604 research outputs found
The Nuclear Reactions in Standard BBN
Nowadays, the Cosmic Microwave Background (CMB) anisotropies studies
accurately determine the baryon fraction omega_b, showing an overall and
striking agreement with previous determinations of omega_b obtained from Big
Bang Nucleosynthesis (BBN). However, a deeper comparison of BBN predictions
with the determinations of the primordial light nuclides abundances shows
slight tensions, motivating an effort to further improve the accuracy of
theoretical predictions, as well as to better evaluate systematics in both
observations and nuclear reactions measurements. We present some results of an
important step towards an increasing precision of BBN predictions, namely an
updated and critical review of the nuclear network, and a new protocol to
perform the nuclear data regression.Comment: 4 pp.,4figs. Few typos corrected and updated refs. to match the
version appearing in the proceedings of Conference ``Nuclei in the Cosmos
VIII'', Vancouver, BC, Canada, 19-23 Jul 2004, published in Nucl. Phys.
Cosmological neutrino mass detection: The best probe of neutrino lifetime
Future cosmological data may be sensitive to the effects of a finite sum of
neutrino masses even as small as ~0.06 eV, the lower limit guaranteed by
neutrino oscillation experiments. We show that a cosmological detection of
neutrino mass at that level would improve by many orders of magnitude the
existing limits on neutrino lifetime, and as a consequence on neutrino secret
interactions with (quasi-)massless particles as in majoron models. On the other
hand, neutrino decay may provide a way-out to explain a discrepancy <~ 0.1 eV
between cosmic neutrino bounds and Lab data.Comment: 5 pages, 1 eps figure; clarifications and references added, improved
discussion, conclusions unchanged. Matches version published in PR
Model-independent dark matter annihilation bound from the diffuse gamma ray flux
An upper limit on the total annihilation cross section of dark matter (DM)
has recently been derived from the observed atmospheric neutrino background. We
show that comparable bounds are obtained for DM masses around the TeV scale by
observations of the diffuse gamma-ray flux by EGRET, because electroweak
bremsstrahlung leads to non-negligible electromagnetic branching ratios, even
if DM particles only couple to neutrinos at tree level. A better mapping and
the partial resolution of the diffuse gamma-ray background into astrophysical
sources by the GLAST satellite will improve this bound in the near future.Comment: 4 pages revtex, 2 figures; minor changes, references added,
conclusions unchanged; Matches published versio
A robust upper limit on N_eff from BBN, circa 2011
We derive here a robust bound on the effective number of neutrinos from
constraints on primordial nucleosynthesis yields of deuterium and helium. In
particular, our results are based on very weak assumptions on the astrophysical
determination of the helium abundance, namely that the minimum effect of
stellar processing is to keep constant (rather than increase, as expected) the
helium content of a low-metallicity gas. Using the results of a recent analysis
of extragalactic HII regions as upper limit, we find that Delta Neff<= 1 at 95
% C.L., quite independently of measurements on the baryon density from cosmic
microwave background anisotropy data and of the neutron lifetime input. In our
approach, we also find that primordial nucleosynthesis alone has no significant
preference for an effective number of neutrinos larger than the standard value.
The ~2 sigma hint sometimes reported in the literature is thus driven by CMB
data alone and/or is the result of a questionable regression protocol to infer
a measurement of primordial helium abundance.Comment: 5 pages, 1 table, 1 figure. Minor improvements and extensions in the
analysis, clarifications and reference added, conclusions slightly
strengthened. Matches version published in Phys. Lett.
Standard and non-standard primordial neutrinos
The standard cosmological model predicts the existence of a cosmic neutrino
background with a present density of about 110 cm^{-3} per flavour, which
affects big-bang nucleosynthesis, cosmic microwave background anisotropies, and
the evolution of large scale structures. We report on a precision calculation
of the cosmic neutrino background properties including the modification
introduced by neutrino oscillations. The role of a possible
neutrino-antineutrino asymmetry and the impact of non-standard
neutrino-electron interactions on the relic neutrinos are also briefly
discussed.Comment: 4 pages, no figures. Contribution to the proceedings of SNOW 2006,
Stockholm, May 2-6, 2006. Typos corrected, updated reference
Measuring the 13-mixing angle and the CP phase with neutrino telescopes
The observed excess of high-energy cosmic rays from the Galactic plane in the
energy range \sim 10^18 eV may be naturally explained by neutron primaries
generated in the photo-dissociation of heavy nuclei. In this scenario, neutrons
with lower energy decay before reaching the Earth and produce a detectable flux
in a 1 km^3 neutrino telescope. The initial flavor composition of these
neutrinos, \phi(\bar\nu_e):\phi(\bar\nu_\mu):\phi(\bar\nu_\tau)=1:0:0, offers
the opportunity to perform a combined \bar\nu_\mu/\bar\nu_\tau appearance and
\bar\nu_e disappearance experiment. The observable ratio
\phi(\bar\nu_\mu)/\phi(\bar\nu_e+\bar\nu_\tau) of fluxes arriving on Earth
depends appreciably on the 13-mixing angle \theta_13 and the leptonic CP phase
\delta_CP, opening thus a new experimental avenue to measure these two
quantities.Comment: 4 pages, 2 eps figures. Enlarged discussion, references added.
Matches version to appear in PR
Conservative effects in spin-transfer-driven magnetization dynamics
It is shown that under appropriate conditions spin-transfer-driven
magnetization dynamics in a single-domain nanomagnet is conservative in nature
and admits a specific integral of motion, which is reduced to the usual
magnetic energy when the spin current goes to zero. The existence of this
conservation law is connected to the symmetry properties of the dynamics under
simultaneous inversion of magnetisation and time. When one applies an external
magnetic field parallel to the spin polarization, the dynamics is transformed
from conservative into dissipative. More precisely, it is demonstrated that
there exists a state function such that the field induces a monotone relaxation
of this function toward its minima or maxima, depending on the field
orientation. These results hold in the absence of intrinsic damping effects.
When intrinsic damping is included in the description, a competition arises
between field-induced and damping-induced relaxations, which leads to the
appearance of limit cycles, that is, of magnetization self-oscillations.Comment: 5 pages, 3 figure
Bremsstrahlung gamma rays from light Dark Matter
We discuss the often-neglected role of bremsstrahlung processes on the
interstellar gas in computing indirect signatures of Dark Matter (DM)
annihilation in the Galaxy, particularly for light DM candidates in the
phenomenologically interesting O(10) GeV mass range. Especially from directions
close to the Galactic Plane, the expected gamma-ray spectrum is altered via two
effects: directly, by the photons emitted in the bremsstrahlung process on the
interstellar gas by energetic electrons which are among the DM annihilation
byproducts; indirectly, by the modification of the same electron spectrum, due
to the additional energy loss process in the diffusion-loss equation (e.g. the
resulting inverse Compton emission is altered). We quantify the importance of
the bremsstrahlung emission in the GeV energy range, showing that it is the
dominant component of the gamma-ray spectrum for some cases. We also find that,
in regions in which bremsstrahlung dominates energy losses, the related
gamma-ray emission is only moderately sensitive to possible large variations in
the gas density. Still, we stress that, for computing precise spectra in the
(sub-)GeV range, it is important to obtain a reliable description of the inner
Galaxy gas distribution as well as to compute self-consistently the gamma
emission and the solution to the diffusion-loss equation. For example, these
are crucial issues to quantify and interpret meaningfully gamma-ray map
`residuals' in terms of (light) DM annihilations.Comment: 17 pages, 5 figures, 1 table; references added, changed to match the
published versio
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
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