34 research outputs found
Damping the neutrino flavor pendulum by breaking homogeneity
The most general case of self-induced neutrino flavor evolution is described
by a set of kinetic equations for a dense neutrino gas evolving both in space
and time. Solutions of these equations have been typically worked out assuming
that either the time (in the core-collapse supernova environment) or space (in
the early universe) homogeneity in the initial conditions is preserved through
the evolution. In these cases one can gauge away the homogeneous variable and
reduce the dimensionality of the problem. In this paper we investigate if small
deviations from an initial postulated homogeneity can be amplified by the
interacting neutrino gas, leading to a new flavor instability. To this end, we
consider a simple two flavor isotropic neutrino gas evolving in time, and
initially composed by only and with equal densities. In the
homogeneous case, this system shows a bimodal instability in the inverted mass
hierarchy scheme, leading to the well studied flavor pendulum behavior. This
would lead to periodic pair conversions . To break space homogeneity, we introduce small amplitude
space-dependent perturbations in the matter potential. By Fourier transforming
the equations of motion with respect to the space coordinate, we then
numerically solve a set of coupled equations for the different Fourier modes.
We find that even for arbitrarily tiny inhomogeneities, the system evolution
runs away from the stable pendulum behavior: the different modes are excited
and the space-averaged ensemble evolves towards flavor equilibrium. We finally
comment on the role of a time decaying neutrino background density in weakening
these results.Comment: (7 pages, 5 eps figures. Figure improved. Final version appeared in
PRD
Self-induced flavor instabilities of a dense neutrino stream in a two-dimensional model
We consider a simplifed model for self-induced flavor conversions of a dense
neutrino gas in two dimensions, showing new solutions that spontaneously break
the spatial symmetries of the initial conditions. As a result of the symmetry
breaking induced by the neutrino-neutrino interactions, the coherent behavior
of the neutrino gas becomes unstable. This instability produces large spatial
variations in the flavor content of the ensemble. Furthermore, it also leads to
the creation of domains of different net lepton number flux. The transition of
the neutrino gas from a coherent to incoherent behavior shows an intriguing
analogy with a streaming flow changing from laminar to turbulent regime. These
finding would be relevant for the self-induced conversions of neutrinos
streaming-off a supernova core.Comment: (v2: revised version: 8 pages, 7 eps figures. To appear on Physical
Review D as Rapid Communication. Discussion enlarged. Two Appendices added.
Unveiling secret interactions among sterile neutrinos with big-bang nucleosynthesis
Short-baseline neutrino anomalies suggest the existence of low-mass ( m \sim
O(1)~eV) sterile neutrinos \nu_s. These would be efficiently produced in the
early universe by oscillations with active neutrino species, leading to a
thermal population of the sterile states seemingly incompatible with
cosmological observations. In order to relieve this tension it has been
recently speculated that new "secret" interactions among sterile neutrinos,
mediated by a massive gauge boson X (with M_X << M_W), can inhibit or suppress
the sterile neutrino thermalization, due to the production of a large matter
potential term. We note however, that they also generate strong collisional
terms in the sterile neutrino sector that induce an efficient sterile neutrino
production after a resonance in matter is encountered, increasing their
contribution to the number of relativistic particle species N_ eff. Moreover,
for values of the parameters of the \nu_s-\nu_s interaction for which the
resonance takes place at temperature T\lesssim few MeV, significant distortions
are produced in the electron (anti)neutrino spectra, altering the abundance of
light element in Big Bang Nucleosynthesis (BBN). Using the present
determination of He and deuterium primordial abundances we determine the
BBN constraints on the model parameters. We find that H/H density ratio
exclude much of the parameter space if one assume a baryon density at the best
fit value of Planck experiment, \Omega_B h^2= 0.02207, while bounds become
weaker for a higher \Omega_B h^2=0.02261, the 95 % C.L. upper bound of Planck.
Due to the large error on its experimental determination, the helium mass
fraction Y_p gives no significant bounds.Comment: v2: revised version. Minor changes: figures improved, references
updated. Matches the version to appear in Phys. Rev.
Cosmogenic neutrino fluxes under the effect of active-sterile secret interactions
Ultra High Energy cosmogenic neutrinos may represent a unique opportunity to
unveil possible new physics interactions once restricted to the neutrino sector
only. In the present paper we study the observable effects of a secret
active-sterile interactions, mediated by a pseudoscalar, on the expected flux
of cosmogenic neutrinos. The results show that for masses of sterile neutrinos
and pseudoscalars of hundreds MeV, necessary to evade cosmological,
astrophysical and elementary particle constraints, the presence of such new
interactions can significantly change the energy spectrum of cosmogenic
neutrinos at Earth in the energy range from PeV to ZeV. Interestingly, the
distortion of the spectrum results to be detectable at GRAND apparatus if the
scalar mediator mass is around 250 MeV and the UHECRs are dominated by the
proton component. Larger mediator masses or a chemical composition of UHECRs
dominated by heavier nuclei would require much larger cosmic rays apparatus
which might be available in future.Comment: 10 pages, 3 figure
Cosmic microwave background constraints on secret interactions among sterile neutrinos
Secret contact interactions among eV sterile neutrinos, mediated by a massive
gauge boson (with ), and characterized by a gauge coupling
, have been proposed as a mean to reconcile cosmological observations and
short-baseline laboratory anomalies. We constrain this scenario using the
latest Planck data on Cosmic Microwave Background anisotropies, and
measurements of baryon acoustic oscillations (BAO). We consistently include the
effect of secret interactions on cosmological perturbations, namely the
increased density and pressure fluctuations in the neutrino fluid, and still
find a severe tension between the secret interaction framework and cosmology.
In fact, taking into account neutrino scattering via secret interactions, we
derive our own mass bound on sterile neutrinos and find (at 95% CL) eV or eV from Planck alone or in combination with BAO,
respectively. These limits confirm the discrepancy with the laboratory
anomalies. Moreover, we constrain, in the limit of contact interaction, the
effective strength to be from Planck
(Planck+BAO). This result, together with the mass bound, strongly disfavours
the region with MeV and relatively large coupling , previously indicated as a possible solution to the small scale dark
matter problem.Comment: 15 pages, 3 figures, 4 table
Collisional production of sterile neutrinos via secret interactions and cosmological implications
Secret interactions among sterile neutrinos have been recently proposed as an
escape-route to reconcile eV sterile neutrino hints from short-baseline
anomalies with cosmological observations. In particular models with coupling
g_X \gtrsim 10^{-2} and gauge boson mediators with MeV
lead to large matter potential suppressing the sterile neutrino production
before the neutrino decoupling. With this choice of parameter ranges, big bang
nucleosynthesis is left unchanged and gives no bound on the model. However, we
show that at lower temperatures when active-sterile oscillations are no longer
matter suppressed, sterile neutrinos are still in a collisional regime, due to
their secret self-interactions. The interplay between vacuum oscillations and
collisions leads to a scattering-induced decoherent production of sterile
neutrinos with a fast rate. This process is responsible for a flavor
equilibration among the different neutrino species. We explore the effect of
this large sterile neutrino population on cosmological observables. We find
that a signature of strong secret interactions would be a reduction of the
effective number of neutrinos at matter radiation equality down
to 2.7. Moreover, for MeV sterile neutrinos would be
free-streaming before becoming non-relativistic and they would affect the
large-scale structure power spectrum. As a consequence, for this range of
parameters we find a tension of a eV mass sterile state with cosmological
neutrino mass bounds.Comment: (v2: 8 pages, 2 eps figures. Revised version: Major changes. Title
changed. Added a Section on the impact of secret interactions on . Cosmological mass bounds revised. References updated.
Multi-momentum and multi-flavour active-sterile neutrino oscillations in the early universe: role of neutrino asymmetries and effects on nucleosynthesis
We perform a study of the flavour evolution in the early universe of a
multi-flavour active-sterile neutrino system with parameters inspired by the
short-baseline neutrino anomalies. In a neutrino-symmetric bath a "thermal"
population of the sterile state would quickly grow, but in the presence of
primordial neutrino asymmetries a self-suppression as well as a resonant
sterile neutrino production can take place, depending on temperature and chosen
parameters. In order to characterize these effects, we go beyond the usual
average momentum and single mixing approximations and consider a multi-momentum
and multi-flavour treatment of the kinetic equations. We find that the
enhancement obtained in this case with respect to the average momentum
approximation is significant, up to \sim 20 % of a degree of freedom. Such
detailed and computationally demanding treatment further raises the asymmetry
values required to significantly suppress the sterile neutrino production, up
to large and preferentially net asymmetries |L_{\nu}| > O(10^{-2}). For such
asymmetries, however, the active-sterile flavour conversions happen so late
that significant distortions are produced in the electron (anti)neutrino
spectra. The larger |L_{\nu}|, the more the impact of these distortions takes
over as dominant cosmological effect, notably increasing the 4 He abundance in
primordial nucleosynthesis (BBN). The standard expression of the primordial
yields in terms of the effective number of neutrinos and asymmetries is also
greatly altered. We numerically estimate the magnitude of such effects for a
few representative cases and comment on possible implications for forthcoming
cosmological measurements.Comment: v2 (12 pages, 4 eps figures) revised version. Comments added,
references updated. Matches the version published in PR
No collective neutrino flavor conversions during the supernova accretion phase
The large neutrino fluxes emitted with a distinct flavor hierarchy from
core-collapse supernovae (SNe) during the post-bounce accretion phase, offer
the best opportunity to detect effects from neutrino flavor oscillations. We
perform a dedicated study of the SN neutrino flavor evolution during the
accretion phase, using results from recent neutrino radiation hydrodynamics
simulations. In contrast to what expected in the presence of only
neutrino-neutrino interactions, we find that the multi-angle effects associated
with the dense ordinary matter suppress collective oscillations. This is
related to the high matter densities during the accretion phase in
core-collapse SNe of massive iron-core progenitors. The matter suppression
implies that neutrino oscillations will start outside the neutrino transport
region and therefore will have a negligible impact on the neutrino heating and
the explosion dynamics. Furthermore, the possible detection of the next
galactic SN neutrino signal from the accretion phase, based on the usual
Mikheyev- Smirnov-Wolfenstein effect in the SN mantle and Earth matter effects,
can reveal the neutrino mass hierarchy in the case that the mixing angle
is not very small.Comment: (4 pages, 4 eps figures, v2 revised version. Discussion clarified.
Matches the version published on PRL
The strongest bounds on active-sterile neutrino mixing after Planck data
Light sterile neutrinos can be excited by oscillations with active neutrinos
in the early universe. Their properties can be constrained by their
contribution as extra-radiation, parameterized in terms of the effective number
of neutrino species N_ eff, and to the universe energy density today \Omega_\nu
h^2. Both these parameters have been measured to quite a good precision by the
Planck satellite experiment. We use this result to update the bounds on the
parameter space of (3+1) sterile neutrino scenarios, with an active-sterile
neutrino mass squared splitting in the range (10^{-5} - 10^2 ) eV^2. We
consider both normal and inverted mass orderings for the active and sterile
states. For the first time we take into account the possibility of two
non-vanishing active-sterile mixing angles. We find that the bounds are more
stringent than those obtained in laboratory experiments. This leads to a strong
tension with the short-baseline hints of light sterile neutrinos. In order to
relieve this disagreement, modifications of the standard cosmological scenario,
e.g. large primordial neutrino asymmetries, are required.Comment: v2 (9 pages, 10 eps figures) revised version. Discussion enlarged.
Included bounds from the Planck limit on the sterile neutrino mass.
References update