937 research outputs found
Softening Transitions with Quenched 2D Gravity
We perform extensive Monte Carlo simulations of the 10-state Potts model on
quenched two-dimensional gravity graphs to study the effect of
quenched connectivity disorder on the phase transition, which is strongly first
order on regular lattices. The numerical data provides strong evidence that,
due to the quenched randomness, the discontinuous first-order phase transition
of the pure model is softened to a continuous transition.Comment: 3 pages, LaTeX + 1 postscript figure. Talk presented at
LATTICE96(other models). See also
http://www.cond-mat.physik.uni-mainz.de/~janke/doc/home_janke.htm
Square Gravity
We simulate the Ising model on dynamical quadrangulations using a
generalization of the flip move for triangulations with two aims: firstly, as a
confirmation of the universality of the KPZ/DDK exponents of the Ising phase
transition, worthwhile in view of some recent surprises with other sorts of
dynamical lattices; secondly, to investigate the transition of the Ising
antiferromagnet on a dynamical loosely packed (bipartite) lattice. In the
latter case we show that it is still possible to define a staggered
magnetization and observe the antiferromagnetic analogue of the transition.Comment: LaTeX file and 7 postscript figures bundled together with uufile
Super AutoDipole
The publicly available package for an automated dipole subtraction,
AutoDipole, is extended to include the SUSY dipoles in the MSSM. All fields in
the SM and the MSSM are available. The code is checked against the analytical
expressions for a simple process. The extended package makes it possible to
compute the QCD NLO corrections to SUSY multi-parton processes like the stop
pair production plus jets at the LHC.Comment: 16 pages, 1 figure, v2: a few typos to match the published version in
Eur. Phys. J.
Cooling atomic motion with quantum interference
We theoretically investigate the quantum dynamics of the center of mass of
trapped atoms, whose internal degrees of freedom are driven in a
-shaped configuration with the lasers tuned at two-photon resonance.
In the Lamb-Dicke regime, when the motional wave packet is well localized over
the laser wavelenght, transient coherent population trapping occurs, cancelling
transitions at the laser frequency. In this limit the motion can be efficiently
cooled to the ground state of the trapping potential. We derive an equation for
the center-of-mass motion by adiabatically eliminating the internal degrees of
freedom. This treatment provides the theoretical background of the scheme
presented in [G. Morigi {\it et al}, Phys. Rev. Lett. {\bf 85}, 4458 (2000)]
and implemented in [C.F. Roos {\it et al}, Phys. Rev. Lett. {\bf 85}, 5547
(2000)]. We discuss the physical mechanisms determining the dynamics and
identify new parameters regimes, where cooling is efficient. We discuss
implementations of the scheme to cases where the trapping potential is not
harmonic.Comment: 11 pages, 3 figure
Ising model on 3D random lattices: A Monte Carlo study
We report single-cluster Monte Carlo simulations of the Ising model on
three-dimensional Poissonian random lattices with up to 128,000 approx. 503
sites which are linked together according to the Voronoi/Delaunay prescription.
For each lattice size quenched averages are performed over 96 realizations. By
using reweighting techniques and finite-size scaling analyses we investigate
the critical properties of the model in the close vicinity of the phase
transition point. Our random lattice data provide strong evidence that, for the
available system sizes, the resulting effective critical exponents are
indistinguishable from recent high-precision estimates obtained in Monte Carlo
studies of the Ising model and \phi^4 field theory on three-dimensional regular
cubic lattices.Comment: 35 pages, LaTex, 8 tables, 8 postscript figure
Resonance fluorescence of a trapped three-level atom
We investigate theoretically the spectrum of resonance fluorescence of a
harmonically trapped atom, whose internal transitions are --shaped and
driven at two-photon resonance by a pair of lasers, which cool the
center--of--mass motion. For this configuration, photons are scattered only due
to the mechanical effects of the quantum interaction between light and atom. We
study the spectrum of emission in the final stage of laser--cooling, when the
atomic center-of-mass dynamics is quantum mechanical and the size of the wave
packet is much smaller than the laser wavelength (Lamb--Dicke limit). We use
the spectral decomposition of the Liouville operator of the master equation for
the atomic density matrix and apply second order perturbation theory. We find
that the spectrum of resonance fluorescence is composed by two narrow sidebands
-- the Stokes and anti-Stokes components of the scattered light -- while all
other signals are in general orders of magnitude smaller. For very low
temperatures, however, the Mollow--type inelastic component of the spectrum
becomes visible. This exhibits novel features which allow further insight into
the quantum dynamics of the system. We provide a physical model that interprets
our results and discuss how one can recover temperature and cooling rate of the
atom from the spectrum. The behaviour of the considered system is compared with
the resonance fluorescence of a trapped atom whose internal transition consists
of two-levels.Comment: 11 pages, 4 Figure
Dijet Rapidity Gaps in Photoproduction from Perturbative QCD
By defining dijet rapidity gap events according to interjet energy flow, we
treat the photoproduction cross section of two high transverse momentum jets
with a large intermediate rapidity region as a factorizable quantity in
perturbative QCD. We show that logarithms of soft gluon energy in the interjet
region can be resummed to all orders in perturbation theory. The resummed cross
section depends on the eigenvalues of a set of soft anomalous dimension
matrices, specific to each underlying partonic process, and on the
decomposition of the scattering according to the possible patterns of hard
color flow. We present a detailed discussion of both. Finally, we evaluate
numerically the gap cross section and gap fraction and compare the results with
ZEUS data. In the limit of low gap energy, good agreement with experiment is
obtained.Comment: 37 pages, Latex, 17 figure
Resonant and off-resonant transients in electromagnetically induced transparency: Turn-on and turn-off dynamics
Published versio
A Measurement of Psi(2S) Resonance Parameters
Cross sections for e+e- to hadons, pi+pi- J/Psi, and mu+mu- have been
measured in the vicinity of the Psi(2S) resonance using the BESII detector
operated at the BEPC. The Psi(2S) total width; partial widths to hadrons,
pi+pi- J/Psi, muons; and corresponding branching fractions have been determined
to be Gamma(total)= (264+-27) keV; Gamma(hadron)= (258+-26) keV, Gamma(mu)=
(2.44+-0.21) keV, and Gamma(pi+pi- J/Psi)= (85+-8.7) keV; and Br(hadron)=
(97.79+-0.15)%, Br(pi+pi- J/Psi)= (32+-1.4)%, Br(mu)= (0.93+-0.08)%,
respectively.Comment: 8 pages, 6 figure
Measurements of the Mass and Full-Width of the Meson
In a sample of 58 million events collected with the BES II detector,
the process J/ is observed in five different decay
channels: , , (with ), (with
) and . From a combined fit of all five
channels, we determine the mass and full-width of to be
MeV/ and
MeV/.Comment: 9 pages, 2 figures and 4 table. Submitted to Phys. Lett.
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