A Search for Relativistic Magnetic Monopoles with the IceCube 22-String Detector

Abstract

Magnetic monopoles are particles which act as a source for divergent magnetic

fields, equivalent to a proton's electric field. Beyond simply adding the final symmetry

to Maxwell's equations, their existence would solve numerous outstanding

problems in the particle physics community. However, no conclusive evidence for

their existence has been found.

Magnetic monopoles possess many unique characteristics that allow for detection

from a variety of experimental methods. One property is the large scaling of the

Cherenkov radiation (∼ 8300) compared to electrically charged particles. Magnetic

monopoles are postulated to be extremely heavy (&#8764; 10<super>4</super>&#8722;10<super>17</super> GeV). However, they would be topologically stable and accelerated via magnetic field lines throughout the universe, potentially reaching energies &#8764; 10<super>15</super> GeV. Therefore, searches for relativistic magnetic monopoles incident on Earth are an important piece to the overall experimental search.

The IceCube neutrino observatory, located at the South Pole, offers a novel

environment to search for these particles. IceCube is a km<super>3</super grid of light sensors

buried deep within the Antarctic Ice Shelf and represents the most colossal neutrino

telescope in the world. The large instrumented volume and relatively clear glacial

ice allows for a significant improvement in sensitivity to the bright tracks relativistic

magnetic monopoles would exhibit.

The main background comes from large muon bundles produced in air showers

generated by the highest energy cosmic rays. The depth of the detector allows for

a limited rejection of these events from the Southern Hemisphere, while the Earth

acts as an opaque shield to these events traveling from the Northern Hemisphere.

In contrast, a large range of potential magnetic monopole masses and energies considered

(M & 10<super>7</super> GeV, E & 10<super>11</super> GeV) can travel completely through the Earth while remaining relativistic.

This dissertation details the first search performed for these relativistic magnetic

monopoles with IceCube data. The data is from 2007, when IceCube operated

as a partially completed detector with an instrumented volume of &#8764;0.2 km<super>3</super>. It

considers monopoles at four discrete speeds: &#946; = 0.76, 0.8, 0.9, 0.995, ranging from

just above the Cherenkov threshold in ice to a boost factor of 10. Discrimination

between a potential magnetic monopole signal and background is achieved by considering

the brightness and direction of the event. After an initial search revealed

deficiencies in the simulated background model, a more conservative analysis produces

limits that are &#8764; 10 x better than previous searches. The final limits are then

transformed to be a limit on an isotropic flux at the Earth's surface, due to the

dependence on direction to the overall sensitivity of the analysis.