F. Vissani: Supernova Neutrinos, risks and opportunities
Vissani started by explaining he wanted to discuss the issue of what we want to do in the field of supernova neutrinos.
If supernovae track milky way’s matter distribution, the typical distance is 10+-5 kparsec. A supernova at 10 kiloparsecs will not cause risks to our life. The issue is when a supernova will occur. The best estimate has been for a while of 50+-20 years. However, no supernova was seen in Andromeda for 130 years, so the estimate is being moved to about 60 years on average.
An ordinary water cherenkov detector of 1kton mass looks at the electron antineutrino reaction on protons. Then also the elastic scattering off electron is important as it is directional, but has of course much smaller cross section.
What we know of the expected time distribution ? One observes that the bulk of arrival time is below half a second, during the accretion stage of the supernova; earlier there is a neutronization phase when neutrinos can escape, in the first tenth of a second, with a strong peak in flux.
One needs a parametrized flux to analyze the data, in time and energy. Something was attempted in connection with SN1987A by Abbott, De Rujula, Walker NPB 1988, and Loredo and Lamb. For that supernova, results from Kamiokande, IMB and Baksan indicate a luminous initial phase. Time distribution studies will be a major goal of multi-kiloton detectors in the future.
The energy distribution is also uncertain; neutrinos carry away 2-3×10^53 erg from the SN; there is approximate energy equipartition among the six species, and quasi-thermal spectra with an average energy of 12 MeV. But emission parameters are uncertain. Several parameterizations exist in the literature.
Scintillators come to our rescue. With cherenkov-type detector we study the inverse beta decay, but with scintillators we have many possible reactions to use for detection. The neutral current part, not affected by oscillations, is very important and promising.
In some models, such as that of mirror neutrinos of Berezinsky et al, NPB 2003, or pseudo-dirac neutrinos of Beacom et al., PRL 2004, only half of the neutrino flux may reach us, due to new oscillations. Ordinary neutrinos speak with mirror ones and half of them are invisible to us as they reach the Earth. In such a scenario one should revise the 1987A SN neutrino data interpretation. This is not excluded by current data until we improve the theory or if we measure neutral current signals.
To summarize, the general picture of SN neutrino emission seems reliable but the uncertainties are considerable and not precisely quantified; it would be useful to haver reliable predictions. One risk is to observe a signal but not being able to really understand it with the current hardware: it would be shocking to realize that our detectors were not sufficient after observing a clean, close-by supernova explosion.