19,571 research outputs found
CPT- and B-Violation: The p-pbar Sector
The CPT symmetry of relativistic quantum field theory requires the total
lifetimes of particles and antiparticles be equal. Detection of pbar lifetime
shorter than tau_p > O(10^32) yr would signal breakdown of CPT invariance, in
combination with B-violation. The best current limit on tau_pbar, inferred from
cosmic ray measurements, is about one Myr, placing lower limits on
CPT-violating scales that depend on the exact mechanism. Paths to CPT breakdown
within and outside ordinary quantum mechanics are sketched. Many of the
limiting CPT-violating scales in pbar decay lie within the weak-to-Planck
range.Comment: 6 pages, LaTeX, .sty file included; based on contribution to CPT98
Conference; minor changes, accepted by Mod. Phys. Lett.
The superconducting phase transition and gauge dependence
The gauge dependence of the renormalization group functions of the
Ginzburg-Landau model is investigated. The analysis is done by means of the
Ward-Takahashi identities. After defining the local superconducting order
parameter, it is shown that its exponent is in fact gauge independent.
This happens because in the Landau gauge is the only gauge having a
physical meaning, a property not shared by the four-dimensional model where any
gauge choice is possible. The analysis is done in both the context of the
-expansion and in the fixed dimension approach. It is pointed out the
differences that arise in both of these approaches concerning the gauge
dependence.Comment: RevTex, 3 pages, no figures; accepted for publication in PRB; this
paper is a short version of cond-mat/990527
The VLT-FLAMES Tarantula Survey X: Evidence for a bimodal distribution of rotational velocities for the single early B-type stars
Aims: Projected rotational velocities (\vsini) have been estimated for 334
targets in the VLT-FLAMES Tarantula survey that do not manifest significant
radial velocity variations and are not supergiants. They have spectral types
from approximately O9.5 to B3. The estimates have been analysed to infer the
underlying rotational velocity distribution, which is critical for
understanding the evolution of massive stars.
Methods: Projected rotational velocities were deduced from the Fourier
transforms of spectral lines, with upper limits also being obtained from
profile fitting. For the narrower lined stars, metal and non-diffuse helium
lines were adopted, and for the broader lined stars, both non-diffuse and
diffuse helium lines; the estimates obtained using the different sets of lines
are in good agreement. The uncertainty in the mean estimates is typically 4%
for most targets. The iterative deconvolution procedure of Lucy has been used
to deduce the probability density distribution of the rotational velocities.
Results: Projected rotational velocities range up to approximately 450 \kms
and show a bi-modal structure. This is also present in the inferred rotational
velocity distribution with 25% of the sample having \ve100\,\kms
and the high velocity component having \ve\,\kms. There is no
evidence from the spatial and radial velocity distributions of the two
components that they represent either field and cluster populations or
different episodes of star formation. Be-type stars have also been identified.
Conclusions: The bi-modal rotational velocity distribution in our sample
resembles that found for late-B and early-A type stars. While magnetic braking
appears to be a possible mechanism for producing the low-velocity component, we
can not rule out alternative explanations.Comment: to be publisged in A&
The Kentucky Noisy Monte Carlo Algorithm for Wilson Dynamical Fermions
We develop an implementation for a recently proposed Noisy Monte Carlo
approach to the simulation of lattice QCD with dynamical fermions by
incorporating the full fermion determinant directly. Our algorithm uses a
quenched gauge field update with a shifted gauge coupling to minimize
fluctuations in the trace log of the Wilson Dirac matrix. The details of tuning
the gauge coupling shift as well as results for the distribution of noisy
estimators in our implementation are given. We present data for some basic
observables from the noisy method, as well as acceptance rate information and
discuss potential autocorrelation and sign violation effects. Both the results
and the efficiency of the algorithm are compared against those of Hybrid Monte
Carlo.
PACS Numbers: 12.38.Gc, 11.15.Ha, 02.70.Uu Keywords: Noisy Monte Carlo,
Lattice QCD, Determinant, Finite Density, QCDSPComment: 30 pages, 6 figure
Numerical Study of the Antiferrromagnetic Spin Chain with Bond Alternation
We study the quantum spin chain with bond alternation {\cal H}=\sum _i
(1-(-1)^i\delta)\vect{S}_i\cdot \vect{S}_{i+1} by the density matrix
renormalization group method recently proposed by Steven R. White
(\PRL{69}{3844}{1993}). We find a massless point at .
We also find the edge states in the region under the open
boundary condition, which disappear in the region . At the
massless point, the spin wave velocity is and the central
charge is . Our results indicate that a continuous phase
transition occurs at the massless point accompanying
breaking of the hidden symmetry.Comment: 9 pages and 1 PostScript figure, Revtex 3.0 (Minor corrections in
TEX-file format to remove possible compilatory troubles.
Speeding up finite step-size updating of full QCD on the lattice
We propose various improvements of finite step-size updating for full QCD on
the lattice that might turn finite step-size updating into a viable alternative
to the hybrid Monte Carlo algorithm. These improvements are noise reduction of
the noisy estimator of the fermion determinant, unbiased inclusion of the
hopping parameter expansion and a multi-level Metropolis scheme. First
numerical tests are performed for the 2 dimensional Schwinger model with two
flavours of Wilson fermions and for QCD two flavours of Wilson fermions and
Schr"odinger functional boundary conditions.Comment: 22 pages, 1 figur
Exact Ground States in Spin Systems with Orbital Degeneracy
We present exact ground states in spin models with orbital generacy in one
and higher dimensions. A method to obtain the exact ground states of the models
when the Hamiltonians are composed of the products of two commutable operators
is proposed. For the case of the spin-1/2 model with two-fold degeneracy some
exact ground states are given, such as the Valence-Bond (VB), the magnetically
ordered, and the orbitally ordered states under particular parameter regimes.
We also find the models with the higher spin and degeneracy which have the new
types of VB ground states in the spin and the orbital sectors.Comment: 4 pages(JPSJ.sty), 2 figures(EPS), to appear in J. Phys. Soc. Jpn.
68, No.2 (1999) 32
S(k) for Haldane Gap Antiferromagnets: Large-scale Numerical Results vs. Field Theory and Experiment
The structure function, S(k), for the s=1, Haldane gap antiferromagnetic
chain, is measured accurately using the recent density matrix renormalization
group method, with chain-length 100. Excellent agreement with the nonlinear
model prediction is obtained, both at where a single
magnon process dominates and at where a two magnon process
dominates. We repeat our calculation with crystal field anisotropy chosen to
model NENP, obtaining good agreement with both field theory predictions and
recent experiments. Correlation lengths, gaps and velocities are determined for
both polarizations.Comment: 11 pages, 3 postscript figures included, REVTEX 3.0, UBCTP-93-02
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