721 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.
Neutrinos and Cosmology: an update
We review the current cosmological status of neutrinos, with particular
emphasis on their effects on Big Bang Nucleosynthesis, Large Scale Structure of
the universe and Cosmic Microwave Background Radiation measurements.Comment: 4 pages, 1 figure, to appear in the proceedings of IFAE, Catania 200
"Discrepant hardenings" in cosmic ray spectra: a first estimate of the effects on secondary antiproton and diffuse gamma-ray yields
Recent data from CREAM seem to confirm early suggestions that primary cosmic
ray (CR) spectra at few TeV/nucleon are harder than in the 10-100 GeV range.
Also, helium and heavier nuclei spectra appear systematically harder than the
proton fluxes at corresponding energies. We note here that if the measurements
reflect intrinsic features in the interstellar fluxes (as opposed to local
effects) appreciable modifications are expected in the sub-TeV range for the
secondary yields, such as antiprotons and diffuse gamma-rays. Presently, the
ignorance on the origin of the features represents a systematic error in the
extraction of astrophysical parameters as well as for background estimates for
indirect dark matter searches. We find that the spectral modifications are
appreciable above 100 GeV, and can be responsible for ~30% effects for
antiprotons at energies close to 1 TeV or for gamma's at energies close to 300
GeV, compared to currently considered predictions based on simple extrapolation
of input fluxes from low energy data. Alternatively, if the feature originates
from local sources, uncorrelated spectral changes might show up in antiproton
and high-energy gamma-rays, with the latter ones likely dependent from the
line-of-sight.Comment: 6 pages, 3 figures. Clarifications and references added, conclusions
unchanged. Matches published versio
A loophole to the universal photon spectrum in electromagnetic cascades: application to the "cosmological lithium problem"
The standard theory of electromagnetic cascades onto a photon background
predicts a quasi-universal shape for the resulting non-thermal photon spectrum.
This has been applied to very disparate fields, including non-thermal big bang
nucleosynthesis (BBN). However, once the energy of the injected photons falls
below the pair-production threshold the spectral shape is very different, a
fact that has been overlooked in past literature. This loophole may have
important phenomenological consequences, since it generically alters the BBN
bounds on non-thermal relics: for instance it allows to re-open the possibility
of purely electromagnetic solutions to the so-called "cosmological lithium
problem", which were thought to be excluded by other cosmological constraints.
We show this with a proof-of-principle example and a simple particle physics
model, compared with previous literature.Comment: 5 pages, 2 figures, typos corrected; matches version published in
PRL. (Version 1 of this article was submitted to arxiv on Jan. 8th, kept on
hold by arxiv moderators due to unspecified classification doubts for almost
one month.
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
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