Prospect for relic neutrino searches

Neutrinos from the Big Bang are theoretically expected to be the most abundant particles in the Universe after the photons of the Cosmic Microwave Background (CMB). Unlike the relic photons, relic neutrinos have not so far been observed. The Cosmic Neutrino Background (CνB) is the oldest relic from the Big Bang, produced a few seconds after the Bang itself. Due to their impact in cosmology, relic neutrinos may be revealed indireclty in the near future through cosmological observations. In this talk we concentrate on other proposals, made in the last 30 years, to try to detect the CνB directly, either in laboratory searches (through tiny accelerations they produce on macroscopic targets) or through astrophysical observations (looking for absorption dips in the flux of Ultra-High Energy (UHE) neutrinos, due to the annihilation of these neutrinos with relic neutrinos at the Z-resonance). We concentrate mainly on the first possibility. We show that, given present bounds on neutrino masses, lepton number in the Universe and gravitational clustering of neutrinos, all expected laboratory effects of relic neutrinos are far from observability, awaiting future technological advances to reach the necessary sensitivity. The problem for astrophysical searches is that sources of UHE neutrinos at the extreme energies required may not exist. If they do exist, we could reveal the existence, and possibly the mass spectrum, of relic neutrinos, with detectors of UHE neutrinos (such as ANITA, Auger, EUSO, OWL, RICE and SalSA). PACS numbers: 14.60.Lm; 14.60.Pq; 95.35.+d; 95.85.Ry Talk given at the Nobel Symposium on Neutrino Physics, Enkoping, Sweden, Augus 19-24, 2004. Email address: [email protected]