IceCube sets best limits for mildly relativistic magnetic monopoles

Magnetic monopoles are thought to have been created during the early stages of the universe. These monopoles would have been accelerated by intergalactic magnetic fields and could reach relativistic velocities, i.e. velocities near the speed of light (c). Similar to electrically charged particles, magnetic monopoles produce Cherenkov light when traveling through ice at such high speeds. If they do in fact exist, IceCube could be the first experiment to detect them. It is the large instrumented volume in IceCube that makes it such a suitable detector for searches of exotic particles.

The IceCube Collaboration today presents a search for relativistic and mildly relativistic monopoles using two years of data. No monopole candidate was observed, but IceCube data allowed setting very stringent limits for the range of velocities studied. These results have been submitted today to European Physical Journal C.

Monopole sensitivity
Sensitivities (magenta) and final limits (red) of both analysis at certain characteristic velocities compared to other limits. The lines are only drawn to guide the eyes. Image: IceCube Collaboration

“Magnetic monopoles crossing the Antarctic ice will leave different traces in IceCube than other known particles,” says Jonas Posselt, who graduated from the University of Wuppertal with a thesis on the topic. Last year, the collaboration presented a search for nonrelativistic monopoles, which may catalyze the decay of nucleons. That search led to the lowest limits on the flux, which ruled out the possibility that monopoles are a dominant form of dark matter in our universe.

For monopoles with speeds above the Cherenkov threshold in ice, approximately 0.76c, the magnetic charge will induce Cherenkov light, which will be much brighter than that of electrically charged particles. A monopole with minimal magnetic charge will generate a few thousand times more Cherenkov radiation than a particle with the electrical charge of an electron. Moreover, monopoles with velocities greater than 0.4c also produce Cherenkov light indirectly since they knock off electrons from ice molecules that can be accelerated to speeds above the Cherenkov threshold.

Event view of a simulated magnetic monopole with a velocity of 0.83c using both direct and indirect Cherenkov light. The red line shows the reconstructed track. Image: IceCube Collaboration

The search for relativistic monopoles, above the Cherenkov threshold, was based on data recorded from May 2008 to May 2009, when IceCube was taking data with 40 strings only. For mildly relativistic monopoles, greater than 0.51c, data from the first year of the completed detector—May 2011 to May 2012—was used.

“These analyses set the strongest limits for the magnetic monopole flux on a wide range of velocities. The limits are nearly two orders of magnitude below previous limits,” says Anna Pollmann, a graduate student at the University of Wuppertal and a corresponding author of this paper.

These searches are breaking ground for improved results that will benefit from the sensitivity increase expected from new IceCube data and from the proposed extension to the IceCube Neutrino Observatory.

+ Info: “Searches for Relativistic Magnetic Monopoles in IceCube,” IceCube Collaboration: M. G. Aartsen et al. European Physical Journal C76 (2016) 133,