next up previous
Next: Supernova implications Up: VIOLENT ASTROPHYSICAL EVENTS Previous: VIOLENT ASTROPHYSICAL EVENTS

High energy neutrinos

There are many possible sources of ultra high energy astrophysical neutrinos, including gamma ray bursts (GRB), active galactic nuclei (AGN), and particle physics exotica. They are a particularly useful probe because high energy $\gamma$ rays tend to be absorbed in the astrophysical source, while protons are magnetically deflected. Also, protons above $10^{20}$ eV can be observed only from local sources because of the Greisen-Zatsepin-Kuzmin (GZK) cutoff (scattering from the CMB).

Waxman [55], Halzen [56], and Perrone [57] described the theoretical expectations for GRBs, and the need for km$^3$ scale detectors. GRBs are now understood to be mainly at cosmological distances, and arise from the expansion of a relativistic fireball. The expanding shock accelerates protons and high energy $e^-$, which emit the $\gamma$'s by synchrotron emission. Outstanding issues are the fireball progenitor (e.g., coalescence of neutron stars or of a neutron star and black hole) and the $e-p$ coupling mechanism.

Reactions such as $\gamma p \mbox{$\rightarrow$}\pi^+ n$ from the GRB photons are expected to produce a burst of 100 TeV $\nu$'s, followed by the production of $10^{18}$ eV $\nu$'s for about 10 s associated with the expansion of the shock into the interstellar medium (the afterglow). $p n \mbox{$\rightarrow$}\pi^+$ processes also lead to lower energy (10 GeV) neutrinos. Estimates of the fluxes suggest the need for a km$^3$ detector for a reasonable event rate of order 10's/yr. One also expects a few/yr from the diffuse scattering of protons from the CMB photons. The $\gamma$'s from $\pi^0$ decay are absorbed and cascade to lower energies $\mathrel{\mathpalette\lower2.pt\vbox{\baselineskip0pt \lineskip-.5pt
\ialign{$\mathsurround=0pt ;\hfil ... TeV. These may be ultimately observable.

The observed ultra high energy cosmic ray protons may also be due to this mechanism (those above $10^{20}$ would have to be due to nearby GRBs to evade the GZK cutoff). One can turn this argument around: any mechanism that produces high energy $\nu$'s from $\gamma p$ (or $p N$) interactions in an optically thin source should produce high energy $p$'s as well as $\nu$'s. The observed $p$ flux therefore limits the possible $\nu$ flux (the Bahcall-Waxman bound), invalidating some early optimistic estimates. The bound can be evaded in optically thick sources, but most plausible models are thin.

There are other possible sources of high energy $\nu$'s. These include the annihilation of WIMPs in the earth, the sun, or the galactic center; monopole annihilation; the decay of topological defects (lattice calculations are being carried out by the TRENTO group [58]); or local astrophysical sources. High energy neutrinos may also produce $Z$'s by annihilation with the relic neutrinos.

One spectacular prediction [55] for oscillations is that an initial $\nu_\mu$ source should yield equal numbers of $\nu_\mu$ and $\nu_\tau$ for $\mbox{$\Delta m^2$}> 10^{-17}$ eV$^2$ for maximal mixing ( $\mbox{$\sin^2 2 \theta$}\sim 1$).

The present experimental status and future prospects were reviewed by Halzen [56], Vignaud [59], van Dantzig [60], and de Marzo [61]. The MACRO and Lake Baikal detectors had areas $A < 10^3$ m$^2$. (Baikal observed some events). In the $A \sim 10^4$ m$^4$ range, AMANDA (S. Pole) has been running for some time, and observed 193 atmospheric $\nu$'s (it has mainly vertical sensitivity), and NESTOR (near Greece) is under development. The larger AMANDA II with $A \sim 10^5$ m$^2$ is taking data. It may eventually be succeeded by the km$^3$ ICE-CUBE. Also in the $A \sim 10^5$ m$^2$ range is ANTARES [60] in the Mediterranean, which should run in 2002-2003. This should be succeeded by ANTARES II. Another project, NEMO [61] is under study. It could be deployed near Sicily, or merge with ANTARES II.


next up previous
Next: Supernova implications Up: VIOLENT ASTROPHYSICAL EVENTS Previous: VIOLENT ASTROPHYSICAL EVENTS
Paul Langacker 2001-09-27