789 research outputs found
Extra dimensions, orthopositronium decay, and stellar cooling
In a class of extra dimensional models with a warped metric and a single
brane the photon can be localized on the brane by gravity only. An intriguing
feature of these models is the possibility of the photon escaping into the
extra dimensions. The search for this effect has motivated the present round of
precision orthopositronium decay experiments. We point out that in this
framework a photon in plasma should be metastable. We consider the
astrophysical consequences of this observation, in particular, what it implies
for the plasmon decay rate in globular cluster stars and for the core-collapse
supernova cooling rate. The resulting bounds on the model parameter exceed the
possible reach of orthopositronium experiments by many orders of magnitude.Comment: 13 pages, no figure
Red giant bound on the axion-electron coupling reexamined
If axions or other low-mass pseudoscalars couple to electrons (``fine
structure constant'' ) they are emitted from red giant stars by the
Compton process and by bremsstrahlung .
We construct a simple analytic expression for the energy-loss rate for all
conditions relevant for a red giant and include axion losses in evolutionary
calculations from the main sequence to the helium flash. We find that
\alpha_a\lapprox0.5\mn(-26) or m_a\lapprox 9\,\meV/\cos^2\beta lest the red
giant core at helium ignition exceed its standard mass by more than
0.025\,\MM_\odot, in conflict with observational evidence. Our bound is the
most restrictive limit on , but it does not exclude the possibility
that axion emission contributes significantly to the cooling of ZZ~Ceti stars
such as G117--B15A for which the period decrease was recently measured.Comment: 11 pages, uuencoded and compressed postscript fil
Pseudoscalar Conversion and X-rays from the Sun
We investigate the detection of a pseudoscalar that couples
electromagnetically via an interaction . In
particular, we focus on the conversion of pseudoscalars produced in the sun's
interior in the presence of the sun's external magnetic dipole field and
sunspot-related magnetic fields. We find that the sunspot approach is superior.
Measurements by the SXT on the Yohkoh satellite can measure the coupling
constant down to --, provided the
pseudoscalar mass eV, which makes it competitive with
other astrophysical approaches.Comment: 15 pages, RevTex file. Figures available upon request to
[email protected]. (please include full mailing address in
request). Submitted to Physics Letters
Lepton asymmetry and primordial nucleosynthesis in the era of precision cosmology
We calculate and display the primordial light-element abundances as a
function of a neutrino degeneracy parameter \xi common to all flavors. It is
the only unknown parameter characterizing the thermal medium at the primordial
nucleosynthesis epoch. The observed primordial helium abundance Y_p is the most
sensitive cosmic ``leptometer.'' Adopting the conservative Y_p error analysis
of Olive and Skillman implies -0.04 \alt \xi \alt 0.07 whereas the errors
stated by Izotov and Thuan imply \xi=0.0245+-0.0092 (1 sigma). Improved
determinations of the baryon abundance have no significant impact on this
situation. A determination of Y_p that reliably distinguishes between a
vanishing or nonvanishing \xi is a crucial test of the cosmological standard
assumption that sphaleron effects equilibrate the cosmic lepton and baryon
asymmetries.Comment: 5 pages, 2 figures; minor changes, references added, replaced to
match the published version in PRD (Brief Reports
Earth matter effects in supernova neutrinos: Optimal detector locations
A model-independent experimental signature for flavor oscillations in the
neutrino signal from the next Galactic supernova (SN) would be the observation
of Earth matter effects. We calculate the probability for observing a Galactic
SN shadowed by the Earth as a function of the detector's geographic latitude.
This probability depends only mildly on details of the Galactic SN
distribution. A location at the North Pole would be optimal with a shadowing
probability of about 60%, but a far-northern location such as Pyhasalmi in
Finland, the proposed site for a large-volume scintillator detector, is almost
equivalent (58%). We also consider several pairs of detector locations and
calculate the probability that only one of them is shadowed, allowing a
comparison between a shadowed and a direct signal. For the South Pole combined
with Kamioka this probability is almost 75%, for the South Pole combined with
Pyhasalmi it is almost 90%. One particular scenario consists of a large-volume
scintillator detector located in Pyhasalmi to measure the geo-neutrino flux in
a continental location and another such detector in Hawaii to measure it in an
oceanic location. The probability that only one of them is shadowed exceeds 50%
whereas the probability that at least one is shadowed is about 80%. We provide
an online tool to calculate different shadowing probabilities for the one- and
two-detector cases.Comment: v2: 17 pages, 6 eps figures. Typos removed, matches the published
version. Online tool to calculate the Earth shadowing probabilities available
at http://www.mppmu.mpg.de/supernova/shadowing . High-resolution color
version of fig_2a and fig_2b available at
http://www.mppmu.mpg.de/supernova/shadowing/ma
Decoherence in supernova neutrino transformations suppressed by deleptonization
In the dense-neutrino region at 50-400 km above the neutrino sphere in a
supernova, neutrino-neutrino interactions cause large flavor transformations.
We study when the multi-angle nature of the neutrino trajectories leads to
flavor decoherence between different angular modes. We consider a two-flavor
mixing scenario between nu_e and another flavor nu_x and assume the usual
hierarchy F(nu_e)>F{antinu_e)>F(nu_x)=F(antinu_x) for the number fluxes. We
define epsilon=(F(nu_e)-F(antinu_e))/(F(antinu_e)-F(antinu_x)) as a measure for
the deleptonization flux which is the one crucial parameter. The transition
between the quasi single-angle behavior and multi-angle decoherence is abrupt
as a function of epsilon. For typical choices of other parameters, multi-angle
decoherence is suppressed for epsilon>0.3, but a much smaller asymmetry
suffices if the neutrino mass hierarchy is normal and the mixing angle small.
The critical epsilon depends logarithmically on the neutrino luminosity. In a
realistic supernova scenario, the deleptonization flux is probably enough to
suppress multi-angle decoherence.Comment: 17 pages, 12 figures. Misprint in Eq (14) correcte
Self-induced decoherence in dense neutrino gases
Dense neutrino gases exhibit collective oscillations where "self-maintained
coherence" is a characteristic feature, i.e., neutrinos of different energies
oscillate with the same frequency. In a non-isotropic gas, however, the flux
term of the neutrino-neutrino interaction has the opposite effect of causing
kinematical decoherence of neutrinos propagating in different directions, an
effect that is at the origin of the "multi-angle behavior" of neutrinos
streaming off a supernova core. We cast the equations of motion in a form where
the role of the flux term is manifest. We study in detail the symmetric case of
equal neutrino and antineutrino densities where the evolution consists of
collective pair conversions ("bipolar oscillations"). A gas of this sort is
unstable in that an infinitesimal anisotropy is enough to trigger a run-away
towards flavor equipartition. The "self-maintained coherence" of a perfectly
isotropic gas gives way to "self-induced decoherence."Comment: Revtex, 16 pages, 12 figure
Detecting Axion-Like Particles With Gamma Ray Telescopes
We propose that axion-like particles (ALPs) with a two-photon vertex,
consistent with all astrophysical and laboratory bounds, may lead to a
detectable signature in the spectra of high-energy gamma ray sources. This
occurs as a result of gamma rays being converted into ALPs in the magnetic
fields of efficient astrophysical accelerators according to the "Hillas
criterion", such as jets of active galactic nuclei or hot spots of radio
galaxies. The discovery of such an effect is possible by GLAST in the 1-100 GeV
range and by ground based gamma ray telescopes in the TeV range.Comment: corrected typos, one plot modified, material rearranged for
clarification. Conclusions unchanged. Matches version published in Phys. Rev.
Let
On some singularities of the correlation functions that determine neutrino opacities
Certain perturbation graphs in the calculation of the effects of the medium
on neutrino scattering in supernova matter have a nonintegrable singularity in
a physical region. A number of papers have addressed the apparent pathology
through an ansatz that invokes higher order (rescattering) effects. Taking the
Gamow-Teller terms as an example, we display an expression for the spin-spin
correlation function that determines the cross-sections. It is clear from the
form that there are no pathologies in the order by order perturbation
expansion. Explicit formulae are given for a simple case, leading to an answer
that is very different from one given by other authors.Comment: 8 page
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