Two particles shake up physics

While it is too early to claim victory, that does not prevent hope. But what exactly? “The properties of muons, such as mesons, and everything that makes up matter are predicted by the Standard Model of particle physics, recalls Yasmine Amhis. Our goal is to titillate him in order to catch him at fault. To discover its limits. Both of these anomalies could mean we’re not far from it. “ The standard model, developed at the start of the 1970s, groups together the 17 so-called elementary particles, because a priori unbreakable. They constitute the bricks of a construction game allowing to create all the atoms existing in the Universe. Some of these bricks are particles of matter, and others of radiation, called bosons. This means that they carry the forces that bind the particles together. For example, the strong interaction ensures the cohesion of the nucleus of atoms. It is carried by the aptly named gluons. The electromagnetic interaction, at the origin of the electric and magnetic forces, is carried by the photons. There are three fundamental interactions in all, plus the gravitational interaction which always escapes description by particle physics.

Before looking at its limits, we must first recognize the extraordinary robustness of the standard model. Not only does it offer a precise description of the behavior of elementary particles, but it has made it possible to predict the existence of new ones, even before we are able to produce them. The most famous is the Higgs boson, imagined in 1964 by theorists Robert Brout, François Englert and Peter Higgs, and finally observed at the LHC in 2012. So why try to scratch such a beautiful building? “The standard model describes very well classical matter and its interactions, confirms Olcyr Sumensari, theoretical physicist at the Irène Joliot-Curie Physics Laboratory of 2 Infinities (IJCLab), in Orsay (Essonne). But we know from the standard model of cosmology that it only represents 5% of everything that makes up the Universe. However, it does not say anything about dark matter (25%), nor about dark energy (70%) at the origin of the acceleration of the expansion of the Universe. Likewise, he predicts that neutrinos, the most abundant particles in the Universe, emitted during radioactive processes, are devoid of mass. However, we discovered in 1998 that they had one, although much smaller than those of other known particles. The Standard Model therefore seems solid, but we can see it as the foundation of a more fundamental theory that we have not yet mastered. The big challenge today is to try to find experimental clues that would help us formulate this theory. “ But questioning the Standard Model has serious consequences. As Yasmine Amhis points out: “We can find other particles of matter, but also other particles of radiation. Which would mean bringing to light… a new force!”

In search of unknown particles

From this point of view, the anomaly of the magnetic moment of the muon could be particularly illuminating. If the measured value does not correspond to that expected, it means that the muon is heckled by other particles which inhabit the “void”. Because in the infinitely small, the void never is. As the surface of the ocean is traversed by waves sometimes forming drops, the vacuum is agitated unceasingly by particles which burst out of a “sea” of energy filling all the space, then disintegrate. What are the particles that disturb the muon in this way? “To calculate its magnetic moment, all interactions with known particles have been considered. If this difference between theory and measurement is confirmed, then the disturbances necessarily come from unknown particles”, deduced Olcyr Sumensari. The measure does not seem to be called into question. And if it was necessary to revise the prediction instead, the theoretical calculation being very difficult to carry out?

However, at the same time as Fermilab published its result, an international team of theorists gathered within the “Budapest-Marseille-Wuppertal” collaboration published a new theoretical value of the magnetic moment compatible with the measurements! This new approach is currently the subject of intense verification. On the experimental side, a measurement of the magnetic moment with a method different from Fermilab, should take place at the Proton Accelerator Research Complex, in Japan, by 2025. On the B mesons side, the theoretical value is not, this time, put into effect. doubt: these are the measures that need to be confirmed. Not only is the counting of those taken at the LHC continuing, but another B-meson decay experiment, Belle II, is taking place in Japan right now. If it confirms the LHCb results, then theorists already have some ideas to explain them … “We can imagine a very massive particle, nicknamed leptoquark, which would interact more with muons than with electrons, expliqué Olcyr Sumensari . It can also be the manifestation of a fifth force, carried by a boson called Z ‘, which would act even more on muons than on electrons. In both cases, it would be a revolution, because muons and electrons are identical for the forces described by the standard model. There is no theoretical reason for such a difference in behavior. “ Not only the counting continues at the LHC, but other publications are expected on the decays of different types of mesons, a big family! “With LHCb, we are also studying B +, B0, lambda b mesons… We hope to publish at least one more result before the end of the year. And more will follow next year”, confirms Yasmine Amhis. With the hope that mesons, like muons, will confirm their strange behavior, opening the doors to new particle physics.

Towards the accelerators of the future

To discover new particles, it is necessary to generate more and more powerful collisions, in order to reach areas of energy still largely unexplored. To imagine these colliders of the future, European scientists in the field launched the I.FAST project on May 1. With a budget of 18.7 million euros financed by the European Union, it will bring together 48 organizations from 14 countries and will be coordinated by CERN in Geneva. “This collaboration between accelerator physicists has existed for a long time, explains Jean-Luc Biarrotte, who represents the CNRS within I. FAST. On the other hand, the fact that it now welcomes industrialists in the field is new and promising. ” Among the technologies that will have to evolve, superconducting magnets figure prominently. Thus, the Future Circular Collider (FCC), a 100 km ring which could one day succeed the LHC, on the same site, will need 16 Tesla magnets, twice as many as those of the LHC. “For the moment, we do not know how to do, notes Jean-Luc Biarrotte. Developing such magnets will require new type of superconducting materials. “ Another project: muon colliders. These particles being 200 times more massive than electrons, they could be used for the development of relatively compact very high energy colliders. “But in this field, almost everything remains to be invented: the muon factory, the formation of a usable beam, then its acceleration which must be hyper-rapid because they are particles with a very short lifespan … This type of collider will probably not see the light of day before 2050. “

Observe particles sometimes very fugitive requires having enough energy to create them during collisions between stable particles. Hence the race for gigantism of the colliders (in orange). Their performance is still far from reaching the energy levels involved during the Big Bang, 13.8 billion years ago, where according to some hypotheses, all the forces of physics were merged into one. Credit: BRUNO BOURGEOIS

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