92 research outputs found
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
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.
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.
"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.
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
Dark Matter annihilations in halos and high-redshift sources of reionization of the universe
It is well known that annihilations in the homogeneous fluid of dark matter
(DM) can leave imprints in the cosmic microwave background (CMB) anisotropy
power spectrum. However, the relevance of DM annihilations in halos for
cosmological observables is still subject to debate, with previous works
reaching different conclusions on this point. Also, all previous studies used a
single type of parameterization for the astrophysical reionization, and
included no astrophysical source for the heating of the intergalactic medium.
In this work, we revisit these problems. When standard approaches are adopted,
we find that the ionization fraction does exhibit a very particular (and
potentially constraining) pattern, but the currently measurable optical depth
to reionization is left almost unchanged: In agreement with the most of the
previous literature, for plausible halo models we find that the modification of
the signal with respect to the one coming from annihilations in the smooth
background is tiny, below cosmic variance within currently allowed parameter
space. However, if different and probably more realistic treatments of the
astrophysical sources of reionization and heating are adopted, a more
pronounced effect of the DM annihilation in halos is possible. We thus conclude
that within currently adopted baseline models the impact of the virialised DM
structures cannot be uncovered by CMB power spectra measurements, but a larger
impact is possible if peculiar models are invoked for the redshift evolution of
the DM annihilation signal or different assumptions are made for the
astrophysical contributions. A better understanding (both theoretical and
observational) of the reionization and temperature history of the universe,
notably via the 21 cm signal, seems the most promising way for using halo
formation as a tool in DM searches, improving over the sensitivity of current
cosmological probes.Comment: 30 pages, 11 figures. v2: extended version (notably astrophysical
source effects significantly expanded), references added, main conclusions
unchanged. Matches version accepted by JCA
A fresh look at linear cosmological constraints on a decaying dark matter component
We consider a cosmological model in which a fraction of the Dark Matter
(DM) is allowed to decay in an invisible relativistic component, and compute
the resulting constraints on both the decay width (or inverse lifetime)
and from purely gravitational arguments. We report a full
derivation of the Boltzmann hierarchy, correcting a mistake in previous
literature, and compute the impact of the decay --as a function of the
lifetime-- on the CMB and matter power spectra. From CMB only, we obtain that
no more than 3.8 % of the DM could have decayed in the time between
recombination and today (all bounds quoted at 95 % CL). We also comment on the
important application of this bound to the case where primordial black holes
constitute DM, a scenario notoriously difficult to constrain. For lifetimes
longer than the age of the Universe, the bounds can be cast as Gyr. For the first time, we also checked that
degeneracies with massive neutrinos are broken when information from the large
scale structure is used. Even secondary effects like CMB lensing suffice to
this purpose. Decaying DM models have been invoked to solve a possible tension
between low redshift astronomical measurements of and and the ones inferred by Planck. We reassess this claim finding that with
the most recent BAO, HST and data extracted from the CFHT survey,
the tension is only slightly reduced despite the two additional free
parameters, loosening the bound to Gyr.
The bound however improves to Gyr if only
data consistent with the CMB are included. This highlights the importance of
establishing whether the tension is due to real physical effects or unaccounted
systematics, for settling the reach of achievable constraints on decaying DM.Comment: 30p, 11 figures, comments welcom
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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