83 research outputs found
An ansatz for spacetimes of zero gravitational mass : global monopoles and textures
We propose a geometric ansatz, a restriction on Euclidean / Minkowski
distance in the embedding space being propotional to distance in the embedded
space, to generate spacetimes with vanishing gravitational mass (). It turns out that these spacetimes can represent
global monopoles and textures. Thus the ansatz is a prescription to generate
zero mass spacetimes that could describe topological defects, global monopoles
and textures.Comment: 9 pages, LaTeX versio
Momentum-dependent contributions to the gravitational coupling of neutrinos in a medium
When neutrinos travel through a normal matter medium, the electron neutrinos
couple differently to gravity compared to the other neutrinos, due to the
presence of electrons in the medium and the absence of the other charged
leptons. We calculate the momentum-dependent part of the matter-induced
gravitational couplings of the neutrinos under such conditions, which arise at
order , and determine their contribution to the neutrino dispersion
relation in the presence of a gravitational potential .
These new contributions vanish for the muon and tau neutrinos. For electron
neutrinos with momentum , they are of the order of the usual Wolfenstein
term times the factor , for high energy
neutrinos. In environments where the gravitational potential is substantial,
such as those in the vicinity of Active Galactic Nuclei, they could be the
dominant term in the neutrino dispersion relation. They must also be taken into
account in the analysis of possible violations of the Equivalence Principle in
the neutrino sector, in experimental settings involving high energy neutrinos
traveling through a matter background.Comment: Minor corrections in the references; one reference adde
Coherent Neutrino Interactions in a Dense Medium
Motivated by the effect of matter on neutrino oscillations (the MSW effect)
we study in more detail the propagation of neutrinos in a dense medium. The
dispersion relation for massive neutrinos in a medium is known to have a
minimum at nonzero momentum p \sim (G_F\rho)/\sqrt{2}. We study in detail the
origin and consequences of this dispersion relation for both Dirac and Majorana
neutrinos both in a toy model with only neutral currents and a single neutrino
flavour and in a realistic "Standard Model" with two neutrino flavours. We find
that for a range of neutrino momenta near the minimum of the dispersion
relation, Dirac neutrinos are trapped by their coherent interactions with the
medium. This effect does not lead to the trapping of Majorana neutrinos.Comment: 28 pages, 6 figures, Latex; minor changes, one reference added;
version to appear in Phys. Rev.
O(4) texture with a cosmological constant
We investigate O(4) textures in a background with a positive cosmological
constant. We find static solutions which co-move with the expanding background.
There exists a solution in which the scalar field is regular at the horizon.
This solution has a noninteger winding number smaller than one. There also
exist solutions in which scalar-field derivatives are singular at the horizon.
Such solutions can complete one winding within the horizon. If the winding
number is larger than some critical value, static solutions including the
regular one are unstable under perturbations.Comment: 25 pages, revtex, 6 eps figure
Maximum lepton asymmetry from active-sterile neutrino oscillations in the Early Universe
A large lepton asymmetry could be generated in the Early Universe by
oscillations of active to sterile neutrinos with a small mixing angle sin 2
\theta < 10^-2. The final order of magnitude of the lepton asymmetry \eta is
mainly determined by its growth in the last stage of evolution when the MSW
resonance dominates the kinetic equations. In this paper we present a simple
way of calculating the maximum possible lepton asymmetry which can be created.
Our results are in good agreement to previous calculations. Furthermore, we
find that the growth of asymmetry does not obey any particular power law. We
find that the maximum possible asymmetry at the freeze-out of the n/p ratio at
T \sim 1 MeV strongly depends on the mass-squared difference \delta m^2: the
asymmetry is negligible for \delta m^2 \ll 1 eV^2 and reaches asymptotically
large values for \delta m^2 \ge 50 eV^2.Comment: 14 pp, 4 figure
Electron-, Mu-, and Tau-Number Conservation in a Supernova Core
We study if the neutrino mixing parameters suggested by the atmospheric
neutrino anomaly imply chemical equilibrium between mu- and tau-flavored
leptons in a supernova (SN) core. The initial flavor-conversion rate would
indeed be fast if the nu_mu-nu_tau-mixing angle were not suppressed by
second-order refractive effects. The neutrino diffusion coefficients are
different for nu_mu, anti-nu_mu, nu_tau and anti-nu_tau so that neutrino
transport will create a net mu and tau lepton number density. This will
typically lead to a situation where the usual first-order refractive effects
dominate, further suppressing the rate of flavor conversion. Altogether,
neutrino refraction has the nontrivial consequence of guaranteeing the separate
conservation of e, mu, and tau lepton number in a SN core on the infall and
cooling time scales, even when neutrino mixing angles are large.Comment: Slightly expanded version with improved presentation, no changes of
substanc
Neutrino-Neutrino Scattering and Matter-Enhanced Neutrino Flavor Transformation in Supernovae
We examine matter-enhanced neutrino flavor transformation
() in the region above the neutrino
sphere in Type II supernovae. Our treatment explicitly includes contributions
to the neutrino-propagation Hamiltonian from neutrino-neutrino forward
scattering. A proper inclusion of these contributions shows that they have a
completely negligible effect on the range of - vacuum
mass-squared difference, , and vacuum mixing angle, , or
equivalently , required for enhanced supernova shock re-heating.
When neutrino background effects are included, we find that -process
nucleosynthesis from neutrino-heated supernova ejecta remains a sensitive probe
of the mixing between a light and a with a
cosmologically significant mass. Neutrino-neutrino scattering contributions are
found to have a generally small effect on the
parameter region probed by -process nucleosynthesis. We point out that the
nonlinear effects of the neutrino background extend the range of sensitivity of
-process nucleosynthesis to smaller values of .Comment: 38 pages, tex, DOE/ER/40561-150-INT94-00-6
Further studies on relic neutrino asymmetry generation II: a rigorous treatment of repopulation in the adiabatic limit
We derive an approximate relic neutrino asymmetry evolution equation that
systematically incorporates repopulation processes from the full quantum
kinetic equations (QKEs). It is shown that in the collision dominant epoch, the
said equation reduces precisely to the expression obtained previously from the
static/adiabatic approximation. The present treatment thus provides a rigorous
justification for the seemingly incongruous assumptions of a negligible
repopulation function and instantaneous repopulation sometimes employed in
earlier works.Comment: RevTeX, 11 pages, no figure
Textures and Newtonian Gravity
Newtonian theory is used to study the gravitational effects of a texture, in
particular the formation of massive structures.Comment: 4 pages, 4 ps figures, REVTEX, accepted for publication in PR
Neutrino-Mixing-Generated Lepton Asymmetry and the Primordial He Abundance
It has been proposed that an asymmetry in the electron neutrino sector may be
generated by resonant active-sterile neutrino transformations during Big Bang
Nucleosynthesis (BBN). We calculate the change in the primordial He yield
resulting from this asymmetry, taking into account both the time evolution
of the and distribution function and the spectral
distortions in these. We calculate this change in two schemes: (1) a lepton
asymmetry directly generated by mixing with a lighter right-handed
sterile neutrino ; and (2) a lepton asymmetry generated by a
or transformation
which is subsequently partially converted to an asymmetry in the
sector by a matter-enhanced active-active neutrino
transformation. In the first scheme, we find that the percentage change in
is between -1% and 9% (with the sign depending on the sign of the asymmetry),
bounded by the Majorana mass limit m_{\nu_e}\la 1 eV. In the second scheme,
the maximal percentage reduction in is 2%, if the lepton number asymmetry
in neutrinos is positive; Otherwise, the percentage increase in is \la 5%
for m^2_{\nu_\mu,\nu_\tau}-m^2_{\nu_s}\la 10^4 eV. We conclude that the
change in the primordial He yield induced by a neutrino-mixing-generated
lepton number asymmetry can be substantial in the upward direction, but limited
in the downward direction.Comment: 15 pages, 7 figures, submitted to PR
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