Relic Neutrino Background Research Articles

High energy cosmic rays from neutrinos

We discuss recent models in which neutrinos, which are assumed to have mass in the eV range, originate the highest energy cosmic rays by interaction with the enhanced density in the galactic halo of the relic cosmic neutrino background. We make an analytical calculation of the required neutrino fluxes to show that the parameter space for these models is constrained by horizontal air shower searches and by the total number of background neutrinos, so that only models which have fairly unnatural halo sizes and enhanced densities are allowed.

Extremely High Energy Neutrinos, Neutrino Hot Dark Matter, and the Highest Energy Cosmic Rays

Extremely high energy (up to 10**(22) eV) cosmic neutrino beams initiate high energy particle cascades in the background of relic neutrinos from the Big Bang. We perform numerical calculations to show that such cascades could contribute more than 10% to the observed cosmic ray flux above 10**(19) eV if neutrinos have masses in the electron volt range. The required intensity of primary neutrinos could be consistent with astrophysical models for their production if the maximum neutrino energy reaches to 10**(22) eV and the massive neutrino dark matter is locally clustered. Future observations of ultra high energy cosmic rays will lead to an indirect but practical search for neutrino dark matter.

Evolution of the cosmic gas and the relic supernova neutrino background

Using the redshift evolution of the cosmic gas, as inferred from QSO absorption line studies, we calculate the past supernova rate and the relic supernova neutrino background. Using this new technique we find the predicted relic neutrino flux at low energies to be at least an order of magnitude below previous estimates. We argue that the evolution of the cosmic gas is consistent with a large decrease in the number of early-type galaxies at redshifts $\sim 3$, and that this evolution is the source of the reduction in the predicted neutrino flux. Additional observational constraints from recent redshift surveys lead us to propose a modified model for the evolution of the cosmic gas in which significant infall at low redshift occurs. We discuss the possible relevance of our calculations to the X-ray emitting metal-enriched gas observed in the intergalactic medium.

EFFECTS OF NEUTRINO OSCILLATION ON THE SUPERNOVA RELIC NEUTRINO BACKGROUND

We investigate to what extent the oscillation or conversion of neutrinos enhances the expected event rate of the supernova relic neutrino background (SRN) at the SuperKamiokande detector (SK). The SRN [Formula: see text] can be almost completely exchanged with vμ-like neutrinos by the MSW oscillation under the inverse mass hierarchy with Δm2~ 10−8–105[eV2], or by the magnetic moment of Majorana neutrinos with μv≳10−12μB and Δm2~10−4–10° [eV2]. In the standard calculation of the SRN flux, the event rate of the SRN [Formula: see text] at the SK in the observable energy range of 15–40 MeV can be enhanced from 1.2 yr−1 to 2.4 yr−1 if all [Formula: see text] are exchanged with vμ-like neutrinos. The enhancement is prominent especially in the high energy range (≳ 25 MeV). In the astrophysically optimistic calculation, the event rate becomes as high as 9.4 yr−1. Because the theoretical upper bound of the SRN events without oscillation is about 5 yr−1 taking into account the various astrophysical uncertainties, we might have to resort to the neutrino oscillation if more than 5 events in a year, as well as a significantly harder spectrum, were observed in the SK.

Spectrum of the Supernova Relic Neutrino Background and Evolution of Galaxies

The spectrum of the supernova relic neutrino background (SRN) from collapse-driven supernovae ever occurred in the universe is calculated by using a realistic, time-dependent supernova rate derived from a standard model of galaxy evolution based on the population synthesis method. The SRN spectrum we show here is the most realistic at present, because the largest uncertainty in previous theoretical predictions has come from unrealistic assumptions of the supernova rate so far made. The SRN is one of the targets of the Superkamiokande (SK) detector which will be constructed in a year and the SRN, if at all detected, would provide a new tool to probe the history of supernova explosions in the universe. The expected event rate at the SK is therefore calculated in this paper. Our major results include: (1) the supernova rate is much higher in the early phase of evolution of galaxies and there appears a hump in the SRN spectrum in the low-energy region of $\ltilde 5$ MeV, (2) the SRN flux depends on the Hubble constant ($H_0$) in a way approximately proportional to $H_0^2$ and only weakly on the density parameter of the universe ($\Omega_0$) and a cosmological constant ($\lambda_0$), and (3) the plausible event rate at the SK is 1.2 yr$^{-1}$ in the observable energy range. Such a low event rate is due mainly to a quite low supernova rate at present which is averaged over the morphological types of galaxies. The most optimistic rate in our model is found to be 4.7 yr$^{-1}$, and if more events are detected, we will have to reconsider our current understanding of collapse-driven supernovae and evolution of galaxies.

Spectrum of the relic neutrino background from past supernovae and cosmological models

It is greatly expected that the relic neutrino background from past supernovae is detected by Superkamiokande (SK) which is now under construction. We calculate the spectrum and the event rate at SK systematically by using the results of simulations of a supernova explosion and reasonable supernova rates. We also investigate the effect of a cosmological constant, $\Lambda$, on the spectrum, since some recent cosmological observations strongly suggest the existence of $\Lambda$. We find following results. 1) The spectrum has a peak at about 3 MeV, which is much lower than that of previous estimate ($6 \sim$ 10 MeV). 2) The event rate at SK in the range from 10 MeV to 50 MeV, where the relic neutrinos from past supernovae is dominant, is about $25 \, {h_{50}}^2 \, \left( \frac{R_{SN}}{0.1 {\rm yr^{-1}}} \right) \left(\frac{n_G \, h_{50}^{-3}}{0.02{\rm Mpc}^{-3}}\right)$ events per year, where $R_{SN}$ is the supernova rate in a galaxy, $n_G$ is the number density of galaxies, and $h_{50} = H_0 /$ (50km/s/Mpc), where $H_0$ is the Hubble constant. 3) The event rate is almost insensitive to $\Lambda$. The flux increases in the low energy side ($< 10$ MeV) with increasing $\Lambda$, but decreases in the high energy side (10 MeV $<$) in models in which the integrated number of supernovae in one galaxy is fixed.

New detectors for neutrinos and dark matter

Hypothetical particles in the GeV mass range and with typical Galactic velocity 10 -3 c would have MeV range momentum, similar to that of solar or supernova neutrinos. Thus elastic collisions with target nuclei would in each case give keV range nuclear recoils. There would also be a cross-section enhancement arising from full or partial coherence over the constituent nucleons. Detectors for these low energy nuclear recoils can be based on ionization, scintillation or low temperature phonon techniques. Radioactive background in the detector materials provides the main obstacle to detecting low event rates and significant effort is now being made to develop more advanced ideas which will distinguish the nuclear recoil events from background. Examples are simultaneous measurement of ionization and phonon energy in semiconductors, and photon timing or wavelength filtering in scintillators. Several groups are actively constructing underground dark matter detectors with targets in the 1-100 kg range. Solar and supernova neutrino detectors based on coherent scattering would have much lower target masses (by factors 20—100) than conventional detectors but would still require a substantial scale-up of these new techniques. Experiments with reactor neutrinos will provide a first step in verifying coherent neutrino scattering. Further scale-up to allow extra-galactic neutrino detection is feasible in principle and a possible challenge for the 21st century. Macroscopic coherent detection of the relic neutrino background may also become possible with foreseeable new technology.

Has a cosmological neutrino background from gravitational stellar collapse been detected?

The rate of neutrino-induced events with observed energy below 100 MeV in the Kamiokande proton-decay detector is much higher than expected from atmospheric neutrinos. However, it is shown here that this rate is consistent with a cosmological background of relic neutrinos that was formed by gravitational stellar collapses that have been taking place since galaxy and star formation.

On neutron star cooling by relic neutrinos

The maximum possible enhancement in the cooling rate of neutron stars due to the relic neutrino background predicted by the big bang model of the universe is found to be too small to be of any possible use as a detection mechanism for the relic neutrinos.