Neutrino masses in astroparticle physics - MPP Theory Group

706 G.G. Raffelt / New Astronomy Reviews 46 (2002) 699–708The neutrino burst from a future galactic SN could to the sub-eV mass scale (Halzen and Jaczko, 1996;yield more restrictive limits because one would Choubey and King, 2002).expect up to 8000 events in a detector like Super-Kamiokande for a typical galactic distance of around10 kpc. With such a high-statistics signal the relevant 7. Discussion and summarytime-scale Dt is the fast rise-time of around 100 msrather than the overall burst duration of several The compelling detection of flavor oscillations inseconds. Therefore, one is sensitive to smaller mass- the solar and atmospheric neutrino data have trigesthan the SN 1987A burst, despite the shorter gered a new era in neutrino physics. In the laboratorybaseline. From detailed Monte-Carlo simulations one will proceed with precision experiments aimed atTotani (1998) infers that Super-Kamiokande would measuring the details of the mixing matrix. Futurebe sensitive to about mn* 3 eV, almost independent- tritium decay experiments may well be able to probely of the exact distance. (At a larger distance one the overall neutrino mass scale down to the 0.3 eVgains baseline but loses statistics, two effects that range, but if the absolute masses are smaller, it willcancel for a given detector size.)be very difficult to measure them, and the overallConceivably this sensitivity could be improved if a mass scale may remain the most important unknowngravitational wave signal could be detected preced- quantity in neutrino physics for a long time to come.ing the neutrinos, signifying the instant of the stellar Unfortunately, it is unlikely that astrophysicalcollapse (Fargion, 1981; Arnaud et al., 2002). In this time-of-flight methods will help much. Foreseeablecase one may be sensitive to about 1 eV.SN neutrino detectors are sensitive to eV masses, butIt is also conceivable that a SN collapses to a not the sub-eV range. On the bright side this meansblack hole some short time after the original col- that the measured neutrino light-curve of a futurelapse. In this case the neutrino signal would abruptly galactic SN will faithfully represent the sourceterminate (within Dt & 0.5 ms), thereby defining a without much modifications by neutrino dispersion.very short time scale. Beacom et al. (2000, 2001) Cosmological large-scale structure data at presentfound that Super-Kamiokande would be sensitive to provide the most restrictive limit on neutrino massesmn* 1.8 eV.of omn, 2.5 eV, corresponding to mn, 0.8 eV in aIn the foreseeable future megatonne neutrino degenerate mass scenario. A rigorous relationshipdetectors may be constructed to search for proton between the cosmic hot dark matter fraction Vnanddecay and to perform precision long-baseline oscilla- mndepends on the cosmic neutrino density nn. If thetion measurements with neutrino beams. Such detec- solar LMA solution is correct, big-bang nutorswould have about 30 times the fiducial volume cleosynthesis constrains nnwithout further assump-of Super-Kamiokande. The exact mnsensitivity for tions about the neutrino chemical potentials. In thesuch an instrument has not been worked out. With a LMA case neutrinos reach de-facto flavor equilibmegatonnedetector one could measure SN neutrinos rium before the epoch of weak-interaction freeze out.throughout the local group of galaxies. From An- While neutrinos do not play a dominant role fordromeda at a distance of 750 kpc one would get dark matter or structure formation, the mass andaround 50 events. Using the overall signal duration mixing schemes suggested by the oscillation experiforDt yields a sensitivity in the few eV range. ments are nicely consistent with leptogenesisThe only conceivable time-of-flight technique that scenarios for creating the cosmic baryon asymmetry.could probe the sub-eV range involves Gamma-Ray Therefore, massive neutrinos may be closely relatedBursts (GRBs) which have been speculated to be to the baryons in the universe, not the dark matter.strong neutrino sources. If the neutrino emission If extremely-high energy neutrinos will be obshowstime structure on the millisecond scale, and served in future, the Z-burst scenario provides aassuming a cosmological distance of 1 Gpc, one handle on the cosmic background neutrinos and theirwould be sensitive to neutrino masses m * 0.1 eV E/ mass through the observed cosmic rays near theGeV. Therefore, observing millisecond time structure GZK cutoff.in sub-GeV neutrinos from a GRB would be sensitive The great advance in our knowledge of neutrino