The model of physics called into question

The numbers wouldn’t even tickle a scale hand. Except that the physicists did not use a balance and that even if the mass considered is tiny, the difference between theory and practice is enormous. Thursday, April 7, an international study, published in the journal Science and co-signed by nearly 400 researchers, came to shake the foundations of physics. The problem ? The W boson is overweight compared to theoretical predictions.

This boson is an elementary particle, whose existence had been predicted in the 1960s and finally proven in 1983 by experiments at CERN, the European organization for nuclear research. In detail, it conveys the weak interaction, one of the four fundamental forces of physics. “The weak interaction is responsible for radioactivity and the production of energy by the sun, for exampledescribes Yves Sirois, particle physicist at Polytechnique. It acts at a very short distance and causes the instability of matter. »

A solid and coherent standard model

To describe this interaction, but also all the other fundamental forces of matter and the different particles, physicists have built a model over the decades and discoveries. This so-called “standard” model aims to explain almost everything that surrounds us. “It is a fundamental theoretical approach which, for 60 years, summarizes the whole history of physicscontinues the specialist who participated in the discovery of another boson, the famous Higgs boson. The model is extremely solid and scalable. He was able to integrate the discoveries as they were. »

→ CHRONICLE. Higgs boson, science news

Solid, reliable, practical, the standard model is also consistent. The parameters are somehow codependent, and by knowing the mass of some particles, it is possible to predict the mass of others. In the case studied, the mass of the W boson can thus be deduced from two other known values. Except that by comparing the prediction and the reality, the physicists found a difference: the boson weighs heavier than expected.

Results to come from CERN

Could the error come from the prediction? Maybe the values ​​used to deduce the mass of the W boson are not the right ones? Unlikely, retort experts in the field. And even if there is an inaccuracy on these two reference values, it would not be enough to explain the observed difference.

→ EXPLANATION. CERN revolutionizes particle physics

A measurement error then? Maybe, maybe not. “The study was carried out from the collisions of particles produced in an American accelerator, the CDF in Chicago, which stopped in 2011describes Guillaume Unal, coordinator of a particle detection program at CERN. It took ten years of analyzes to arrive at these results and they are very robust. » Scientists are still awaiting other results in the coming years, this time from CERN facilities, for confirmation. “Better to have more than one measurerecognizes Yves Sirois. If the tension remains in these results too, it is extremely interesting and upsetting. »

Unknown elementary particles?

The difference between real measurements and theoretical predictions would then signify the existence of interactions or elementary particles, hitherto unknown. “The heavier a particle, the more difficult it is to detect because it takes a lot of energy to produce it in an accelerator.simplifies Guillaume Unal. For now, the only certain thing is that there are phenomena that are not explained by the standard model. This does not mean that it should be abandoned, but it needs to be extended. »

In recent years, other studies have cracked the standard model. Above all, this model does not include gravitation (the force behind Earth’s gravity), because it does not “stick” with other forces. In the universe, astrophysicists have also had to invent dark matter, whose existence is still hypothetical, to explain phenomena affecting stars and galaxies. Again, the Standard Model neither predicts nor explains this dark matter. As one specialist humbly puts it, “We don’t know everything yet”.


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