| Grand unified theories predict the existence of magnetic monopoles, elementary particles with a magnetic charge g = hc/4{dollar}pi{dollar}e and a mass greater than 10{dollar}sp{lcub}15{rcub}{dollar}GeV. The large mass precludes creating the particles in accelerators so we search for them in cosmic rays. Superconducting detectors are ideal for the search since their sensitivity is calculated from classical properties and is independent of the particle's mass, velocity, electric charge, or magnetic dipole moment.; We are now operating a superconducting detector for cosmic ray magnetic monopoles originally designed with a cross section of 1.5 m{dollar}sp2{dollar} (averaged over 4{dollar}pi{dollar} solid angle) for double coincident events. The detector consists of eight independent gradiometer detection loops on the surface of an octagonal cylinder. The conductor is niobium-titanium foil, laid in grooves in a circuit-board support panel. The current sensors are rf SQUIDs (Superconducting QUantum Interference Devices) biased at 190 MHz. The signal to noise ratio for a single Dirac charge is greater than 40 in a 0.1 Hz bandwidth, and we have data from 165 days of this low-noise operation. Open circuits in portions of two loops and occasional coupling of disturbances in adjacent panels reduce the active sensing area to 1.1 m{dollar}sp2{dollar}. A closed-cycle helium liquifier, with the pressure regulated at the dewar, eliminates helium transfers and increases the stability of the data. Anti-coincidence instrumentation includes strain gauges, a flux-gate magnetometer, an ultrasonic motion detector, and a wide-band rms rf voltmeter. The exposure to date represents a limit on the flux of cosmic ray magnetic monopoles of 1.2 {dollar}times{dollar} 10{dollar}sp{lcub}-12{rcub}{dollar}cm{dollar}sp{lcub}-2{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar}sr{dollar}sp{lcub}-1{rcub}{dollar} at a 90% confidence level, eliminating most monopole plasma oscillation theories. This limit is roughly three orders of magnitude above the mass-independent Parker bound on monopole abundance (based on galactic field arguments) and larger detectors are needed to reach this bound. |
