10,899 research outputs found
Bose-Einstein condensation in an optical lattice
In this paper we develop an analytic expression for the critical temperature
for a gas of ideal bosons in a combined harmonic lattice potential, relevant to
current experiments using optical lattices. We give corrections to the critical
temperature arising from effective mass modifications of the low energy
spectrum, finite size effects and excited band states. We compute the critical
temperature using numerical methods and compare to our analytic result. We
study condensation in an optical lattice over a wide parameter regime and
demonstrate that the critical temperature can be increased or reduced relative
to the purely harmonic case by adjusting the harmonic trap frequency. We show
that a simple numerical procedure based on a piecewise analytic density of
states provides an accurate prediction for the critical temperature.Comment: 10 pages, 5 figure
The binary mass transfer origin of the red blue straggler sequence in M30
Two separated sequences of blue straggler stars (BSSs) have been revealed by
Ferraro et al. (2009) in the color-magnitude diagram (CMD) of the Milky Way
globular cluster M30. Their presence has been suggested to be related to the
two BSS formation channels (namely, collisions and mass-transfer in close
binaries) operating within the same stellar system. The blue sequence was
indeed found to be well reproduced by collisional BSS models. In contrast, no
specific models for mass transfer BSSs were available for an old stellar system
like M30. Here we present binary evolution models, including case-B mass
transfer and binary merging, specifically calculated for this cluster. We
discuss in detail the evolutionary track of a binary, which
spends approximately 4 Gyr in the BSS region of the CMD of a 13 Gyr old
cluster. We also run Monte-Carlo simulations to study the distribution of mass
transfer BSSs in the CMD and to compare it with the observational data. Our
results show that: (1) the color and magnitude distribution of synthetic mass
transfer BSSs defines a strip in the CMD that nicely matches the observed red
BSS sequence, thus providing strong support to the mass transfer origin for
these stars; (2) the CMD distribution of synthetic BSSs never attains the
observed location of the blue BSS sequence, thus reinforcing the hypothesis
that the latter formed through a different channel (likely collisions); (3)
most () of the synthetic BSSs are produced by mass-transfer models,
while the remaining requires the contribution from merger models.Comment: 8 pages, 5 figures, accepted to Ap
A Variational Principle Based Study of KPP Minimal Front Speeds in Random Shears
Variational principle for Kolmogorov-Petrovsky-Piskunov (KPP) minimal front
speeds provides an efficient tool for statistical speed analysis, as well as a
fast and accurate method for speed computation. A variational principle based
analysis is carried out on the ensemble of KPP speeds through spatially
stationary random shear flows inside infinite channel domains. In the regime of
small root mean square (rms) shear amplitude, the enhancement of the ensemble
averaged KPP front speeds is proved to obey the quadratic law under certain
shear moment conditions. Similarly, in the large rms amplitude regime, the
enhancement follows the linear law. In particular, both laws hold for the
Ornstein-Uhlenbeck process in case of two dimensional channels. An asymptotic
ensemble averaged speed formula is derived in the small rms regime and is
explicit in case of the Ornstein-Uhlenbeck process of the shear. Variational
principle based computation agrees with these analytical findings, and allows
further study on the speed enhancement distributions as well as the dependence
of enhancement on the shear covariance. Direct simulations in the small rms
regime suggest quadratic speed enhancement law for non-KPP nonlinearities.Comment: 28 pages, 14 figures update: fixed typos, refined estimates in
section
Branching ratios and CP asymmetries of decays in the pQCD approach
We calculate the branching ratios and CP violating asymmetries of the four B
\to K \etap decays in the perturbative QCD (pQCD) factorization approach.
Besides the full leading order contributions, the partial next-to-leading order
(NLO) contributions from the QCD vertex corrections, the quark loops, and the
chromo-magnetic penguins are also taken into account. The NLO pQCD predictions
for the CP-averaged branching ratios are , Br(B^\pm \to K^\pm \etar) \approx 51.0 \times 10^{-6},
, and Br(B^0 \to K^0 \etar)
\approx 50.3 \times 10^{-6}. The NLO contributions can provide a 70%
enhancement to the LO Br(B \to K \etar), but a 30% reduction to the LO , which play the key role in understanding the observed pattern of
branching ratios. The NLO pQCD predictions for the CP-violating asymmetries,
such as \acp^{dir} (K^0_S \etar) \sim 2.3% and \acp^{mix}(K^0_S \etar)\sim
63%, agree very well with currently available data. This means that the
deviation \Delta S=\acp^{mix}(K^0_S \etar) - \sin{2\beta} in pQCD approach is
also very small.Comment: 31 pages, 11 ps/eps figures, typos corrected. A little modificatio
The spacetime structure of MOND with Tully-Fisher relation and Lorentz invariance violation
It is believed that the modification of Newtonian dynamics (MOND) is possible
alternate for dark matter hypothesis. Although Bekenstein's TeVeS supplies a
relativistic version of MOND, one may still wish a more concise covariant
formulism of MOND. In this paper, within covariant geometrical framwork, we
present another version of MOND. We show the spacetime structure of MOND with
properties of Tully-Fisher relation and Lorentz invariance violation.Comment: 6 pages. arXiv admin note: substantial text overlap with
arXiv:1111.1383 and arXiv:1108.344
Nucleation in scale-free networks
We have studied nucleation dynamics of the Ising model in scale-free networks
with degree distribution by using forward flux sampling
method, focusing on how the network topology would influence the nucleation
rate and pathway. For homogeneous nucleation, the new phase clusters grow from
those nodes with smaller degree, while the cluster sizes follow a power-law
distribution. Interestingly, we find that the nucleation rate decays
exponentially with the network size , and accordingly the critical nucleus
size increases linearly with , implying that homogeneous nucleation is not
relevant in the thermodynamic limit. These observations are robust to the
change of and also present in random networks. In addition, we have
also studied the dynamics of heterogeneous nucleation, wherein impurities
are initially added, either to randomly selected nodes or to targeted ones with
largest degrees. We find that targeted impurities can enhance the nucleation
rate much more sharply than random ones. Moreover, scales as and for targeted and
random impurities, respectively. A simple mean field analysis is also present
to qualitatively illustrate above simulation results.Comment: 7 pages, 5 figure
Mutual selection in network evolution: the role of the intrinsic fitness
We propose a new mechanism leading to scale-free networks which is based on
the presence of an intrinsic character of a vertex called fitness. In our
model, a vertex is assigned a fitness , drawn from a given probability
distribution function . During network evolution, with rate we add a
vertex of fitness and connect to an existing vertex of fitness
selected preferentially to a linking probability function
which depends on the fitnesses of the two vertices involved and, with rate
we create an edge between two already existed vertices with fitnesses
and , with a probability also preferential to the connection
function . For the proper choice of , the resulting networks
have generalized power laws, irrespective of the fitness distribution of
vertices.Comment: ws-ijmpc.te
Baryon production and net-proton distributions in relativistic heavy ion collisions
The higher order moments of the net-baryon distributions in relativistic
heavy ion collisions are useful probes for the QCD critical point and
fluctuations. We study the net-proton distributions and their moments in a
simple model which considers the baryon stopping and pair production effects in
the processes. It is shown that a single emission source model can explain the
experimental data well. Centrality and energy dependence of the distributions
and higher moments is discussed.Comment: 5 pages in RevTex, 8 eps figures, to be appeared in PR
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