14,664 research outputs found
Radiation can never again dominate Matter in a Vacuum Dominated Universe
We demonstrate that in a vacuum-energy-dominated expansion phase,
surprisingly neither the decay of matter nor matter-antimatter annihilation
into relativistic particles can ever cause radiation to once again dominate
over matter in the future history of the universe.Comment: updated version, as it will appear in Phys. Rev D. Title change, and
some other minor alteration
Thermal effects on slow-roll dynamics
A description of the transition from the inflationary epoch to radiation
domination requires the understanding of quantum fields out of thermal
equilibrium, particle creation and thermalisation. This can be studied from
first principles by solving a set of truncated real-time Schwinger-Dyson
equations, written in terms of the mean field (inflaton) and the field
propagators, derived from the two-particle irreducible effective action. We
investigate some aspects of this problem by considering the dynamics of a
slow-rolling mean field coupled to a second quantum field, using a \phi^2\chi^2
interaction. We focus on thermal effects. It is found that interactions lead to
an earlier end of slow-roll and that the evolution afterwards depends on
details of the heatbath.Comment: 25 pages, 11 eps figures. v2: paper reorganized, title changed,
conclusions unchanged, to appear in PR
Scattering of Dirac and Majorana Fermions off Domain Walls
We investigate the interaction of fermions having both Dirac and left-handed
and right-handed Majorana mass terms with vacuum domain walls. By solving the
equations of motion in thin-wall approximation, we calculate the reflection and
transmission coefficients for the scattering of fermions off walls.Comment: 6 pages, 1 figure, some typos corrected, one reference added, major
revisions, title changed, version accepted for publication in Phys. Rev.
Leptogenesis with Dirac Neutrinos
We describe a "neutrinogenesis" mechanism whereby, in the presence of
right-handed neutrinos with sufficiently small pure Dirac masses,
(B+L)-violating sphaleron processes create the baryon asymmetry of the
Universe, even when B=L=0 initially. It is shown that the resulting neutrino
mass constraints are easily fulfilled by the neutrino masses suggested by
current experiments. We present a simple toy model which uses this mechanism to
produce the observed baryon asymmetry of the Universe. (PostScript Errors
corrected in latest Version).Comment: 4 pages, Latex (using amsmath,feynmp,graphicx), 4 figure
Resonant decay of parity odd bubbles in hot hadronic matter
We investigate the decay of metastable states with broken CP-symmetry which
have recently been proposed by Kharzeev, Pisarski and Tytgat to form in hot
hadronic matter. We consider the efficiency of the amplification of the
-field via parametric resonance, taking the backreaction into account.
For times of the order , we find a particle density of about
and a correlation length of . The
corresponding momentum spectra show a non-thermal behaviour.Comment: 11 pages latex file with 4 gif - figures. Uses elsart.cls (included
Potentially Large One-loop Corrections to WIMP Annihilation
We compute one-loop corrections to the annihilation of non--relativistic
particles due to the exchange of a (gauge or Higgs) boson with
mass in the initial state. In the limit this leads to
the "Sommerfeld enhancement" of the annihilation cross section. However, here
we are interested in the case \mu \lsim m_\chi, where the one--loop
corrections are well--behaved, but can still be sizable. We find simple and
accurate expressions for annihilation from both and wave initial
states; they differ from each other if . In order to apply our
results to the calculation of the relic density of Weakly Interacting Massive
Particles (WIMPs), we describe how to compute the thermal average of the
corrected cross sections. We apply this formalism to scalar and Dirac fermion
singlet WIMPs, and show that the corrections are always very small in the
former case, but can be very large in the latter. Moreover, in the context of
the Minimal Supersymmetric Standard Model, these corrections can decrease the
relic density of neutralinos by more than 1%, if the lightest neutralino is a
strongly mixed state.Comment: 25 pages, 8 figures. Added an appendix showing that the approximation
works well in a scalar toy model. To be published in PRD
Transverse-Mass Spectra in Heavy-Ion Collisions at energies E_{lab} = 2--160 GeV/nucleon
Transverse-mass spectra of protons, pions and kaons produced in collisions of
heavy nuclei are analyzed within the model of 3-fluid dynamics. It was
demonstrated that this model consistently reproduces these spectra in wide
ranges of incident energies E_{lab}, from 4A GeV to 160A GeV, rapidity bins and
centralities of the collisions. In particular, the model describes the
"step-like" dependence of kaon inverse slopes on the incident energy. The key
point of this explanation is interplay of hydrodynamic expansion of the system
with its dynamical freeze-out.Comment: 13 pages, 16 figures, summary is extended, version accepted by Phys.
Rev.
New Upper Limits on the Tau Neutrino Mass from Primordial Helium Considerations
In this paper we reconsider recently derived bounds on tau neutrinos,
taking into account previously unaccounted for effects. We find that, assuming
that the neutrino life-time is longer than , the constraint
rules out masses in the range
for Majorana neutrinos and
for Dirac neutrinos. Given that the present
laboratory bound is 35 MeV, our results lower the present bound to and
for Majorana and Dirac neutrinos respectively.Comment: 9 pages (2 figures available upon request), UM-AC-93-0
Turning off the Lights: How Dark is Dark Matter?
We consider current observational constraints on the electromagnetic charge
of dark matter. The velocity dependence of the scattering cross-section through
the photon gives rise to qualitatively different constraints than standard dark
matter scattering through massive force carriers. In particular, recombination
epoch observations of dark matter density perturbations require that
, the ratio of the dark matter to electronic charge, is less than
for , rising to for .
Though naively one would expect that dark matter carrying a charge well below
this constraint could still give rise to large scattering in current direct
detection experiments, we show that charged dark matter particles that could be
detected with upcoming experiments are expected to be evacuated from the
Galactic disk by the Galactic magnetic fields and supernova shock waves, and
hence will not give rise to a signal. Thus dark matter with a small charge is
likely not a source of a signal in current or upcoming dark matter direct
detection experiments.Comment: 19 pages, 2 figures; v2 - figures fixed, references adde
Further Considerations on the CP Asymmetry in Heavy Majorana Neutrino Decays
We work out the thermodynamic equations for the decays and scatterings of
heavy Majorana neutrinos including the constraints from unitarity. The
Boltzmann equations depend on the CP asymmetry parameter which contains both, a
self-energy and a vertex correction. At thermal equilibrium there is no net
lepton asymmetry due to the CPT theorem and the unitarity constraint. We show
explicitly that deviations from thermal equilibrium create the lepton
asymmetry.Comment: 16 pages, LaTeX, 1 eps figure, 1 ps figur
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