysis then predicts the form of the pion
masses and the flavor-parity breaking condensate within the Aoki phase. Our
analysis also applies for non-perturbatively improved Wilson fermions.Comment: 22 pages, LaTeX, 5 figures (added several references and a comment
Chiral perturbation theory with Wilson-type fermions including effects: degenerate case
We have derived the quark mass dependence of , and
, using the chiral perturbation theory which includes the effect
associated with the explicit chiral symmetry breaking of the Wilson-type
fermions, in the case of the degenerate quarks. Distinct features of
the results are (1) the additive renormalization for the mass parameter
in the Lagrangian, (2) corrections to the chiral log ()
term, (3) the existence of more singular term, , generated by
contributions, and (4) the existence of both and terms
in the quark mass from the axial Ward-Takahashi identity, . By
fitting the mass dependence of and , obtained by the
CP-PACS collaboration for full QCD simulations, we have found that the
data are consistently described by the derived formulae. Resumming the most
singular terms , we have also derived the modified formulae, which
show a better control over the next-to-leading order correction.Comment: 21 pages, 4 figures (10 eps files), Revtex4, some discussions and
references added, the final version to appear in PR
Operator product expansion and the short distance behavior of 3-flavor baryon potentials
The short distance behavior of baryon-baryon potentials defined through
Nambu-Bethe-Salpeter wave functions is investigated using the operator product
expansion. In a previous analysis of the nucleon-nucleon case, corresponding to
the SU(3) channels and , we argued that the potentials
have a repulsive core. A new feature occurs for the case of baryons made up of
three flavors: manifestly asymptotically attractive potentials appear in the
singlet and octet channels. Attraction in the singlet channel was first
indicated by quark model considerations, and recently been found in numerical
lattice simulations. The latter have however not yet revealed asymptotic
attraction in the octet channels; we give a speculative explanation for this
apparent discrepancy.Comment: 11 pages, 2 figure
Pion scattering in Wilson ChPT
We compute the scattering amplitude for pion scattering in Wilson chiral
perturbation theory for two degenerate quark flavors. We consider two different
regimes where the quark mass m is of order (i) a\Lambda_QCD^2 and (ii)
a^2\Lambda_QCD^3. Analytic expressions for the scattering lengths in all three
isospin channels are given. As a result of the O(a^2) terms the I=0 and I=2
scattering lengths do not vanish in the chiral limit. Moreover, additional
chiral logarithms proportional to a^2\ln M_{\pi}^2 are present in the one-loop
results for regime (ii). These contributions significantly modify the familiar
results from continuum chiral perturbation theory.Comment: 20 pages, 4 figures. V3: Comments on finite size effects and the
axial vector current added, one more reference. To be published in PR
I=2 Pion scattering length with improved actions on anisotropic lattices
scattering length in the I=2 channel is calculated within quenched
approximation using improved gauge and improved Wilson fermion actions on
anisotropic lattices. The results are extrapolated towards the chiral, infinite
volume and continuum limit. This result improves our previous result on the
scattering length. In the chiral, infinite volume and continuum limit, we
obtain , which is consistent with the result from
Chiral Perturbation Theory, the experiment and results from other lattice
calculations.Comment: 7 pages, 2 figures, typeset wit elsart.cl
A small tabletop experiment for a direct measurement of the speed of light
A small tabletop experiment for a direct measurement of the speed of light to
an accuracy of few percent is described. The experiment is accessible to a wide
spectrum of undergraduate students, in particular to students not majoring in
science or engineering. The experiment may further include a measurement of the
index of refraction of a sample. Details of the setup and equipment are given.
Results and limitations of the experiment are analyzed, partly based on our
experience in employing the experiment in our student laboratories. Safety
considerations are also discussed.Comment: 4pages, 5fig
New Dissipation Relaxation Phenomenon in Oscillating Solid He-4
We describe the first observations on the time-dependent dissipation when the
drive level of a torsional oscillator containing solid He-4 is abruptly
changed. The relaxation of dissipation in solid He-4 shows rich dynamical
behavior including exponential and logarithmic time-dependent decays,
hysteresis, and memory effects.Comment: 4 pages, 4 figure
Oscillation Frequency Dependence of Non-Classical Rotation Inertia of Solid He
The non-classical rotational inertia fraction of the identical cylindrical
solid He below 300 mK is studied at 496 and 1173 Hz by a double resonance
torsional oscillator. Below 35 mK, the fraction is the same at sufficiently low
rim velocities. Above 35 mK, the fraction is greater for the higher than the
lower mode. The dissipation peak of the lower mode occurs at a temperature
4 mK lower than that of the higher mode. The drive dependence of the two
modes shows that the reduction of the fraction is characterized by critical
velocity, \textit{not} amplitude nor acceleration.Comment: 4 pages, 4 figure
Extreme Enhancements of r-process Elements in the Cool Metal-Poor Main-Sequence Star SDSS J2357-0052
We report the discovery of a cool metal-poor, main-sequence star exhibiting
large excesses of r-process elements. This star is one of two newly discovered
cool subdwarfs (effective temperatures of 5000 K) with extremely low
metallicity ([Fe/H]<-3) identified from follow-up high-resolution spectroscopy
of metal-poor candidates from the Sloan Digital Sky Survey. SDSS J2357-0052 has
[Fe/H]=-3.4 and [Eu/Fe]=+1.9, and exhibits a scaled solar r-process abundance
pattern of heavy neutron-capture elements. This is the first example of an
extremely metal-poor, main-sequence star showing large excesses of r-process
elements; all previous examples of the large r-process-enhancement phenomena
have been associated with metal-poor giants. The metallicity of this object is
the lowest, and the excess of Eu ([Eu/Fe]) is the highest, among the
r-process-enhanced stars found so far. We consider possible scenarios to
account for the detection of such a star, and discuss techniques to enable
searches for similar stars in the future.Comment: 16 pages, 3 figures, 2 tables, ApJL in pres
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
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