273 research outputs found
The initial singularity of ultrastiff perfect fluid spacetimes without symmetries
We consider the Einstein equations coupled to an ultrastiff perfect fluid and
prove the existence of a family of solutions with an initial singularity whose
structure is that of explicit isotropic models. This family of solutions is
`generic' in the sense that it depends on as many free functions as a general
solution, i.e., without imposing any symmetry assumptions, of the
Einstein-Euler equations. The method we use is a that of a Fuchsian reduction.Comment: 16 pages, journal versio
Stress field around arbitrarily shaped cracks in two-dimensional elastic materials
The calculation of the stress field around an arbitrarily shaped crack in an
infinite two-dimensional elastic medium is a mathematically daunting problem.
With the exception of few exactly soluble crack shapes the available results
are based on either perturbative approaches or on combinations of analytic and
numerical techniques. We present here a general solution of this problem for
any arbitrary crack. Along the way we develop a method to compute the conformal
map from the exterior of a circle to the exterior of a line of arbitrary shape,
offering it as a superior alternative to the classical Schwartz-Cristoffel
transformation. Our calculation results in an accurate estimate of the full
stress field and in particular of the stress intensity factors K_I and K_{II}
and the T-stress which are essential in the theory of fracture.Comment: 7 pages, 4 figures, submitted for PR
Collisions of particles in locally AdS spacetimes I. Local description and global examples
We investigate 3-dimensional globally hyperbolic AdS manifolds containing
"particles", i.e., cone singularities along a graph . We impose
physically relevant conditions on the cone singularities, e.g. positivity of
mass (angle less than on time-like singular segments). We construct
examples of such manifolds, describe the cone singularities that can arise and
the way they can interact (the local geometry near the vertices of ).
We then adapt to this setting some notions like global hyperbolicity which are
natural for Lorentz manifolds, and construct some examples of globally
hyperbolic AdS manifolds with interacting particles.Comment: This is a rewritten version of the first part of arxiv:0905.1823.
That preprint was too long and contained two types of results, so we sliced
it in two. This is the first part. Some sections have been completely
rewritten so as to be more readable, at the cost of slightly less general
statements. Others parts have been notably improved to increase readabilit
Grain Surface Models and Data for Astrochemistry
AbstractThe cross-disciplinary field of astrochemistry exists to understand the formation, destruction, and survival of molecules in astrophysical environments. Molecules in space are synthesized via a large variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. A broad consensus has been reached in the astrochemistry community on how to suitably treat gas-phase processes in models, and also on how to present the necessary reaction data in databases; however, no such consensus has yet been reached for grain-surface processes. A team of ∼25 experts covering observational, laboratory and theoretical (astro)chemistry met in summer of 2014 at the Lorentz Center in Leiden with the aim to provide solutions for this problem and to review the current state-of-the-art of grain surface models, both in terms of technical implementation into models as well as the most up-to-date information available from experiments and chemical computations. This review builds on the results of this workshop and gives an outlook for future directions
Bose-Einstein Correlations of Neutral and Charged Pions in Hadronic Z Decays
Bose-Einstein correlations of both neutral and like-sign charged pion pairs
are measured in a sample of 2 million hadronic Z decays collected with the L3
detector at LEP. The analysis is performed in the four-momentum difference
range 300 MeV < Q < 2 GeV. The radius of the neutral pion source is found to be
smaller than that of charged pions. This result is in qualitative agreement
with the string fragmentation model
Observation of Scaling Violations in Scaled Momentum Distributions at HERA
Charged particle production has been measured in deep inelastic scattering
(DIS) events over a large range of and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, has been investigated in the current fragmentation region of the Breit
frame. The results show clear evidence, in a single experiment, for scaling
violations in scaled momenta as a function of .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
D* Production in Deep Inelastic Scattering at HERA
This paper presents measurements of D^{*\pm} production in deep inelastic
scattering from collisions between 27.5 GeV positrons and 820 GeV protons. The
data have been taken with the ZEUS detector at HERA. The decay channel
(+ c.c.) has been used in the study. The
cross section for inclusive D^{*\pm} production with
and is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region
{ GeV and }. Differential cross
sections as functions of p_T(D^{*\pm}), and are
compared with next-to-leading order QCD calculations based on the photon-gluon
fusion production mechanism. After an extrapolation of the cross section to the
full kinematic region in p_T(D^{*\pm}) and (D^{*\pm}), the charm
contribution to the proton structure function is
determined for Bjorken between 2 10 and 5 10.Comment: 17 pages including 4 figure
Study of Inclusive Strange-Baryon Production and Search for Pentaquarks in Two-Photon Collisions at LEP
Measurements of inclusive production of the Lambda, Xi- and Xi*(1530) baryons
in two-photon collisions with the L3 detector at LEP are presented. The
inclusive differential cross sections for Lambda and Xi- are measured as a
function of the baryon transverse momentum, pt, and pseudo-rapidity, eta. The
mean number of Lambda, Xi- and Xi*(1530) baryons per hadronic two-photon event
is determined in the kinematic range 0.4 GeV < pt< 2.5 GeV, |eta| < 1.2.
Overall agreement with the theoretical models and Monte Carlo predictions is
observed. A search for inclusive production of the pentaquark theta+(1540) in
two-photon collisions through the decay theta+ -> proton K0s is also presented.
No evidence for production of this state is found
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