1,641 research outputs found
QCD and Hadron Dynamics
Perturbative QCD predicts and describes various features of multihadron
production. An amazing similarity between observable hadron systems and
calculable underlying parton ensembles justifies the attempts to use the
language of quarks and gluons down to small momentum scales, to approach the
profound problems that are commonly viewed as being entirely non-perturbative.Comment: Talk at the Royal Society meeting "Structure of Matter", London, May
200
Stable Exact Solutions in Cosmological Models with Two Scalar Fields
The stability of isotropic cosmological solutions for two-field models in the
Bianchi I metric is considered. We prove that the sufficient conditions for the
Lyapunov stability in the Friedmann-Robertson-Walker metric provide the
stability with respect to anisotropic perturbations in the Bianchi I metric and
with respect to the cold dark matter energy density fluctuations. Sufficient
conditions for the Lyapunov stability of the isotropic fixed points of the
system of the Einstein equations have been found. We use the superpotential
method to construct stable kink-type solutions and obtain sufficient conditions
on the superpotential for the Lyapunov stability of the corresponding exact
solutions. We analyze the stability of isotropic kink-type solutions for string
field theory inspired cosmological models.Comment: 23 pages, v3:typos corrected, references adde
General relativistic analysis of peculiar velocities
We give a careful general relativistic and (1+3)-covariant analysis of
cosmological peculiar velocities induced by matter density perturbations in the
presence of a cosmological constant. In our quasi-Newtonian approach,
constraint equations arise to maintain zero shear of the non-comoving
fundamental worldlines which define a Newtonian-like frame, and these lead to
the (1+3)-covariant dynamical equations, including a generalized Poisson-type
equation. We investigate the relation between peculiar velocity and peculiar
acceleration, finding the conditions under which they are aligned. In this case
we find (1+3)-covariant relativistic generalizations of well-known Newtonian
results.Comment: 8 pages, LaTeX2e (iopart); minor changes, matches version accepted
for publication by Classical and Quantum Gravit
Why is Spacetime Lorentzian?
We expand on the idea that spacetime signature should be treated as a
dynamical degree of freedom in quantum field theory. It has been argued that
the probability distribution for signature, induced by massless free fields, is
peaked at the Lorentzian value uniquely in D=4 dimensions. This argument is
reviewed, and certain consistency constraints on the generalized signature
(i.e. the tangent space metric
\eta_{ab}(x)=\mbox{diag}[e^{i\theta(x)},1,1,1]) are derived. It is shown that
only one dynamical "Wick angle" can be introduced in the
generalized signature, and the magnitude of fluctuations away from Lorentzian
signature is estimated to be of order
, where is the Planck length, and is the length scale of
the Universe. For massless fields, the case of D=2 dimensions and the case of
supersymmetry are degenerate, in the sense that no signature is preferred. Mass
effects lift this degeneracy, and we show that a dynamical origin of Lorentzian
signature is also possible for (broken) supersymmetry theories in D=6
dimensions, in addition to the more general non-supersymmetric case in D=4
dimensions.Comment: 26 pages, plain LaTeX, NBI-HE-93-3
Absorption of phi mesons in near-threshold proton-nucleus reactions
In the framework of the nuclear spectral function approach for incoherent
primary proton--nucleon and secondary pion--nucleon production processes we
study the inclusive meson production in the interaction of 2.83 GeV
protons with nuclei. In particular, the A-dependences of the absolute and
relative meson yields are investigated within the different scenarios
for its in-medium width as well as for the cross section ratio . Our model calculations take into account
the acceptance window of the ANKE facility used in a recent experiment
performed at COSY. They show that the pion--nucleon production channel
contributes distinctly to the creation in heavy nuclei in the chosen
kinematics and, hence, has to be taken into consideration on close examination
of the dependences of the phi meson yields on the target mass number with the
aim to get information on its width in the medium. They also demonstrate that
the experimentally unknown ratio has a weak effect on the A-dependence of the relative meson
production cross section at incident energy of present interest, whereas it is
found to be appreciably sensitive to the phi in-medium width, which means that
this relative observable can indeed be useful to help determine the above width
from the direct comparison the results of our calculations with the future data
from the respective ANKE-at-COSY experiment.Comment: 16 pages, 9 figure
Bianchi Type I Massive String Magnetized Barotropic Perfect Fluid Cosmological Model in General Relativity
Bianchi type I massive string cosmological model with magnetic field of
barotropic perfect fluid distribution through the techniques used by Latelier
and Stachel, is investigated. To get the deterministic model of the universe,
it is assumed that the universe is filled with barotropic perfect fluid
distribution. The magnetic field is due to electric current produced along
x-axis with infinite electrical conductivity. The behaviour of the model in
presence and absence of magnetic field together with other physical aspects is
further discussed.Comment: 10 pages, no figure. Chin. Phys. Lett., Vol. 24, No. 8 (2007), to
appea
Null Energy Condition Violation and Classical Stability in the Bianchi I Metric
The stability of isotropic cosmological solutions in the Bianchi I model is
considered. We prove that the stability of isotropic solutions in the Bianchi I
metric for a positive Hubble parameter follows from their stability in the
Friedmann-Robertson-Walker metric. This result is applied to models inspired by
string field theory, which violate the null energy condition. Examples of
stable isotropic solutions are presented. We also consider the k-essence model
and analyse the stability of solutions of the form .Comment: 27 pages, references added, accepted for publication in Phys. Rev.
Second-order perturbations of cosmological fluids: Relativistic effects of pressure, multi-component, curvature, and rotation
We present general relativistic correction terms appearing in Newton's
gravity to the second-order perturbations of cosmological fluids. In our
previous work we have shown that to the second-order perturbations, the density
and velocity perturbation equations of general relativistic zero-pressure,
irrotational, single-component fluid in a flat background coincide exactly with
the ones known in Newton's theory. Here, we present the general relativistic
second-order correction terms arising due to (i) pressure, (ii)
multi-component, (iii) background curvature, and (iv) rotation. In case of
multi-component zero-pressure, irrotational fluids under the flat background,
we effectively do not have relativistic correction terms, thus the relativistic
result again coincides with the Newtonian ones. In the other three cases we
generally have pure general relativistic correction terms. In case of pressure,
the relativistic corrections appear even in the level of background and linear
perturbation equations. In the presence of background curvature, or rotation,
pure relativistic correction terms directly appear in the Newtonian equations
of motion of density and velocity perturbations to the second order. In the
small-scale limit (far inside the horizon), relativistic equations including
the rotation coincide with the ones in Newton's gravity.Comment: 41 pages, no figur
The Oscillating Universe: an Alternative to Inflation
The aim of this paper is to show, that the 'oscillating universe' is a viable
alternative to inflation. We remind that this model provides a natural solution
to the flatness or entropy and to the horizon problem of standard cosmology. We
study the evolution of density perturbations and determine the power spectrum
in a closed universe. The results lead to constraints of how a previous cycle
might have looked like. We argue that most of the radiation entropy of the
present universe may have originated from gravitational entropy produced in a
previous cycle.
We show that measurements of the power spectrum on very large scales could in
principle decide whether our universe is closed, flat or open.Comment: revised version for publication in Classical and Quantum Gravity, 23
pages, uuencoded compressed tarred Latex file with 7 eps figures included,
fig.8 upon reques
Minijet corrections to Higgs production
We study higher order corrections to Higgs production with an associated jet
at SSC energies, using the resummation of the leading logarithmic contributions
to multiple gluon emissions due to Lipatov and collaborators. We find a
considerable enhancement of Higgs production at large transverse momenta.Comment: 15 page
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