10,908 research outputs found
Dark matter and Higgs boson physics
A vector-like colorless fermion doublet and a singlet added to the Standard
Model allow a consistent interpretation of dark matter in terms of the lightest
neutral particle, as they may help in obtaining successful gauge coupling
unification. We analyze in detail the mass range of the lightest neutral
particle below the W mass, i.e. in a range of the parameters where the physics
of the Standard Model Higgs boson may be substantially affected either directly
or indirectly.Comment: 17 pages, 7 figures. v3: published version (small corrections
Revised Canonical Quantum Gravity via the Frame Fixing
We present a new reformulation of the canonical quantum geometrodynamics,
which allows to overcome the fundamental problem of the frozen formalism and,
therefore, to construct an appropriate Hilbert space associate to the solution
of the restated dynamics. More precisely, to remove the ambiguity contained in
the Wheeler-DeWitt approach, with respect to the possibility of a (3 +
1)-splitting when the space-time is in a quantum regime, we fix the reference
frame (i.e. the lapse function and the shift vector) by introducing the
so-called kinematical action; as a consequence the new super-Hamiltonian
constraint becomes a parabolic one and we arrive to a Schroedinger-like
approach for the quantum dynamics. In the semiclassical limit our theory
provides General Relativity in the presence of an additional energy-momentum
density contribution coming from no longer zero eigenvalues of the Hamiltonian
constraints; the interpretation of these new contributions comes out in natural
way as soon as it is recognized that the kinematical action can be recasted in
such a way it describes a pressureless, but, in general, non geodesic perfect
fluid.Comment: 24 pages, 0 figures, to appear on Int. Jour. Mod. Phys.
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
An Open Inflationary Model for Dimensional Reduction and its Effects on the Observable Parameters of the Universe
Assuming that higher dimensions existed in the early stages of the universe
where the evolution was inflationary, we construct an open, singularity-free,
spatially homogeneous and isotropic cosmological model to study the effects of
dimensional reduction that may have taken place during the early stages of the
universe. We consider dimensional reduction to take place in a stepwise manner
and interpret each step as a phase transition. By imposing suitable boundary
conditions we trace their effects on the present day parameters of the
universe.Comment: 5 pages, accepted for publication in Int. J. of Mod. Phys.
Exploring the Expansion History of the Universe
Exploring the recent expansion history of the universe promises insights into
the cosmological model, the nature of dark energy, and potentially clues to
high energy physics theories and gravitation. We examine the extent to which
precision distance-redshift observations can map out the history, including the
acceleration-deceleration transition, and the components and equations of state
of the energy density. We consider the ability to distinguish between various
dynamical scalar field models for the dark energy, as well as higher dimension
and alternate gravity theories. Finally, we present a new, advantageous
parametrization for the study of dark energy.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Letter
Singlet fermion dark matter and electroweak baryogenesis with radiative neutrino mass
The model of radiative neutrino mass with dark matter proposed by one of us
is extended to include a real singlet scalar field. There are then two
important new consequences. One is the realistic possibility of having the
lightest neutral singlet fermion (instead of the lightest neutral component of
the dark scalar doublet) as the dark matter of the Universe. The other is a
modification of the effective Higgs potential of the Standard Model, consistent
with electroweak baryogenesis.Comment: 9 pages, no figure
Particle-Antiparticle Asymmetry Due to Non-Renormalizable Effective Interactions
We consider a model for generating a particle-antiparticle asymmetry through
out-of-equilibrium decays of a massive particle due to non-renormalizable,
effective interactions.Comment: preliminary version, 38 pages; LaTeX source, epsf.sty and EPS files
included in tar archiv
v4: A small, but sensitive observable for heavy ion collisions
Higher order Fourier coefficients of the azimuthally dependent single
particle spectra resulting from noncentral heavy ion collisions are
investigated. For intermediate to large transverse momenta, these anisotropies
are expected to become as large as 5 %, and should be clearly measurable. The
physics content of these observables is discussed from two different extreme
but complementary viewpoints, hydrodynamics and the geometric limit with
extreme energy loss.Comment: as published: typos corrected, Fig. 3 slightly improved in numerics
and presentatio
Abundance of Cosmological Relics in Low-Temperature Scenarios
We investigate the relic density n_\chi of non-relativistic long-lived or
stable particles \chi in cosmological scenarios in which the temperature T is
too low for \chi to achieve full chemical equilibrium. The case with a heavier
particle decaying into \chi is also investigated. We derive approximate
solutions for n_\chi(T) which accurately reproduce numerical results when full
thermal equilibrium is not achieved. If full equilibrium is reached, our ansatz
no longer reproduces the correct temperature dependence of the \chi number
density. However, it does give the correct final relic density, to an accuracy
of about 3% or better, for all cross sections and initial temperatures.Comment: 26 pages, 8 figures, comments added, to appear in Phys. Rev.
Nonthermal Supermassive Dark Matter
We discuss several cosmological production mechanisms for nonthermal
supermassive dark matter and argue that dark matter may be elementary particles
of mass much greater than the weak scale. Searches for dark matter should not
be limited to weakly interacting particles with mass of the order of the weak
scale, but should extend into the supermassive range as well.Comment: 11 page LaTeX file. No major changes. Version accepted by PR
- …