Supernova Neutrino Flux Research Articles

SEARCH FOR EXTRATERRESTRIAL ANTINEUTRINO SOURCES WITH THE KamLAND DETECTOR

We present the results of a search for extraterrestrial electron antineutrinos ($\bar{\nu}_{e}$'s) in the energy range $8.3 MeV < E_{\bar{\nu}_{e}} < 31.8 MeV$ using the KamLAND detector. In an exposure of 4.53 kton-year, we identify 25 candidate events. All of the candidate events can be attributed to background, most importantly neutral current atmospheric neutrino interactions, setting an upper limit on the probability of $^{8}$B solar $\nu_{e}$'s converting into $\bar{\nu}_{e}$'s at $5.3 \times 10^{-5}$ (90% C.L.), if we assume an undistorted $\bar{\nu}_{e}$ shape. This limit corresponds to a solar $\bar{\nu}_{e}$ flux of $93 cm^{-2} s^{-1}$ or an event rate of $1.6 events (kton-year)^{-1}$ above the energy threshold $(E_{\bar{\nu}_{e}} > 8.3 MeV)$. The present data also allows us to set more stringent limits on the diffuse supernova neutrino flux and on the annihilation rates for light dark matter particles.

The neutrino signal at HALO: learning about the primary supernova neutrino fluxes and neutrino properties

Core-collapse supernova neutrinos undergo a variety of phenomena when they travel from the high neutrino density region and large matter densities to the Earth. We perform analytical calculations of the supernova neutrino fluxes including collective effects due to the neutrino-neutrino interactions, the Mikheev-Smirnov-Wolfenstein (MSW) effect due to the neutrino interactions with the background matter and decoherence of the wave packets as they propagate in space. We predict the numbers of one- and two-neutron charged and neutral-current electron-neutrino scattering on lead events. We show that, due to the energy thresholds, the ratios of one- to two-neutron events are sensitive to the pinching parameters of neutrino fluxes at the neutrinosphere, almost independently of the presently unknown neutrino properties. Besides, such events have an interesting sensitivity to the spectral split features that depend upon the presence/absence of energy equipartition among neutrino flavors. Our calculations show that a lead-based observatory like the Helium And Lead Observatory (HALO) has the potential to pin down important characteristics of the neutrino fluxes at the neutrinosphere, and provide us with information on the neutrino transport in the supernova core.

Multiangle effects in self-induced oscillations for different supernova neutrino fluxes

The non-isotropic nature of the neutrino emission from a supernova (SN) core might potentially affect the flavor evolution of the neutrino ensemble, via neutrino-neutrino interactions in the deepest SN regions. We investigate the dependence of these "multi-angle effects" on the original SN neutrino fluxes in a three-flavor framework. We show that the pattern of the spectral crossings (energies where F_\nu_e= F_\nu_x, and F_\bar\nu_e= F_\bar\nu_x) is crucial in determining the impact of multi-angle effects on the flavor evolution. For neutrino spectra presenting only a single-crossing, synchronization of different angular modes prevails over multi-angle effects, producing the known "quasi single-angle" evolution. Conversely, in the presence of spectra with multiple crossing energies, synchronization is not stable. In this situation, multi-angle effects would produce a sizable delay in the onset of the flavor conversions, as recently observed. We show that, due to the only partial adiabaticity of the evolution at large radii, the multi-angle suppression can be so strong to dramatically affect the final oscillated neutrino spectra. In particular three-flavor effects, associated with the solar parameters, could be washed-out in multi-angle simulations.

Fast time variations of supernova neutrino fluxes and their detectability

In the delayed explosion scenario of core-collapse supernovae, the accretion phase shows pronounced convective overturns and a low-multipole hydrodynamic instability, the standing accretion shock instability. These effects imprint detectable fast time variations on the emerging neutrino flux. Among existing detectors, IceCube is best suited to this task, providing an event rate of $\ensuremath{\sim}1000\text{ }\text{ }{\mathrm{ms}}^{\ensuremath{-}1}$ during the accretion phase for a fiducial SN distance of 10 kpc, comparable to what could be achieved with a megaton water Cherenkov detector. If the standing accretion shock instability activity lasts for several hundred ms, a Fourier component with an amplitude of 1% of the average signal clearly sticks out from the shot noise. We analyze in detail the output of axially symmetric hydrodynamical simulations that predict much larger amplitudes up to frequencies of a few hundred Hz. If these models are roughly representative for realistic SNe, fast time variations of the neutrino signal are easily detectable in IceCube or future megaton-class instruments. We also discuss the information that could be deduced from such a measurement about the physics in the SN core and the explosion mechanism of the SN.

Search for CP violation in the lepton sector

One of the major open issues in neutrino physics is the possible existence of CP violation in the neutrino sector. Such an observation would have an important impact in various domains of physics, from high energy physics to cosmology. Its search requires future accelerator neutrino facilities producing intense and pure neutrino beams such as “beta-beams”. Here we review the different beta-beam scenarios proposed so far and discuss the present status, with a particular emphasis on the original baseline scenario and its feasibility. Alternative strategies for the CP violation search are to be pursued as well. A possibility is to search for CP violation effects in astrophysical environments. Here we present recent analytical and numerical results obtained in the context of core-collapse supernovae. In particular, we point out the conditions under which there can be CP violating effects in dense media and show numerical results on the supernova (anti-)neutrino fluxes and on the electron fraction, relevant for the r -process nucleosynthesis.

RESONANT FLAVOR CONVERSION OF SUPERNOVA NEUTRINOS AND NEUTRINO PARAMETERS

The unknown neutrino parameters may leave detectable signatures in the supernova (SN) neutrino flux. However, even the contribution from the MSW flavor transition alone could cause ambiguity in the interpretation to the neutrino signals because of the uncertain local density profile of the SN matter and the model-dependent SN neutrino spectral parameters. A specific parametrization to the unknown local density profile is proposed in this work, and the contribution from the standard MSW effect is investigated through a multi-detector analysis of the SN neutrinos. In establishing the model-independent scheme, results based on the existing spectral models are included. The limitation of the analysis is also discussed.

Neutrino coherent scattering rates at direct dark matter detectors

Neutrino-induced recoil events may constitute a background to direct dark matter searches, particularly for those detectors that strive to reach the ton-scale and beyond. This paper discusses the expected neutrino-induced background spectrum due to several of the most important sources, including solar, atmospheric, and diffuse supernova neutrinos. The largest rate arises from 8B produced solar neutrinos, providing upwards of ∼103 events per ton-year over all recoil energies for the heaviest nuclear targets. However the majority of these 8B events are expected to be below the recoil threshold of modern detectors. The remaining neutrino sources are found to constitute a background to the weakly interacting massive particles (WIMP)-induced recoil rate only if the WIMP-nucleon cross section is less than 10−12 pb. Finally the sensitivity to the diffuse supernova neutrino flux for non-electron neutrino flavors is discussed, and projected flux limits are compared with existing flux limits.

Parameter degeneracy in flavor-dependent reconstruction of supernova neutrinofluxes

We re-examine the possibility of reconstructing the initial fluxes of supernova neutrinosemitted in a future core-collapse galactic supernova explosion and detected in amegaton-sized water Cherenkov detector. A novel key element in our method is theinclusion, in addition to the total and the average energies of each neutrino species, of a‘pinching’ parameter characterizing the width of the distribution as a fit parameter. Weuncover in this case a continuous degeneracy in the reconstructed parameters of supernovaneutrino fluxes at the neutrinosphere. We analyze in detail the features of this degeneracyand show how it occurs irrespective of the parameterization used for the distributionfunction. Given that this degeneracy is real we briefly comment on possible steps towardsresolving it, which necessarily requires going beyond the setting presented here.

Upper limits on the diffuse supernova neutrino flux from the SuperKamiokandedata

We analyze the 1496 days of SuperKamiokande data to put limits on theνe, , νμ+ντ and components of the diffuse flux of supernova neutrinos, in different energy intervals andfor different neutrino energy spectra. By considering the presence of only onecomponent at a time, we find the following bounds at 90% CL and for neutrino energyE>19.3 MeV:Φνe<73.3–154 cm−2 s−1, , Φνμ+ντ<(1.0–1.4) × 103 cm−2 s−1 and , where the intervals account for varying the neutrino spectrum. In the intervalE = 22.9–36.9 MeV,we find Φνe<39–54 cm−2 s−1, which improves on the existing limit from SNO in the same energy window. Our results forνμ+ντ and improve by about four orders of magnitude the previous best constraints fromLSD.

Erratum: Self-induced spectral splits in supernova neutrino fluxes [Phys. Rev. D76, 081301 (2007)

In the dense-neutrino region above the neutrino sphere of a supernova (r > 400 km), neutrino-neutrino refraction causes collective flavor transformations. They can lead to "spectral splits" where an energy E_split splits the transformed spectrum sharply into parts of almost pure but different flavors. Unless there is an ordinary MSW resonance in the dense-neutrino region, E_split is determined by flavor-lepton number conservation alone. Spectral splits are created by an adiabatic transition between regions of large and small neutrino density. We solve the equations of motion in the adiabatic limit explicitly and provide analytic expressions for a generic example.

Self-induced spectral splits in supernova neutrino fluxes

In the dense-neutrino region above the neutrino sphere of a supernova ($r\ensuremath{\lesssim}400\text{ }\text{ }\mathrm{km}$), neutrino-neutrino refraction causes collective flavor transformations. They can lead to ``spectral splits'' where an energy ${E}_{\mathrm{split}}$ splits the transformed spectrum sharply into parts of almost pure but different flavors. Unless there is an ordinary MSW resonance in the dense-neutrino region, ${E}_{\mathrm{split}}$ is determined by flavor-lepton number conservation alone. Spectral splits are created by an adiabatic transition between regions of large and small neutrino density. We solve the equations of motion in the adiabatic limit explicitly and provide analytic expressions for a generic example.

Supernova neutrinos

A nascent neutron star resulting from stellar collapse is a prodigious source of neutrinos of all flavors. While the most basic features of this neutrino emission can be estimated from simple considerations, the detailed simulation of the neutrinos' decoupling from the hot neutron star is not yet computationally tractable in its full glory, being a time-dependent six-dimensional transport problem. Nevertheless, supernova neutrino fluxes are of great interest in connection with the core-collapse supernova explosion mechanism and supernova nucleosynthesis, and as a potential probe of the supernova environment and of some of the neutrino mixing parameters that remain unknown; hence a variety of approximate transport schemes have been used to obtain results with reduced dimensionality. However, none of these approximate schemes have addressed a recent challenge to the conventional wisdom that neutrino flavor mixing cannot impact the explosion mechanism or r-process nucleosynthesis.

Testing neutrino spectra formation in collapsing stars with the diffuse supernova neutrino flux

I address the question of what can be learned from the observation of the diffuse supernova neutrino flux in the precision phase, at next generation detectors of Megaton scale. An analytical study of the spectrum of the diffuse flux shows that, above realistic detection thresholds of 10 MeV or higher, the spectrum essentially reflects the exponential-times-polynomial structure of the original neutrino spectrum at the emission point. There is only a weak (tens of percent) dependence on the power $\ensuremath{\beta}$ describing the growth of the supernova rate with the redshift. Different original neutrino spectra correspond to large differences in the observed spectrum of events at a water Cherenkov detector: for typical supernova rates, the ratio of the numbers of events in the first and second energy bins (of 5 MeV width) varies in the interval 1.5--4.3 for pure water (energy threshold 18 MeV) and in the range 1--2.5 for water with gadolinium (10 MeV threshold). In the first case, discrimination would be difficult due to the large errors associated with background. With gadolinium, instead, the reduction of the total error down to the 10%--20% level would allow spectral sensitivity, with a dramatic improvement of precision with respect to the SN1987A data. Even in this latter case, for typical neutrino luminosity the dependence on $\ensuremath{\beta}$ is below sensitivity, so that it can be safely neglected in data analysis.

Time-integrated supernova neutrino flux from a nearby cluster

The rate of gravitational collapse (type II) supernovae in our Galaxy is uncertain by a factor of three or more. One way to determine the Galactic supernova rate is through a radiogeochemical experiment (for example, the molybdenum–technetium experiment) that would integrate the neutrino flux over several million years. While such a measurement is designed to integrate the flux over the entire Galaxy, nearby star-forming regions could skew the results. We model the fluence (integrated flux) from a recently identified such region, the Scorpius-Centaurus OB association, and compare with the fluence from the rest of the Galaxy.

The diffuse supernova neutrino flux, supernova rate and SN1987A

I calculate the diffuse flux of electron antineutrinos from all supernovae using the information on the neutrino spectrum from SN1987A and the information on the rate of supernovae from direct supernova observations. The interval of flux allowed at 99% confidence level is ∼0.05–0.35 cm −2 s −1 above the SuperKamiokande (SK) energy cut of 19.3 MeV. This result is at least a factor of ∼4 smaller than the current SK upper limit of 1.2 cm −2 s −1, thus motivating the experimental efforts to lower the detection energy threshold or to upgrade to higher volumes. A Megaton water Cherenkov detector with ∼90% efficiency would record ∼2–44 inverse beta decay events a year depending on the energy cut.

Diffuse neutrino flux from supernovae: Upper limit on the electron neutrino component from the nonobservation of antineutrinos at SuperKamiokande

I derive an upper bound on the electron neutrino component of the diffuse supernova neutrino flux from the constraint on the antineutrino component at SuperKamiokande. The connection between antineutrino and neutrino channels is due to the similarity of the muon and tau neutrino and antineutrino fluxes produced in a supernova, and to the conversion of these species into electron neutrinos and antineutrinos inside the star. The limit on the electron neutrino flux is 5.5 cm^-2 s^-1 above 19.3 MeV of neutrino energy, and is stronger than the direct limit from Mont Blanc by three orders of magnitude. It represents the minimal sensitivity required at future direct searches, and is intriguingly close to the reach of the Sudbury Neutrino Observatory (SNO) and of the ICARUS experiment. The electron neutrino flux will have a lower bound if the electron antineutrino flux is measured. Indicatively, the first can be smaller than the second at most by a factor of 2-3 depending on the details of the neutrino spectra at production.

On the importance of low-energy beta beams for supernova neutrino physics

We show that low-energy beta beams are very well suited to obtain information about supernova-neutrino interactions. Linear combinations of low-energy beta-beam spectra are fitted to supernova-neutrino energy distributions. The resulting synthetic spectra are able to reproduce the response of a nuclear target to an incoming supernova neutrino flux in a very accurate way. This can provide important information about the neutrino response in a terrestrial detector. We illustrate this technique using deuterium and 16O as target material. The procedure provides an easy and straightforward way to apply the results of a beta-beam neutrino-nucleus measurement to a supernova neutrino detector, virtually eliminating potential uncertainties due to nuclear-structure calculations.

Supernova neutrinos can tell us the neutrino mass hierarchy independently of flux models

We demonstrate that the detection of shock modulations of the neutrino spectra from a galactic core-collapse supernova is sufficient to obtain a high significance determination of the neutrino mass hierarchy if the supernova event is observed in both a Mton-class water Cherenkov detector and a 100 kton-class liquid argon detector. Neither detailed supernova neutrino flux modeling nor observation of Earth matter effects is needed for this determination. As a corollary, a nonzero value of θ x will be established.

Effects of degenerate sterile neutrinos on the supernova neutrino flux

We consider the possibility that there exist sterile neutrinos which are closely degenerate in mass with the active neutrinos and mixed with them. We investigate the effects of this kind of active–sterile neutrino mixing on the composition of supernova neutrino flux at the Earth. If an adiabatic MSW-transition between active and sterile neutrinos takes place, it could dramatically diminish the electron neutrino flux.

Detecting the neutrino mass hierarchy with a supernovaat IceCube

IceCube, a future km3 antarctic ice Cherenkov neutrino telescope,is highly sensitive to a galactic supernova (SN) neutrino burst. TheCherenkov light corresponding to the total energy deposited by the SNneutrinos in the ice can be measured relative to backgroundfluctuations with a statistical precision much better than 1%. If theSN is viewed through the Earth, the matter effect on neutrinooscillations can change the signal by more than 5%, depending on theflavour-dependent source spectra and the neutrino mixing parameters.Therefore, IceCube together with another high-statistics experimentlike Hyper-Kamiokande can detect the Earth effect, an observation that would identify specific neutrino mixing scenarios that are difficult to pin down with long-baseline experiments. In particular, the normal mass hierarchy can be clearly detected if the third mixing angle is not too small, sin2θ13≳10−3. The small flavour-dependentdifferences of the SN neutrino fluxes and spectra that are found instate-of-the-art simulations suffice for this purpose. Although theabsolute calibration uncertainty at IceCube may exceed 5%, the Eartheffect would typically vary by a large amount over the duration of theSN signal, obviating the need for a precise calibration. Therefore,IceCube with its unique geographic location and expected longevity canplay a decisive role as a `co-detector' to measure SN neutrinooscillations. It is also a powerful stand-alone SN detector that canverify the delayed-explosion scenario.