IV. High energy neutrinos underground: status of the Case-Wits-Irvine experiment and future prospect

Abstract

It is part of the folklore of cosmic ray physics dating back to the 1940’s that if one could only go deeply enough underground with a suitably large detector, it would be possible to detect neutrinos in the cosmic radiation. I say ‘folklore’ because no estimates of flux were available, the variation of cosmic ray intensity with depth was known to very shallow depths indeed (a few hundred metres of rock), detection techniques were relatively insensitive and nothing was known about interaction cross-sections, let alone the existence of muon neutrinos. The intervening two decades have witnessed developments and discoveries in all these directions so that it became possible to calculate the spectra of high energy (> 1 GeV) neutrinos resulting from the interaction of cosmic ray primaries with the Earth’s atmosphere and to begin speculation regarding high energy extra terrestrial sources. Measure­ments at high energy machines made it possible to put a lower bound on the interaction of neutrinos in the supra machine range. The discovery of the muon neutrino revealed the existence of a product with long range, the muon, which results from the interaction of muon type neutrinos with nuclei. This feature of the neutrino interaction, the muon associated neutrino, is central to all cosmic ray detection schemes. The electron-neutrino interaction observed long ago by the Los Alamos group is relatively ineffective because of the short range of the product electron. The great strides made in the field of particle detection with the development of efficient and relatively inexpensive large area scintillation detectors showed that finite count rates, ca . 10/y, could be expected from detectors measuring ca . 10 2 m 2 in area. Finally, the measurements of the intensity of cosmic rays with depth-pursued most effectively by the groups working at the Kolar Gold Fields in India-showed that neutrino interactions could be sought in existing deep mines without too much trouble from cosmic ray muons which succeeded in penetrating from the surface of the Earth to the detector. The motivation for seeking to measure and understand the high energy neutrino flux from our atmosphere and beyond is twofold: (1) This source, though weak and not under our control, is of much higher energy than available, or is likely to become available, in the laboratory for some time to come. It is generally recognized that energy is a prime factor in probing the structure of the weak interaction. (2) A curiosity regarding the existence and nature of sources of extraterrestrial neutrinos. A more mundane but perfectly valid reason for studying atmospheric neutrinos is a desire to ‘tidy up’ the record of cosmic ray components as they are produced and interact in the Earth’s atmosphere.