New measurement of W-boson mass baffles researchers

So far, consistent with the predictions of the Standard Model, a new measurement of the mass of the W boson, the weak force mediator particle, has once again shocked the world of particle physics. Behind this overvalued figure is a new clue that something is still completely eluding us.

Something is definitely wrong with the bosons. While the mass of the Higgs boson is still a mystery to physicists, now the mass of the W boson, whose measurement has just been refined in an international experiment, seems much heavier than hitherto expected. a theoretical framework that describes nature at its most fundamental level. In a word, practice absolutely does not confirm the theory, which can confuse, and most importantly, please, physicists.

About twice as accurate as the previous one by CERN in 2018, this new measurement of the proton mass W was made after a decade of analysis of data collected by TeVatron, former Fermilab accelerator in the USA. At the helm is the Collider Detector Fermilab II (CDF II) collaboration, which brings together some 400 researchers from around the world. According to the authors of the findings published on Thursday, April 7, 2022 in the journal The science, the results show that the boson mass W is 7 standard deviations larger than the prediction of the Standard Model. Previously estimated in mW = 80370 ± 19 MeV, i.e. a figure consistent with theory, thus reaching mW = 80433.5 ± 9.4 MeV/c2, “a significant voltage measurement with the expectation of a standard model,” the researchers say.

General surprise

An elementary particle predicted in the 1960s and discovered in 1983, the W boson is a fundamental milestone in the Standard Model: it mediates the weak force (W, for “weak“), one of the four forces that govern the behavior of matter in our universe. It turns protons into neutrons and vice versa through the weak nuclear force, which triggers nuclear fusion and allows stars to burn. Therefore, it is an understatement that knowing its exact mass is a parameter limited other observable parameters, such as the charge of electrons and the masses of other particles, is crucial for testing the reliability of the predictions accepted so far.

“Our measurements directly contradict the standard model”

“Our measurement is in direct conflict with the Standard Model, in other words, the most successful quantum theory of matter and forces to date.”said Science and the future Ashutosh Kotwal, physicist at Duke University and director of the CDF II experiment at Fermilab. Ashutosh Kotwal himself admits to being very surprised by this result, which he describes as “wonderful shock”. And for good reason: if confirmed, this difference between the measured value and the value calculated within the framework of the standard model could only be attributed to a new mechanism in nature, “a new fundamental principle that still eludes us”specifies the physicist. “It could be a new particle or a new subatomic interaction that we will be looking for in current and future experiments.”

Sign of new physics?

Among the possibilities currently being considered by experts are supersymmetry (which predicts a partner particle for each of the standard model particles), the influence of unknown particles such as the Higgs boson or even particles from the “dark sector”, or a family of particles that make up, among other things, dark matter . “We know that the Standard Model is incomplete because it cannot solve the problem of dark matter or excess matter over antimatter.”– Ashutosh Kotwal recalls.

Claudio Campagniari, a physicist at the Italian Institute of Nuclear Physics who was not involved in the CDF experiment, says he is, too. “surprised” by this result. What would not surprise this member of the CMS collaboration at the Large Hadron Collider (LHC) at CERN is that this figure will cause a stir in the physics community once it is officially announced, scheduled for 20:00 French time this Thursday. . . . “Now I expect the following to happen: some experts will sort through the details of the methodology, asking themselves the question: “Is something wrong?”; others will look specifically at the calculation of the W boson mass – and more specifically about the various sources of uncertainty that remain in this result – wondering if these uncertainties were not considered too optimistic, so that the actual prediction of the W mass may not be as accurate as we are thinking”.

Other important impressions

Other theorists must also assume that the gap between experiment and theory is real. “They will try to directly answer the question: what new phenomena could quantitatively explain this deviation, as well as others that have recently appeared in various experiments? Even if in reality none of them are as important as this!” Finally, the physicist predicts that some experimenters will think about ways to improve and revalidate measurements of other physical quantities used as input for the theoretical calculation of the mass of the W boson.

For their part, experiments at the LHC, which have collected and continue to collect a lot of data, should also help to see things more clearly, even if W bosons are produced there a little differently than at TeVatron (LHC is a proton-proton collider, and TeVatron is a proton – antiproton collider). Moreover, if it were built, the new electron-positron collider could also measure the mass of the W boson with great accuracy, like other facilities on a smaller scale, but no less sensitive to new particles and interactions. mass W. Despite everything, patience will be needed: it will definitely take several years of data collection to consider obtaining confirmation worthy of the name.

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