Prospects for detecting supernova neutrino flavor oscillations

Abstract

The neutrinos from a Type II supernova provide perhaps our best opportunity to probe cosmologically interesting muon and/or tauon neutrino masses. This is because matter enhanced neutrino oscillations can lead to an anomalously hot ν e spectrum, and thus to enhanced charged current cross sections in terrestrial detectors. Two recently proposed supernova neutrino observatories, OMNIS and LAND, will detect neutrons spalled from target nuclei by neutral and charged current neutrino interactions. As this signal is not flavor specific, it is not immediately clear whether a convincing neutrino oscillation signal can be extracted from such experiments. To address this issue we examine the responses of a series of possible light and heavy mass targets, 9 Be, 23 Na, 35 Cl, and 208 Pb. We find that strategies for detecting oscillations which use only neutron count rates are problematic at best, even if cross sections are determined by ancillary experiments. Plausible uncertainties in supernova neutrino spectra tend to obscure rate enhancements due to oscillations. However, in the case of 208 Pb, a signal emerges that is largely flavor specific and extraordinarily sensitive to the ν e temperature, the emission of two neutrons. This signal and its flavor specificity are associated with the strength and location of the first-forbidden responses for neutral and charge current reactions, aspects of the 208 Pb neutrino cross section that have not been discussed previously. Hadronic spin transfer experiments might be helpful in confirming some of the nuclear structure physics underlying our conclusions. 14.60.Pq, 26.50.+x, 25.30.Pt

Neutrino–nucleus reactions and nuclear structure

“…To quantify this argument, in [56] the cross sections for the 208 Pb(ν e , e − 2n) 206 Bi reaction were calculated in a model combining the RPA for the neutrino-induced response with the statistical model for the decay of the daughter states. The partial cross sections of 55.7 × 10 −42 cm 2 for ν e neutrinos with (T , α) = (4, 0) Fermi-Dirac distributions should increase significantly if neutrino oscillations occur, as pointed out by [57]. For example, one finds for total ν e ↔ ν µ oscillations partial 2n cross sections of 1560 × 10 −42 cm 2 (for neutrino distributions with parameters (T , α) = (8, 0)).…”

“…However, as has been shown in [57], the total neutron rate is by itself not suitable to detect neutrino oscillations, even if results from various detectors with different materials (hence different ratios of charged-to-neutral current cross sections, as discussed above) are combined. Reference [57] points out that in the case of 208 Pb an attractive signal might emerge.…”

“…However, as has been shown in [57], the total neutron rate is by itself not suitable to detect neutrino oscillations, even if results from various detectors with different materials (hence different ratios of charged-to-neutral current cross sections, as discussed above) are combined. Reference [57] points out that in the case of 208 Pb an attractive signal might emerge. Due to the fact that the IAS and large portions of the GT − strength reside in 208 Bi just above the 2-neutron emission threshold, Fuller et al discuss that the 2-neutron emission rate is both flavor-specific and very sensitive to the temperature of the ν e distribution.…”

“…The first study, performed in [36], has been criticized and improved in [57]. These authors estimated the allowed transitions to the charged-current and neutral-current cross sections empirically using data from (p,n) scattering and from the M1 response to fix the GamowTeller contributions to the cross section.…”

“…These authors estimated the allowed transitions to the charged-current and neutral-current cross sections empirically using data from (p,n) scattering and from the M1 response to fix the GamowTeller contributions to the cross section. Reference [57] completed their cross section estimates by calculating the first-forbidden contributions on the basis of the Goldhaber-Teller model. They calculated cross sections which are somewhat larger than the RPA results of [56].…”