Search for GUT magnetic monopoles with the MACRO detector

Citation

Katsavounidis, Erotokritos Charalambous (1996) Search for GUT magnetic monopoles with the MACRO detector. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/csng-d312. https://resolver.caltech.edu/CaltechTHESIS:05222017-155824164

Abstract

Grand Unified Theories (GUT) predict the existence of super heavy magnetic monopoles (~10¹⁶GeV) as stable particle carrying magnetic charge. The extremely large mass of the GUT monopoles requires that they would have been produced during the early formation of the Universe and would probably have survived until today traveling through the Universe at nonrelativistic velocities. Cosmological arguments regarding the abundance of the magnetic monopoles today yield either too many or too few of them. However, simple arguments regarding the survival of the galactic field may set a reliable upper bound on the monopole flux. This is the so-called Parker bound and places an upper limit on the monopole flux of the order of 10^-15cm^-2sr^-1sec^-1 for monopoles with mass ~10¹⁶GeV and typical galactic velocities 10^-3c.

The MACRO detector at Gran Sasso (Italy) is a large underground detector offering large acceptance (~10,000m²sr) and high redundancy for a search for magnetic monopoles. The detector's large acceptance and different detection techniques will allow monopole searches ultimately beyond the astrophysical bound with high sensitivity over all the possible β-range of a monopole. The scintillator system of the full lower MACRO detector has been operational from December 1992 to June 1993 (total live time 160.7 days) and collected data with a trigger specialized to select slow (10^-4c – 10^-3c) moving particles. The waveforms of the candidate events have been recorded with commercially available waveform digitizers. More than 8,000 events that involved slow particle triggers from at least two different detector faces were found in this data set.

In analyzing this data set we have adapted the Haar decomposition (one of the simplest examples of the Wavelet Transform) of the candidate events. Our purpose was to build an algorithm that will effectively discriminate background (i.e., muon, radioactivity and noise) from monopole waveforms. Using various features of the Haar decomposition of the waveforms (like the propagation of the wavelet maxima among scales and the energy content of each scale), we derived simple conditions by means of which we can efficiently select candidate waveforms. More than 80% of the initial dataset was thus rejected and the remaining events were visually scanned and classified. No signal was consistent with a slow magnetic monopole signature, and based on that we have established an upper flux limit on the monopole flux.

The detector's acceptance was obtained via a Monte Carlo simulation on a run-by-run basis, and for the majority of the runs it had an average value of 5200m²sr. The total effective exposure has been 5.6 x 10¹⁴cm²sr¹sec¹, corresponding to an isotropic upper monopole flux limit of 4.1 x 10^-15cm^-2sr^-1sec^-1 at the 90% confidence level. This limit is valid for monopoles in the velocity regime 1.5 x 10^-4c ≾ v ≾ 4 x 10^-3c.

Thesis Files