The NuTel Project: Watching for Tau Showers from a Mountain

Abstract

The Mountain/Valley type ‘‘NuTel’’ detection mechanism is illustrated in Fig. 1, which arouses many questions. First of all, the mountain? We have the (Paulian) image that neutrinos are like ghosts from a nightmare, penetrating light years of material without ever interacting. This may well be so for reactor neutrinos, but we do know that neutrino cross sections grow rapidly with energy. A most striking thing when heard for the first time, is that the center of the Earth becomes opaque for neutrinos with energies beyond 10 eV or so. A mountain becomes a sensible target for neutrinos beyond such energies. Second, whither ? With finite interaction probability, ‘ ! ‘ conversion is brought about by charged current interaction. Electrons are quickly absorbed by the rock and cannot exit the mountain. Muons could penetrate since they are minimally ionizing particles (MIP), but emit very little light in the air hence hard to detect. As better MIPs, the s more readily penetrate, and, after exiting the mountain, they decay. 83% of tau decays have hadrons or an electron in the final state, which can generate air showers in the subsequent valley. As depicted in Fig. 1, a telescope sitting on a second mountain (to allow for shower development) can in principle pick up the nanosecond Cherenkov pulse emitted by the air shower arising from decay. But, energetic s from heaven? Let us first amuse ourselves and contemplate cosmic, energetic neutrinos. What can be neutrino sources? Since cosmic rays (CR) beyond 10 eV have to be extragalactic in origin, one class of candidates are Active Galactic Nuclei (AGN), where huge Black Holes (BH) are the likely engines behind the observed gigantic ‘‘jets’’. The jet acceleration mechanism is not understood. Some believe that it is purely inverse-Compton, hence only rays are emitted. Unfortunately, in the energy range of 10–10 eV, they are strongly absorbed by the CMBR via CMB ! eþe , in less than intergalactic distances. However, any proton content of the acceleration mechanism would produce pions, which would decay into neutrinos. Interestingly, we do see TeV rays from Mkn 421 and Mkn 501, and there are some claims for evidence for rays from 0 production. In any case, detection of VHE neutrinos holds the key to probing AGN jet acceleration mechanism, which is important for astrophysics. What is important for NuTel is that AGNs are point sources. Our Galactic Center (GC) should not be forgotten in this context. Our GC has a 3.6 solar mass BH at its center, in the direction of Sagittarius A . There are some excess events of CRs at EeV energies near GC direction. As charged particles are unlikely directional, while s suffer from interstellar dust, neutrino probes would again be highly desirable. Now then, how does one get cosmic tau neutrinos? We know that, for every charged pion decay, one gets 2 s and one e, but never any , which has to come from charm and bottom hadron production. These have much lower cross section hence the indigenous cosmic flux would be much lower than e and fluxes. Here we have to be very thankful for a tantalizing recent discovery, which links the heavens and the Earth in Fig. 1: maximal – mixing. The latter is deduced from the observed atmospheric neutrino anomaly, but a appearance experiment is still needed. With maximal – mixing, the aforementioned e : : ratio of 1 : 2 : 0 at production (say in AGNs), changes to 1 : 1 : 1 as the flux impinges on Earth. In this sense, the NuTel concept also constitutes a ‘‘ appearance’’ experiment. Finally, we note in passing that, if the NuTel concept can be made to work, it can also test the various thoughts on TeV scale extra dimensions, and the like. In the Standard Model (SM), beyond the W=Z resonance region, the N cross section scales roughly as E. But we may well have surprises in store for us here, which may even help in the detection.