A search for massive particles in the cosmic radiation

Abstract

Two large area (l.29m.(^2))thin water Cerenkov counters have been developed and their properties investigated when amino G acid (a wavelength shifter) is added to the water. With a concentration of I6mg./litre of the acid an increased response of'~5 is achieved, as well as the uniformity improving by a factor of 2, compared with that of a pure water counter. The efficiency of the acid, however, has been found to deteriorate at a rate of 3.5%/month. The velocity response of the counters has been studied and it is found that, for counters of the present geometry, the addition of the acid is essential to maintain the inherent property of Cerenkov counters: that of a sharp velocity cut off. The counters, together with plastic scintillators, neon flash tubes and suitable amounts of absorber have been combined to form a large aperture (~ 0,1 m.(^2)sterad.) telescope capable of mass discrimination over a range of several Gev/c(^2). The properties of, and mass resolution attainable with such a system have been investigated by selecting sub-relativistic sea level cosmic ray protons to traverse the telescope. The mass resolution achieved for protons was a full width at half height of 350 Mev/c(^2), and the intensities of observed protons were found to be in good agreement with measurements of other workers in the same energy region. The telescope has been operated in a search for sub-relativistic massive particles, having integral or fractional charge ('quarks'), in the sea level cosmic radiation (typically M> 1.3 Gev/c(^2)for z = 1; a lower value applying for z < l). One anomalous event has been observed for which an interpretation in terms of the conventional particles is not forthcoming. The most plausible interpretation appears to be in terms of a unit charged particle having a mass significantly greater than 3.3 Gev/c(^2). However the finite, but small, probability of ~ 10(^-5) of the event being spurious, due to the relatively long sensitive time of the neon flash tubes, precludes a definite conclusion as to the existence of massive particles and this one observation has been used to set an upper limit, at the 90% confidence level, to their presence in the sea level cosmic radiation of <1.01 10(^-9)cm.(^-2)sec.(^-1)sterad(^-1) the limit referring to particles incident within well defined velocity bands, these being a function of the particle mass and charge. The implications on the quark intensity at various levels in the atmosphere have been investigated subject to two models of quark production and four plausible, yet widely differing, models of quark propagation. The intensity limit imposed by the present work, and those reported by other workers searching via different methods, have been used to summarise the limits that can be placed on the quark production cross section subject to each of the production and propagation models, and conclusions have been drawn as to the most profitable areas for future quark searches. The present work has also allowed a limit of ~10(^-7) in the mass range 2-50 Gev/c(^2), to be placed on the fraction of U particles in the primary radiation at low energies (the U particle being suggested by Callan and Glashow, 1968, to be massive (M> 4 Gev/c(^2)), weakly interacting and to comprise ~10(^-3) of the primary radiation). Deuterons have been detected at various stages of the work and their, intensity in the sea level cosmic radiation has been evaluated as (DIAGRAM FORMULA)The measured intensities have been shown to be an order of magnitude too large to be consistent with the bulk of production coming through reactions such as NN→dTT, but they are apparently consistent with what would be expected from 'pick-up' reactions. The 'heavy mass telescope" has been modified to investigate the possibility of the direct production of muons, from the interactions of neutral primaries, at a rate much greater than that expected from neutrinos having their 'noimal' cross section. The results of a series of experiments that were performed were suggestive of a very high pion and proton background, and within the uncertainties in the sea level neutron spectrum the observed rates of events were not inconsistent with all of them having been neutron induced. It is concluded that there is no evidence for an excess of muons induced by neutral primaries as has repeatedly been reported by Cowan et al., (l964 - 1969).