Everything has to do with antimatter and to understand it we must go back in time almost a century, to October 1927 , and approach the Institute of Physiology in Brussels, where the fifth Solvay Congress was held under the title ‘Electrons and Photons’ . It was going to be the most important in the history of physics: 17 of the 29 attendees (it was by invitation only) were or were going to be Nobel Prize winners. The title did not reflect the true intention of this meeting, which was to sort out the path that quantum theory was leading. It was there that Einstein said his famous phrase “God does not play dice” and the Danish physicist and father of quantum theory Niels Bohr replied: “Stop telling God what to do” .
This mythical meeting was attended by Paul Adrien Maurice Dirac , an extremely quiet and taciturn young English physicist (his friends coined the dirac as the smallest unit of words that can be said in a conversation), who worked alone every day of the week except on Sunday, he went out for a walk… alone. He was obsessed with being able to unify the two great theories of physics at the beginning of the 20th century: the quantum theory and Einstein’s special relativity . For two years he had tried and failed, but after the Brussels congress he decided that this would be his life’s goal for the next few years. And he succeeded thanks to a creative use of mathematics , obtaining the first equation that unified both theories . Two things were born from it that have been revealed to be fundamental: the first was that spin was naturally deduced, an elusive concept introduced by the German physicist Wolfgang Pauli – a genius who at the tender age of 18 was an expert in relativity – to explain certain experimental results. Even today the interpretation of what spin is is still obscure, but we do know that in the presence of a magnetic field it can be aligned in two directions: up and down. A strange property that in the electron allows, for example, that in hospitals we have magnetic resonances.
The other surprise hidden between the folds of that equation was that the two directions in which the electron spin could be oriented were only half of the solutions , since there were two others that corresponded to mysterious negative energy states of the electron . This meant that the electron could jump from its normal state of positive energy and negative electric charge to one of negative energy and positive electric charge. What the hell was that? No one knew, and some began to think that the Dirac equation was not as good as it was thought. After four years racking his head, Dirac stated that these mysterious solutions corresponded to a new type of particle that had the same mass as the electron but with a positive charge . And the following year, in 1932, a young 27-year-old American professor, Carl David Anderson, found this mysterious particle in his experiments with cosmic rays: we had just discovered antimatter.
To understand it, we must take into account that physicists distinguish one particle from another in the same way that we distinguish one fruit from another, by its properties: size, color, smell and taste. In the case of subatomic particles we have to refer to characteristics such as mass, charge, angular momentum and magnetic moment. The first two are easy to understand, but the second two are not. We can assimilate angular momentum to rotation and magnetic moment to the fact that the particles, supposedly spherical, behave like the Earth, with a north and south magnetic pole. So what would an antielectron be? First, it is charge reversed , it is positive instead of negative. And the same thing that happens when we see a ball spin in the mirror, its rotation, the angular momentum, is also reversed. This is called parity inversion. This change in parity would force the antielectron to have its north and south magnetic poles swapped with respect to the electron, but since the charge is also reversed, the magnetic moment does not change. The same happens with the proton and its corresponding antiproton, but not with the neutron. The neutron has no charge , therefore the only possible inversion is parity, which forces its magnetic moment (its magnetic poles) to be changed with respect to the neutron, and this is the only way for physicists to distinguish them. Thus we have the three antiparticles needed to build antimatter. In our case, an antiproton and a positron would form the antihydrogen atom.
The most important feature of antimatter is that if it meets a particle of matter, they annihilate . All of its mass is converted into energy, the quantity of which is given by the most famous formula in history E = mc 2 . And vice versa: if we have enough energy we can create an electron and a positron, or a proton and an antiproton. And this is what we always observe: it is a creation of pairs, a particle and its corresponding antiparticle.
This is where cosmology comes into play. With the explosion that marked the beginning of the universe, as much matter as antimatter had to be created. And this is a problem, because if this 50-50 ratio had been exact, all matter would have been completely annihilated with its corresponding antimatter. But we’re here, right? Something must be wrong with our nice theory…
Faced with such a problem, there are only two solutions : first, that our universe is divided into two , one with stars and planets and the other with anti-stars and anti-planets. This option has a serious drawback: the border between the two universes would be a continuous burst of energy visible to our gamma-ray space telescopes , something that is not observed. The only possible option is that this 50-50 was not exact but that there was a tiny excess of matter, about a billion antiparticles compared to a billion and one of matter. Where that excess came from has worried physicists for more than half a century.
In 1967 the Soviet Andrei Sakharov -the famous dissident and father of the Russian H-bomb- showed that the only way to avoid this problem was that the so-called CP symmetry was not fulfilled . This ensures that if we simultaneously exchange its charge (positive for negative) and its parity (right for left) to a particle, it continues to behave in the same way. Only if this did not happen, Sakharov reasoned, could we justify that there is more matter than antimatter in the Universe. At that time it was unthinkable that Sakharov was right because all physicists were convinced that, as they say, the universe is invariant under CP. But in the mid-1970s it was shown that the CP symmetry was not strictly fulfilled: the electroweak theory (which unifies electromagnetism and the weak force, responsible for a type of radioactive decay, under a single formulation) did not fulfill it. Unfortunately, if things were as that theory says, there would only be enough matter in the universe to make a galaxy.
But in January 2019 the LHCb experiment carried out at CERN in Geneva has observed an incredibly rare and never before observed process in an accelerator: the formation and decay of the Λ b 0 particle and its corresponding antiparticle . According to the standard model, this couple should behave exactly the same, but the data obtained suggests that they are different by up to 20%. The hope of physicists is that this small discrepancy in their behavior will explain why the universe is full of stars.
