25,918 research outputs found

### String Effects on Fermi--Dirac Correlation Measurements

We investigate some recent measurements of Fermi--Dirac correlations by the
LEP collaborations indicating surprisingly small source radii for the
production of baryons in $e^+e^-$-annihilation at the $Z^0$ peak. In the
hadronization models there are besides the Fermi--Dirac correlation effect also
a strong dynamical (anti-)correlation. We demonstrate that the extraction of
the pure FD effect is highly dependent on a realistic Monte Carlo event
generator, both for separation of those dynamical correlations which are not
related to Fermi--Dirac statistics, and for corrections of the data and
background subtractions. Although the model can be tuned to well reproduce
single particle distributions, there are large model-uncertainties when it
comes to correlations between identical baryons. We therefore, unfortunately,
have to conclude that it is at present not possible to make any firm conclusion
about the source radii relevant for baryon production at LEP

### Advective collisions

Small particles advected in a fluid can collide (and therefore aggregate) due
to the stretching or shearing of fluid elements. This effect is usually
discussed in terms of a theory due to Saffman and Turner [J. Fluid Mech., 1,
16-30, (1956)]. We show that in complex or random flows the Saffman-Turner
theory for the collision rate describes only an initial transient (which we
evaluate exactly). We obtain precise expressions for the steady-state collision
rate for flows with small Kubo number, including the influence of fractal
clustering on the collision rate for compressible flows. For incompressible
turbulent flows, where the Kubo number is of order unity, the Saffman-Turner
theory is an upper bound.Comment: 4 pages, 1 figur

### Lagrangian perturbation theory for a superfluid immersed in an elastic neutron star crust

The inner crust of mature neutron stars, where an elastic lattice of
neutron-rich nuclei coexists with a neutron superfluid, impacts on a range of
astrophysical phenomena. The presence of the superfluid is key to our
understanding of pulsar glitches, and is expected to affect the thermal
conductivity and hence the evolution of the surface temperature. The coupling
between crust and superfluid must also be accounted for in studies of neutron
star dynamics, discussions of global oscillations and associated instabilities.
In this paper we develop Lagrangian perturbation theory for this problem,
paying attention to key issues like superfluid entrainment, potential vortex
pinning, dissipative mutual friction and the star's magnetic field. We also
discuss the nature of the core-crust interface. The results provide a
theoretical foundation for a range of interesting astrophysical applications.Comment: 13 pages, no figures, to appear in MNRA

### BRUCE: a program for the detection of transfer-messenger RNA genes in nucleotide sequences

A computer program, BRUCE, was developed for the identification of transfer‐messenger RNA (tmRNA) genes. The program employs heuristic algorithms to search for a tRNAAla‐like secondary structure surrounding a short sequence encoding the tag peptide. In the 57 completely sequenced bacterial genomes where tmRNA genes have been reported previously, BRUCE identified all with no false positives. In addition, BRUCE found 99 of the 100 tmRNAs identified previously in other bacteria, red chloroplasts and cyanelles. The output of the program reports the proposed tRNA secondary structure, the tmRNA gene sequence and the tag peptide

### Investigations into the BFKL Mechanism with a Running QCD Coupling

We present approximations of varying degree of sophistication to the integral
equations for the (gluon) structure functions of a hadron (``the partonic flux
factor'') in a model valid in the Leading Log Approximation with a running
coupling constant. The results are all of the BFKL-type, i.e. a power in the
Bjorken variable x_B^{-\lambda} with the parameter \lambda determined from the
size \alpha_0 of the ``effective'' running coupling \bar{\alpha}\equiv
3\alpha_s/\pi= \alpha_0/\log(k_{\perp}^2) and varying depending upon the
treatment of the transverse momentum pole. We also consider the implications
for the transverse momentum (k_{\perp}) fluctuations along the emission chains
and we obtain an exponential falloff in the relevant \kappa\equiv
\log(k_{\perp}^2)-variable, i.e. an inverse power (k_{\perp}^2)^{-(2+\lambda)}
with the same parameter \lambda. This is different from the BFKL-result for a
fixed coupling, where the distributions are Gaussian in the \kappa-variable
with a width as in a Brownian motion determined by ``the length'' of the
emission chains, i.e. \log(1/x_B). The results are verified by a realistic
Monte Carlo simulation and we provide a simple physics motivation for the
change.Comment: 24 pages, 10 supplementary files, submitted to Physical Review

### The Spin Distribution of Fast Spinning Neutron Stars in Low Mass X-Ray Binaries: Evidence for Two Sub-Populations

We study the current sample of rapidly rotating neutron stars in both
accreting and non-accreting binaries in order to determine whether the spin
distribution of accreting neutron stars in low-mass X-ray binaries can be
reconciled with current accretion torque models. We perform a statistical
analysis of the spin distributions and show that there is evidence for two
sub-populations among low-mass X-ray binaries, one at relatively low spin
frequency, with an average of ~300 Hz and a broad spread, and a peaked
population at higher frequency with average spin frequency of ~575 Hz. We show
that the two sub-populations are separated by a cut-point at a frequency of
~540 Hz. We also show that the spin frequency of radio millisecond pulsars does
not follow a log-normal distribution and shows no evidence for the existence of
distinct sub-populations. We discuss the uncertainties of different accretion
models and speculate that either the accreting neutron star cut-point marks the
onset of gravitational waves as an efficient mechanism to remove angular
momentum or some of the neutron stars in the fast sub-population do not evolve
into radio millisecond pulsars.Comment: Submitted to Ap

### The dynamics of dissipative multi-fluid neutron star cores

We present a Newtonian multi-fluid formalism for superfluid neutron star
cores, focussing on the additional dissipative terms that arise when one takes
into account the individual dynamical degrees of freedom associated with the
coupled "fluids". The problem is of direct astrophysical interest as the nature
of the dissipative terms can have significant impact on the damping of the
various oscillation modes of the star and the associated gravitational-wave
signatures. A particularly interesting application concerns the
gravitational-wave driven instability of f- and r-modes. We apply the developed
formalism to two specific three-fluid systems: (i) a hyperon core in which both
Lambda and Sigma^- hyperons are present, and (ii) a core of deconfined quarks
in the colour-flavour-locked phase in which a population of neutral K^0 kaons
is present. The formalism is, however, general and can be applied to other
problems in neutron-star dynamics (such as the effect of thermal excitations
close to the superfluid transition temperature) as well as laboratory
multi-fluid systems.Comment: RevTex, no figure

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