One of the major goals of science is to try to understand what things are made of and why they are stable (or, in some cases, unstable).
You have come a long way towards that goal. Today we know that all known things are made of particles, which we think are elementary, like two quarks of types (the down and the up), electrons and neutrinos (in descending order of their mass).
A hydrogen atom, for example, is formed by a proton, made up of three quarks, around which an electron moves. There are, however, things that are still unknown, like dark matter, that we are trying to figure out what it is.
There are, on the other hand, four fundamental forces that allow these particles to bond: the strong nuclear force, the weak nuclear force, the electromagnetic force and the gravitational force (in decreasing order of their intensity).
Os quarks that form the proton are bound together by the strong nuclear force. And the electron is connected to the proton by the electromagnetic force.
We also associate particles with these forces: the quarks they bind by exchanging massless particles called "gluons", while the electron and proton bind by exchanging other massless particles called "photons".
The first three forces have already been unified in a single theoretical framework (called the “standard model”), lacking the final unification with the gravitational force. Thus, the Universe is made up of particles, which exchange other particles among themselves.
In fact, the world is a little more complicated, because there are, in addition to the first one, two more generations of matter particles, which are unstable: in a second generation there are two more quarks (o charm and the strange), an electron-like but heavier particle called a “muon” (it is 207 times heavier than the electron) and another heavier neutrino, called muon; and, in a third generation, there are two more quarks (o top and the bottom), another “relative” of the electron, called tauão, and another neutrino, called tauonic.
We still have to add antimatter, although it is rarer than matter: there are antiquarks, antielectrons (or positrons), and antineutrinos. There are, in the next generation, anti-muons.
Physicists, however, are not satisfied with this description. Such a complete unified theory or “theory of everything” (this is the name of a film about the life of Stephen Hawking) is lacking. Albert Einstein, on the other hand, nurtured the dream of a “theory of everything”, although he had limited himself to unsuccessful attempts to unite electromagnetic and gravitational forces.
While in biomedicine scientists struggled to discover new vaccines against the new coronavirus, in physics scientists continued to make efforts to penetrate the mysteries of matter. For that, a precious indication would be an identification of a disagreement between what the theory predicts and the experience.
Experience always rules: if done well, theory will have to change in case of disagreement. Even small disagreements can be relevant, as they were in the past, to advance towards new understandings of the Universe.
Now, in an article published on April 7, 2021, the results of an experiment in particle physics were announced, showing a clear disagreement between predictions and reality.
The experiment was carried out by a team of two hundred researchers in a particle accelerator at Fermilab, a research center near Chicago, in the United States.
They analyzed the magnetic properties of antimuons, the antiparticles of muons, which were made to circulate at almost the speed of light in the accelerator, being subjected to a very intense magnetic field, produced by a large magnet.
Antimuons, which are positive while they exist (since they decay quickly to positrons), “dancing” in the magnetic field because they are small magnets, which respond to the field. And the detectors let you know how this “dance” is.
The experiment is called "g-2" because there is a property of the antimuon called the g factor (a number without units, having nothing to do with the acceleration of gravity that has the same name) that describes the magnetic behavior of the muon and should having, according to theory, a value slightly higher than 2. The most recent experience – with much greater precision than the previous ones – confirmed the existence of this deviation.
The new result seems to indicate that there are new particles, as yet unknown. The measured g-factor is 2,00233184122. It is slightly higher than 2, as expected, but it does not match the theoretical value of 2,00233183620. The difference is very small but significant. If the experiment is flawless, the theory will have to be reformulated.
And a new theory consistent with experience will likely go beyond the standard model, incorporating new building blocks of matter, which no one quite knows at this point. We will then have penetrated more deeply into the mysteries of matter.
As always in science, confirmation of the results obtained is awaited.
Author Carlos Fiolhais is Professor of Physics at the University of Coimbra
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