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Particles and interactions

Particles and interactions

New benchmark set for magnetic monopole searches

16 Jun 2022
Magnetic monopole
True north: schematic representation of a magnetic monopole and its field lines. (Courtesy: Kavli IPMU)

A new benchmark has been set in the search for hypothetical magnetic monopoles produced in the atmosphere through collisions with incoming cosmic rays. Using simulations, a team led by Volodymyr Takhistov at the University of Tokyo compared data gathered by experiments searching for monopoles with the signals that are expected to be produced by cosmic-ray collisions. This allowed the team to set new limits on the existence of magnetic monopoles.

Unlike electrical charges, magnetic poles do not appear to exist independently of their opposing poles. If a bar magnet were broken in two, for example, both parts would simply form new magnets with pairs of opposing poles. Yet as demonstrated by Paul Dirac in 1931, the existence of magnetic monopoles would create symmetry in Maxwell’s equations of electromagnetism and would also be consistent with the quantized nature of the electron’s fundamental charge.

As a result, magnetic monopoles have long been the subject of theoretical predictions and experimental searches, but physicists are no closer to proving their existence. Many of these searches have focused on the prediction that large numbers of monopoles may have been created in the early universe by the Kibble–Zurek mechanism. However, the high uncertainties in the monopole masses predicted by this model, combined with the uncertain influence of cosmic inflation across vast timescales, have prevented any verification of the existence of these magnetic monopoles.

Hypothetical flux

Takhistov’s team has taken a different approach and have explored the possibility that monopoles are created when high-energy cosmic rays collide with Earth’s atmosphere. These collisions happen all the time and therefore a hypothetical flux of magnetic monopoles could be raining down constantly onto the Earth. What is more, these monopoles would be passing through existing particle detectors that are searching for monopoles – such as the Radio Ice Cherenkov Experiment (RICE) at the South Pole.

In their study, the researchers simulated the atmospheric production of cosmic ray monopoles with masses on the electroweak scale: 5–100 TeV/C2. They also looked at how this flux would be attenuated by the atmosphere as it headed towards the surface of the Earth. The team then looked at data from existing experiments that should be able to detect such an atmospheric flux if it indeed existed – including RICE. The researchers also looked at searches for monopoles at the low end of the electroweak scale that were done at the Large Hadron Collider.

These experiments have not made any detections so far, so the researchers we able to put upper limits on the production of magnetic monopoles in the atmosphere.

The team says that its results provide a robust new benchmark for future monopole detection experiments. The researchers also point out that a dedicated search for magnetic monopoles using the IceCube detector at the South Pole could also prove fruitful.

The research is described in Physical Review Letters.

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