Track and trap – the long search for magnetic monopoles
Is there an elementary particle carrying magnetic charge? This fundamental question is being addressed by an experiment currently performed at CERN near Geneva. Recently the MoEDAL research collaboration published its first findings. No discovery has been made so far, but now the experiment will enter in its hot period. Prof. Philippe Mermod and his team at the University of Geneva play a vital role in the search for the hypothesized fundamental magnetic particle.
The Large Hadron Collider (LHC) at CERN is the most powerful particle accelerator in the world. The two largest experiments at the LHC – ATLAS and CMS – discovered the Higgs boson in 2012. But ATLAS and CMS are not the sole experiments, which exploit the high-energy collisions along the LHC ring. One of them is called MoEDAL, short for: Monopole and Exotics Detector At the LHC. The international MoEDAL collaboration brings together more than 60 physicists. There is a strong contribution from the Swiss side, provided by the research group of Prof. Philippe Mermod, based in Geneva. Mermod (38) is an SNSF professor at the Department of Particle Physics of the Faculty of Science at the University of Geneva. He is leading a team of one research associate and two PhD students to perform searches for new physics with the ATLAS and MoEDAL experiments.
A particle with a single magnetic pole
The scientific goal of the MoEDAL experiment is the identification of new ionizing long-living particles. One special interest is the search for magnetic monopoles (isolated magnetic charges, just containing either the magnetic north or the south pole) as symmetric counterparts to the well established electric charges (plus or minus). In fact, so far only magnetic dipoles have been observed, in which the north and south poles cannot be separated. Indeed, in magnetic dipoles, the magnetic field is created by an electric current, instead of originating from single magnetic sources. Notably, the magnetic monopole was predicted by the physicist Paul Dirac in 1931. His theory provides an explanation why electric charge is quantized by simply postulating the existence of a magnetic monopole. Furthermore, magnetic monopoles are needed for theories coping with the unification of the strong, weak and electromagnetic forces, as independently showcased by the Russian physicist Alexander M. Polyakov and the Dutch physicist Gerard 't Hooft in the 1970s.
Unfortunately magnetic monopoles have not been discovered so far, although physicists have hunted them for many decades. With every built particle accelerator accessing a new energy regime, particle physicists tried to get hold of magnetic monopoles. At the LHC, this search can now be done with the highest ever-possible collision energy. In summer 2016 the MoEDAL collaboration published a scientific paper in the 'Journal of High Energy Physics'. The bad news: The magnetic monopole has still not been found. The good news: The researchers succeeded in pioneering a novel method of detection, which narrowed the window where the mentioned particle possibly can be found – if it actually exists. Philippe Mermod says: "With the MoEDAL prototype trapping detector, we could constrain the monopole production cross section (or probability to produce a monopole pair in a collision) in ranges of mass and charge which other experiments are unable to access."
Track and trap
The MoEDAL experiment at the LHC is constructed to record tracks of passage of highly-ionizing particles, as magnetic monopoles are – and to trap them. To manage both tasks – track and trap – MoEDAL uses two types of detectors. The first one is the Nuclear Track Detector (NTD). It is made of plastic foils able to identify highly-ionizing particles after exposure, etching and optical scanning. The second one is the trapping detector, which was primarily developed by Philippe Mermod and his scientific team. "When I joined MoEDAL in 2011 I proposed to extend the experiment with a subdetector which did not exist in MoEDAL at the time, the trapping detector. I showed in a study what we could probe with this new detector. The MoEDAL collaboration was very excited about this proposition and it was realized; and now it provided MoEDAL's first physics results!"
Mermod outlines how the trapping detector works: "We deploy an aluminium volume made of stacked rods near one of the LHC interaction points. If magnetic monopoles go through this volume they will slow down and a certain fraction will be stopped and subsequently trapped inside the rods. Each magnetic monopole is expected to bind to an aluminium nucleus. In this state we can unambiguously detect the presence of magnetic monopoles by analysing the rods with a superconducting magnetometer. This analysis is done by my team at the Laboratory for Natural Magnetism at ETH in Zurich, which possesses the sole devices appropriate for this kind of measurement available in Switzerland." Since the start of the experiment no less than 1278 aluminium samples have been probed in Zurich.
A lot of work has been done to establish the MoEDAL detector in its current setting and to collect the first data. But the most exciting period for MoEDAL scientists will occur in the next months. The findings published in Summer 2016 are based on a LHC collision energy of 8 TeV, from exposure in 2012. In the meantime the energy was augmented to 13 TeV. The higher energy significantly increases the chances to detect new massive particles, as Philippe Mermod emphasizes: "The most interesting results from MoEDAL are coming. We work now at the highest energies ever available at a particle accelerator. Magnetic monopoles, if they exist with masses of the order of a few TeV, should appear in the next years."
The discovery of a new particle would be the climax of Philippe Mermod's scientific career. The Swiss researcher studied physics at the University of Geneva and did his PhD in Uppsala (Sweden). He worked as a postdoc in Stockholm (Sweden) and Oxford (UK), attached to the ATLAS experiment. He returned to Geneva first as an SNSF Ambizione fellow in 2011 and then as an SNSF Professor in 2014. The new MoEDAL trapping detector was built in collaboration with the University of Alberta (Canada) and was scanned in Zurich in cooperation with the ETH Laboratory for Natural Magnetism. The simulation software needed to interpret the results was developed by a group of MoEDAL collaborators from various institutes coordinated by Mermod. The research project is supported by the Swiss National Science Foundation and benefited from a grant from the Marc Birkigt Fund of the Geneva Academic Society.
Author: Benedikt Vogel