Scientists Use Giant Atom Smasher in Search for Magnetic Monopoles
Image by James Pinfold, MoEDAL collaboration.
(Inside Science) -- Scientists are using the intense magnetic fields generated by the world's largest atom smasher to search for one of the most elusive particles of all -- the magnetic monopole, a hypothetical particle with either a "north" or "south" magnetic charge, but which has never been seen.
A study published this month in the journal Nature describes the latest experiments with the MoEDAL instrument -- the Monopole and Exotics Detector at the Large Hadron Collider (LHC) -- which was installed in 2015. So far, it's never found any monopoles, but that might be because previous MoEDAL experiments looked for monopoles created in collisions between particles like protons and neutrons.
However, "we realized there is a different mechanism for producing monopoles, not based on collisions of elementary particles," said Arttu Rajantie, a professor of theoretical physics at Imperial College London and a co-author of the study.
Instead, the latest experiments look for monopoles created by what's called the "Schwinger mechanism" in powerful magnetic fields. If monopoles exist, the mechanism would create them as pairs of particles with opposite poles -- one with a "north" magnetic field and the other with a "south" magnetic field, but moving in opposite directions and otherwise completely independent of each other.
The mechanism is named after the American Nobel Prize-winning physicist Julian Schwinger, who in 1951 theorized that strong electric fields would produce electrically charged particles in the same way. Electric fields have since been shown to produce electron-positron pairs -- so physicists hypothesize a strong magnetic field will similarly create pairs of monopoles, the magnetic counterparts of electrons and positrons.
Crucially, the Schwinger mechanism lets scientists confidently calculate how many monopoles of a given mass and magnetic charge would be produced by a magnetic field of known strength. Rajantie led the calculations for the latest study. While the experiments once again didn't find any monopoles, the calculations have enabled the team to narrow down their search by ruling out the possibility that monopoles have very low mass or less than a certain magnetic charge.
The study's lead author, University of Alabama particle physicist Igor Ostrovskiy, said magnetic monopoles feature in several theories that seek to go beyond the Standard Model of particle physics, which describes three of the four known fundamental forces and all of the known elementary particles (currently there are 31, including the Higgs boson).
"There are strong reasons to believe that the Standard Model of physics is not the whole story," he said in an email. Combined with other evidence, there's "a good indication that monopoles may exist and be worth searching for."
But monopoles only exist in theories so far. The magnetism we take for granted -- it sticks magnets to fridges and generates electricity in turbines -- is caused not by monopoles but either by the quantum spin of subatomic particles in a material or by electric currents. But those processes always create a magnetic dipole -- a magnet with a north pole and a south pole that are impossible to separate.
The technological consequences of discovering monopoles can't be foreseen, but it would have "a transformative impact on physics," Ostrovskiy said. "It would confirm that there are laws of nature not captured by the current ruling theory of physics."
The researchers examined the MoEDAL detectors after the LHC's 2018 run of billions of lead nuclei collisions, which was conducted mainly so physicists could study the quark-gluon plasma created when heavy ions smash into each other.
Rajantie explained that the occasional near-misses of the lead nuclei in the LHC briefly created the most powerful magnetic fields on Earth, and possibly anywhere -- the fields were 10,000 times stronger than those on the surface of spinning neutron stars called magnetars. And although they only lasted less than a septillionth of a second, they were also about 10 million times stronger than the weakest magnetic fields needed by the Schwinger mechanism, so they would have produced monopoles -- that is, if monopoles exist.
The team hoped to trap the stable monopoles in the MoEDAL instrument's detector, which consists of 1,700-pound (800 kilogram) aluminum blocks. The blocks were dismantled after the run and passed through a superconducting magnetic loop to verify if any monopoles had been found.
But they weren't found -- and thanks to the predictions of the Schwinger mechanism, the scientists have now ruled out the possibility that monopoles are lighter than about 75 times the mass of a proton, with fewer than three base units of magnetic charge. Their next step will be to repeat the experiments, after modifying the MoEDAL instrument to detect heavier magnetic monopoles with greater magnetic charges.
The MoEDAL search highlights a difference in terminology between particle physicists and condensed matter physicists. Researchers in the United States, Switzerland and Finland said in 2019 they had imaged monopoles that emerged under specific conditions in a low-temperature magnetic material called a "spin ice" at the Lawrence Berkeley National Laboratory in Berkeley, California.
While the MoEDAL monopoles would create either a north or south magnetic field, the Berkeley monopoles do not, which means they’re not the real thing. They're mathematical analogues of monopoles -- virtual particles, also known as "quasiparticles." But since they behave just like particles with a magnetic charge, that can be useful for determining how real monopoles would behave, Rajantie said.
Stephen Blundell, a condensed matter physicist at the University of Oxford who isn't involved in the latest study, suggested the monopole quasiparticles might be the closest thing to true monopoles that anyone will find.
"Searches for magnetic monopoles are yet to find any, and this new, impressive study is no different. … Maybe they're not there at all?" he said in an email. While the quasiparticle analogues don’t challenge the Standard Model of particle physics, they can show how such monopoles would behave under different conditions. "While these aren't the kind of magnetic monopoles that particle physicists are looking for, they can actually be studied in experiments," Blundell said.