The W boson has embarrassed our best theory of particle physics, and that’s great news

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The standard model of particle physics is a real feat. It was developed in several stages during the second half of the last century thanks to the joint efforts of hundreds of scientists, and it has brought us immense milestones. In fact, so far it has worked so well, it has so accurately described the interaction of the particles that we know, that physicists are having a hard time going any further.

And it is essential to do it if we want to elaborate new physics because the standard model does not explain everything. Not much less. In fact, physicists are convinced that ‘only’ roughly describes 5% of the universe, so there is vast knowledge out there that is currently beyond our reach. Going beyond the best theory of physics we have requires finding cracks in it, and, fortunately, little by little they are appearing.

The physicist and scientific communicator Javier Santaolalla precisely described during the conversation I had with him at the beginning of 2019 why the standard model is our best tool, and, at the same time, our biggest barrier in the search for the long-awaited new physics that we inevitably have to access to expand our knowledge about the laws that govern the universe:

“The Higgs boson has underpinned the standard model, and this is a problem because this theory is so perfect, spherical and seamless that we have no clue what the next layer might look like. And this is a challenge because so far we have managed to make progress thanks to the fact that we have been able to see flaws in our theories. You can only intrude into the dark areas that your theory has not entered when you find a hole in it through which you can enter.

W boson mass discrepancy may be the best thing that ever happened to physics

As Javier explained to us, finding cracks in the standard model is extremely complicated. And it is because it requires carrying out very complex experiments that require taking extraordinarily precise measurements. Even so, little by little physicists are managing to walk this path.

Exactly one year ago, in April 2021, Fermilab, the National Laboratory for High Energy Physics in the United States, released very strong evidence of the weakness of the standard model that it had stumbled upon when trying to explain the results it obtained in the muon g-2 experiment.

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During this test the muons, which are particles with a negative electrical charge, like electrons, but with a higher mass, did not behave as our strongest theory predicted. The measurements that the Fermilab physicists collected experimentally were so precise that they claimed to be reasonably convinced that the muon was being influenced by something that does not explain the standard model.

The analysis of the results of this experiment describes a very significant discrepancy between the mass of the W boson predicted by the standard model and the one measured using the Fermilab CDF II detector.

The surprising thing is that this scenario has just been repeated with a different experiment, which, again, has been carried out in the Fermilab particle accelerator. However, in the collection of the data and the analysis of the results, hundreds of physicists from more than twenty different countries, including Spanish scientists from the CIEMAT and the Physics Institute of Cantabria.

In the article published a few hours ago in the magazine Sciencethe physicists who have participated in the analysis of the results of this experiment describe a very significant discrepancy between the mass of the W boson predicted by the standard model and the one they have measured using Fermilab’s CDF II detector. This instrument has allowed them to take the most precise measurement we have so far of the mass of this particle, and, surprisingly, it is larger than they expected.

fermilab

Fermilab, the American high-energy physics laboratory, has the second most advanced particle accelerator after CERN.

The W boson is, together with the Z, one of the particles responsible for the mediation that takes place in the weak nuclear force, which is one of the four fundamental forces of nature along with the electromagnetic force, gravity and the strong nuclear force. Physicists usually place at this same level the higgs fieldwhich is another fundamental interaction that explains how particles acquire their mass, but to make it easier to understand, texts usually include the four I just mentioned as fundamental forces.

The W boson is, together with the Z, one of the particles responsible for the mediation that takes place in the weak nuclear interaction

The weak nuclear interaction is responsible for the radioactive decay of subatomic particles, and, interestingly, the W and Z bosons involved in it are heavier than the protons and neutrons that we can find in the nucleus of atoms. In fact, the mass of the W boson is about 80 times higher than that of a proton. The collection of data from this experiment has lasted for twenty-six years, and in its analysis the physicists have invested another ten years.

In any case, the most important thing is that this race to the bottom has yielded, as I mentioned a few paragraphs above, a significant disparity between the prediction of the standard model and the mass measured at Fermilab. It is still necessary that this result is endorsed by another experiment, but if it is finally confirmed that the standard model has this crack, we will have taken an important step in the search for new physics.

As Javier Santaolalla explained to us in the opening lines of this article, “until now we have managed to make progress thanks to the fact that we have been able to see flaws in our theories. You can only intrude into the dark areas that your theory has not entered when you find a hole in it through which you can enter. Let us trust that this is indeed one of the gaps that physicists have been searching for for many years.

Pictures | fermilab

More information | Science | CSIC

The standard model of particle physics is a real feat. It was developed in several stages during the second half…

The standard model of particle physics is a real feat. It was developed in several stages during the second half…

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