Massive neutrinos and cosmology

The present experimental results on neutrino flavour oscillations provide evidence for non-zero neutrino masses, but give no hint on their absolute mass scale, which is the target of beta decay and neutrinoless double-beta decay experiments. Crucial complementary information on neutrino masses can be obtained from the analysis of data on cosmological observables, such as the anisotropies of the cosmic microwave background or the distribution of large-scale structure. In this review we describe in detail how free-streaming massive neutrinos affect the evolution of cosmological perturbations. We summarize the current bounds on the sum of neutrino masses that can be derived from various combinations of cosmological data, including the most recent analysis by the WMAP team. We also discuss how future cosmological experiments are expected to be sensitive to neutrino masses well into the sub-eV range.Comment: 122 pages, 23 figures, misprints corrected and references added. Review article to be published in Physics Report

Flavor oscillations in the supernova hot bubble region: Nonlinear effects of neutrino background

The neutrino flux close to a supernova core contributes substantially to neutrino refraction so that flavor oscillations become a nonlinear phenomenon. One unexpected consequence is efficient flavor transformation for anti-neutrinos in a region where only neutrinos encounter an MSW resonance or vice versa. Contrary to previous studies we find that in the neutrino-driven wind the electron fraction Y_e always stays below 0.5, corresponding to a neutron-rich environment as required by r-process nucleosynthesis. The relevant range of masses and mixing angles includes the region indicated by LSND, but not the atmospheric or solar oscillation parameters.Comment: 5 pages, 2 figures (second one available in color and b/w). To be published in PR

Model independent constraints on mass-varying neutrino scenarios

Models of dark energy in which neutrinos interact with the scalar field supposed to be responsible for the acceleration of the universe usually imply a variation of the neutrino masses on cosmological time scales. In this work we propose a parameterization for the neutrino mass variation that captures the essentials of those scenarios and allows to constrain them in a model independent way, that is, without resorting to any particular scalar field model. Using WMAP 5yr data combined with the matter power spectrum of SDSS and 2dFGRS, the limit on the present value of the neutrino mass is $m_0 \equiv m_{\nu}(z=0) < 0.43 (0.28)$ eV at 95% C.L. for the case in which the neutrino mass was lighter (heavier) in the past, a result competitive with the ones imposed for standard (i.e., constant mass) neutrinos. Moreover, for the ratio of the mass variation of the neutrino mass $\Delta m_{\nu}$ over the current mass $m_0$ we found that $\log[|\Delta m_{\nu}|/m_0] < -1.3 (-2.7)$ at 95% C.L. for $\Delta m_{\nu} 0)$, totally consistent with no mass variation. These stringent bounds on the mass variation are not related to the neutrino free-streaming history which may affect the matter power spectrum on small scales. On the contrary, they are imposed by the fact that any significant transfer of energy between the neutrino and dark energy components would lead to an instability contradicting CMB and large scale structure data on the largest observable scales.Comment: 13 pages, 7 figures, 2 tables. Some few comments and references added. To be published in PR

Lepton Flavour Violation in a Left-Right Symmetric Model

We consider in this paper a Left-Right symmetric gauge model in which a global lepton-number-like symmetry is introduced and broken spontaneously at a scale that could be as low as 10^4 GeV or so. The corresponding physical Nambu-Goldstone boson, which we call majoron and denote J, can have tree-level flavour-violating couplings to the charged fermions, leading to sizeable majoron-emitting lepton-flavour-violating weak decays. We consider explicitly a leptonic variant of the model and show that the branching ratios for \mu -> e+J, \tau -> e + J and \tau -> \mu + J decays can be large enough to fall within the sensitivities of future \mu and \tau factories. On the other hand the left-right gauge symmetry breaking scale may be as low as few TeV.Comment: LaTeX, 16 pages, 3 PS figures, uses JHEP.cls, published versio