A Search for Slow Magnetic Monopoles Below the Parker Bound

Citation

Kyriazopoulou, Sophia (2002) A Search for Slow Magnetic Monopoles Below the Parker Bound. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/DPSY-DP97. https://resolver.caltech.edu/CaltechETD:etd-07252008-144747

Abstract

During the years of high school physics, instructors would tell us how asymmetry in Maxwell's equations express the fact that while there are stable particles carrying electric charge in nature, there are no magnets with a single pole. Yet Dirac had suggested the idea of a magnetic monopole decades before I was born, and around the time I was attending high school, Cabrera was starting his experiments to find one. I first learned about the possibility of the existence of such a particle when in graduate school. The idea of proving wrong something I had learned to take for granted, was exciting. Of course, whether a stable particle which carries magnetic charge does exist or not has profound implications for our understanding of the universe, but still, what makes the very idea of a magnetic monopole so attractive to me is that there are not that many basic, everyday things left -- things like the indivisibility of a magnet -- to be confirmed yet.

This work describes a search for the magnetic monopole, as it is postulated in many Grand Unified Theories (GUT). Such monopoles are expected to be heavy (~10¹⁷ GeV), may have a large range of velocities and - from current astrophysical constraints - must be very rare. The most widely accepted upper limit to their flux comes from considerations of the survival of the galactic magnetic field (the Parker Bound 1), and it is of the order of ~10⁻¹⁵ cm⁻²sr⁻¹s⁻¹. This flux regime was explored by MACRO (Monopole Astrophysics and Cosmic Ray Observatory), a large area detector built in Gran Sasso in Italy.

Specifically, this was a search for slow-moving magnetic monopoles with the MACRO detector. MACRO featured a large acceptance (≃ 10,000 m²), and three separate detector systems for monopoles. This search relies primarily on one of these systems: the scintillation detector. A custom-made trigger was developed to detect the passage of slow-moving magnetic monopoles (ν ~ 10⁻⁴ c to 4.3 x 10⁻³ c) through a scintillator tank. This trigger was employed for the initial selection of events over the data-taking period between June 1995 and May 2000. There were around 28 million relevant triggers during this period. These represent potential monopole events; each must be examined carefully to see if all recorded details are consistent with the signature of monopole passage. Analysis of the events and final selection was performed mainly with the help of a system which digitally sampled and recorded the waveforms of photomultipliers monitoring the scintillation detectors. A specially-designed pattern recognition algorithm, following the philosophy of the slow monopole trigger, was developed, in order to identify the signature of slow monopoles in the waveform system, while suppressing background generated due to cosmic-ray muons or electronic noise.

No events were observed that are consistent with the passage of a slow monopole. Detailed data quality checks were performed for each data run and the resulting actual configuration of the detector was used in a Monte Carlo calculation of the detector's acceptance on a run-by-run basis. Based on the exposure time due to MACRO over these five years of data taking, this search sets an upper limit to the flux of magnetic monopoles of

F ≾ 2.5 x 10⁻¹⁶ cm⁻² sr⁻¹ sec⁻¹

for particles in the velocity range ν ~ 10⁻⁴ c to 4.1 x 10⁻³ c.

Thesis Files