On July 12, 2012, there was a landmark announcement: CERN’s Large Hadron Collider (LHC) near Geneva had finally detected the Higgs boson, the last brick missing from the reference frame of particle physics. , the so-called standard model. However, there was still another unknown that physicists have been trying to solve for decades, one that would open the doors to a world populated by hypothetical subatomic entities with names like axions, neutralinos, WIMP or charginos. We are talking about the magnetic monopole . Three years after the success of the Higgs, CERN launched a new experiment, called the Monopole and Exotics Detector (MoEDAL): the hunt for the particle that has eluded physicists since the late 1970s had just begun.
Although magnetism is an old acquaintance – there is archaeological evidence that suggests that the ancient Olmecs used it around 1200 BC. C. in their rituals – was an enigma until the nineteenth century. In 1819, a professor of physics at the University of Copenhagen named Hans Christian Oersted observed that when a compass was brought closer to a wire that conducted electricity, the needle changed direction. But it was the French Andrè-Marie Ampère who ended up laying the foundations of electromagnetism by assuming that if the electric current behaved like a magnet it was because, in some way, it must be a magnet . The great synthesis came from the Scotsman James Clerk Maxwell, who concluded that light, electricity, and magnetism were intertwined and in 1864 enunciated what today are known as Maxwell’s laws .
Now, mere observation tells us that there is a fundamental difference between electricity and magnetism: while in the former electric charges may exist separately, in the latter we always see that they appear in pairs; no matter how much we cut a magnet, we will never isolate the north pole from the south. That is why it is said that there are no magnetic monopoles . Now, aren’t there because we haven’t seen them or because they can’t exist?
This absence bothers some physicists because it breaks a criterion of beauty that they consider sacrosanct: that of symmetry ; makes Maxwell’s laws not totally symmetric. However, they perfectly explain all electromagnetic phenomena, an immense achievement for equations with more than 150 years of history. So, if the theory does not need monopoles to explain what happens around us, why insist?
In 1931, with the quantum theory well established, a lonely English physicist named Paul Dirac demonstrated that magnetic monopoles made the electric charge only take a discrete value, that is, a multiple of a minimum unit. Until then, this fact was capon in the equations of quantum mechanics, and now it appeared naturally. Apparently, the existence of monopoles is necessary for the electron to have the charge that it does . And not only that: it can also help solve the quagmire that particle physics finds itself in.
The Standard Model, slowly built over half a century, describes both the properties of subatomic particles and the workings of three of the four forces of nature. But it is incomplete: it does not incorporate gravity, and it also does not have space – or has not been found – for the mysterious dark matter that fills the universe. The monopole can fill in those gaps.
His search began in the 1970s, when the so-called great unification theories (TGU) appeared, the offspring of physicists’ efforts to find the great cosmic symmetry. In this case, the TGU intend to group under a single formulation three forces of nature: the electromagnetic, the strong –responsible for the cohesion of the atomic nucleus– and the weak –on which a type of radioactive decay depends. This union takes place at extremely high temperatures, which in the universe have only been reached just after the big bang. Well, in 1974, Gerardus’ t Hooft and Alexander Polyakov showed that all TGUs presuppose the existence of monopoles . Furthermore, they tell us what its mass may be: about ten thousand trillion times that of the proton. They also predict that large numbers of them had to be created in the very first moments of life in the universe; practically, one for each proton in existence. But that is not observed.
How to solve the problem? You need a way to quickly lower the temperature of the cosmos so that not so many monopoles are formed. This mechanism was found by the cosmologist Alan Guth in the 1980s, the so-called inflation: the universe saw its expansion accelerated exponentially between 10 36 and 10 32 seconds after the initial explosion . It is a consistent theory, capable of explaining some drawbacks of the big bang paradigm, but it is pure conjecture. As the British cosmologist Martin Rees wrote, those who do not believe much about this exotic physics “are not going to be very impressed with a theoretical argument that serves to explain the absence of particles in itself hypothetical.”
Are not the TGU wrong? For now they are nothing more than speculation, as our particle accelerators are nowhere near the energies needed to check their predictions, and the very few that have been found are not very promising. The interest that these theories generated in the 1970s and 1980s has been lost in favor of more grandiose ones, so to speak.
These are the so-called theories of everything, such as the string theories, which try to include gravity in the search for a single superforce that brings together the four that exist in nature . In this case, the forces and particles would arise as a consequence of the vibration of small and mysterious energy strands. Well, all theories at all say that monopoles should exist. One of the leading experts in this field, Joseph Polchinski, claimed in 2002 that it was “one of the safest bets one can make in physics.” Sixteen years later, and before his death in February 2018, he continued to maintain: “When we have a unified theory of physics, you will find that it is accompanied by magnetic monopoles .”
That is why there are several teams determined to look for them, something that can be done in three ways: first, trying to produce them in particle accelerators; second, to look for them in cosmic rays or trapped in certain materials; and third, to trace indirect evidence of its existence in different astronomical phenomena.
Could they be generated in an accelerator like the LHC? Thanks to Einstein we know that energy and mass are related by the formula E = mc 2. This implies that if we want to create a particle, we will multiply its mass by the square of the speed of light to obtain the necessary energy. In the case of the monopoles predicted by the TGU, it takes a trillion times more than is produced in the LHC. However, it is possible that there are specimens of intermediate mass, and that is where the hunt is headed.
With the rest of the particles, like the Higgs boson, the method is very different. These have a very short life and quickly disintegrate. To find them, physicists must search through the tangle of particles that are produced in the accelerator when two protons collide. On the contrary, the monopole is stable – it can only be destroyed if it meets its antimatter twin – so, once produced, it does not disappear . Due to its magnetic charge, it interacts strongly with the electromagnetic field, and best of all, it does so very differently from other particles.
Unfortunately, due to these differences, most of the experiments installed at the LHC do not match the search for monopoles. For this reason, in December 2009 CERN approved the Monopole and Exotics Detector (MoEDAL) , which began collecting data in 2015 and in which 70 researchers from four continents participate. Their goal is also to find other supermassive exotic particles that can be produced in proton collisions.
To get an idea of what it is, let’s imagine many layers of plastic placed one after another. If we shoot a pellet, it will cross all the plates and in each one of them it will have made a hole; By aligning these sheets well, we will be able to draw the trajectory followed by the projectile. In the case of MoEDAL, we have approximately 400 nuclear trace detectors (NTDs), each of which is stacked with ten layers of plastic. “It’s like a giant camera, and the sheets act as film,” says James Pinfold, a spokesman for MoEDAL. If something is recorded, physicists will toast with champagne, because “no particle of the standard model can leave a trace on our plastic,” says Pinfold.
To complete the hunt, in September 2012 the Magnetic Monopole Trapper was installed in MoEDAL, consisting of an aluminum structure that, “if it catches a monopole, it generates an electric current. There is also no way for it to detect anything else, ”explains Pinfold. In July 2019 they published the first results, the same as those given by other accelerators in the past, such as the Tevatron, LEP and HERA: zero monopoles.
Instead of trying to produce them in the lab, we can also look for the ones that supposedly already exist in nature – those that were created with the big bang are still around. Obviously, how easy it is to find them will depend on how many there are. The Pierre Auger Observatory, in Argentina, is dedicated to observing the cascades of particles and photons that cause cosmic rays when they collide with the atmosphere. There, about 1700 tanks full of water arranged in 3000 km 2 await the passage of a hidden monopole between these space showers.
On the other hand, at the South Pole is the IceCube observatory , of the United States National Science Foundation, which uses pristine Antarctic ice to study the neutrino, a particle that barely interacts with matter. This makes this installation an ideal place to try to see the passage of the heavy and slow monopole of the TGUs. If an Antarctic ice proton collided with one, it would disintegrate and other types of particles would appear, which the IceCube would record promptly.
Monopoles have also been searched inside meteorites , as they could have been stuck in an atom: in 1995 the most extensive study carried out to date was carried out, when a total of 112 kilos of space rock were studied. And finally, volcanic rocks collected at the poles have been scrutinized. This is so because throughout the history of the Earth some monopoles could have been trapped in rocks of the Earth’s mantle and then have slowly risen to the surface in the direction of the geomagnetic poles. In 2013, Swiss scientists examined 23.4 kilos of polar rocks … with no result.
But, as they say, the absence of evidence is not evidence of absence . The rules of logic say that the non-existence of, for example, the tooth fairy cannot be proven. So what is the use of such a negative result? To determine a limit to its abundance. Thus, from the study of meteorites it is deduced that the density of monopoles in the Solar System must be equal to or less than one specimen for every hundred kilos of meteorite rocks. The strictest limits come from the underground experiment MACRO (Monopole, Astrophysics and Cosmic Ray Observatory), which operated in Gran Sasso (Italy) from 1989 to 2000. From this and other investigations it is clear that in the universe there must be less than one monopole per quintillion nucleons (protons and neutrons) . A needle – magnetic – in a haystack.
