Prototype Detector for Ultrahigh Energy Neutrino Detection

Necessary technical experience is being gained from successful construction and deployment of current prototype detectors to search for UHE neutrinos in Antarctica, Lake Baikal in Russia, and the Mediterranean. The prototype detectors have also the important central purpose of determining whether or not UHE neutrinos do in fact exist in nature by observation of at least a few UHE neutrino-induced leptons with properties that are not consistent with expected backgrounds. We discuss here the criteria for a prototype detector to accomplish that purpose in a convincing way even if the UHE neutrino flux is substantially lower than predicted at present. † Submitted to Astroparticle Physics ∗ [email protected] ∗∗ [email protected] ‡ Supported in part by the U.S. Department of Energy Introduction There are several current efforts to construct deeply buried particle detectors of very large dimensions with which to search for ultrahigh energy (UHE) neutrinos from space [1]. These efforts involve prototype detectors aimed at mastering equipment design and techniques for deployment in ice or in deep water. The sensing elements to detect the leptonic products of UHE neutrino interactions in the vicinity of the detector are photomultiplier tubes (PMTs) and their associated circuitry which present new technical problems of power capacity and distribution and of data acquisition insofar as the PMTs are deployed at a large distance from the power source and in an unusual medium. Deployment in ice or in the sea, particularly the deep sea, requires development of new designs[2],[3] and of an infrastructure new to particle physicists. These issues need to be studied empirically by the experience that the prototype detectors are meant to provide. However, the prototype detectors have an important physics purpose in addition to answering the technical questions above. This purpose is to determine whether or not the hypothesized UHE neutrino sources which are the object of the search do in fact exist in nature. Specifically, the purpose of the prototype detectors is to demonstrate the existence of UHE neutrinos by observation of at least a small number of UHE neutrino-induced muons or neutrino-induced electrons with properties that are not consistent with expected, wellunderstood backgrounds. It is difficult to foresee the construction of a detector much larger than a prototype detector in the absence of such a proof of existence. In this note we discuss briefly the criteria to be satisfied by a prototype detector to accomplish that purpose in a convincing way. We rely heavily on the valuable, encyclopedic paper of Gandhi et al.[4], but concentrate specifically on the criteria necessary to achieve an UHE neutrino-induced signal above background subject to the perhaps pessimistic assumption that the sought-for UHE neutrino flux is an order of magnitude lower in intensity than the current predicted values. This is not a mindless assumption because the UHE neutrino flux calculations are strongly dependent on the uncorroborated models chosen to simulate the acceleration mechanisms in extragalactic and cosmic sources. A lower than predicted UHE neutrino flux would be similar in intensity to known backgrounds and difficult to extract convincingly from them without rethinking how the search should be performed and how a useful upper limit on the neutrino flux can be obtained. Our aim is to suggest a minimal detector and to indicate how the location and operation of the detector will discriminate against backgrounds and provide a high probability of observing a few UHE neutrino-induced leptons