Cosmic-ray monopole searches at Stanford using superconducting detectors

A Dirac magnetic charge g passing through a superconducting ring changes the magnetic flux threading the ring by exactly 2 phi/sub 0/, where phi/sub 0/ = 2.07 x 10/sup -7/ G cm/sup -2/ is the magnetic flux emanating from the monopole. Superconductive detectors based on this property directly measure magnetic charge of a passing particle independently of its velocity, mass, electric charge, and higher magnetic multipole moments. The detector response follows from simple and fundamental theoretical considerations. Such detectors are thus natural choices in the search for any flux of cosmic-ray magnetic charges, particularly the nonrelativistic and supermassive monopoles predicted by grand unification theories. A 5-cm diameter single-loop prototype detector based on this principle was operated for a total time of 151 days and produced a single candidate event. The existence of one inconclusive candidate motivated the construction of a new detector with significantly improved sensing area, redundancy, mechanical stability, bandwidth, and signal-to-noise ratio. A three-loop detector satisfying these requirements has been in low-noise operation for over a year now, and the authors present here an update of initial results reported in more detail earlier