Science dedicates the cover to a result of the FermiLab in Chicago, which measured the mass of Carlo Rubbia’s W boson with the collaboration of the National Institute of Nuclear Physics: it could open “a crack” in the standard model that until now has explained the wonderful world of particle physics
The mass of the Rubbia’s W boson is bigger than expected. What could change for matter? Forget for a moment the most famous Higgs boson, discovered at CERN in Geneva. The one finished now on the cover of Science is the W boson, theorized in the 1960s and discovered by Carlo Rubbia who thanks to it shared the Nobel Prize in Physics in 1984 with Simon van der Meer: together with the Z boson it is one of the two particles mediating the weak force and is responsible, among other things, for the nuclear decay processes that feed our Sun. In short: understanding it from the point of view of physics is not easy. But intuiting its importance yes. To link it, or rather to couple it with the Higgs boson, is the weak nuclear force, originally discovered by Enrico Fermi and then made more complete by the introduction of electromagnetism in the theory. This is to explain the importance of Italian physics up to this point. ORThe problem is the measurements. They don’t come back. And, not returning, they question the so-called Standard Model, the reference theory of physics that describe the world of elementary particles and fundamental forces (in short, matter). For this Science has dedicated its newsstand cover to the new measurement of the mass of the W boson particle by the FermiLab of Chicago. The title: “Heavy weight” under a fat W. In fact: too heavy.
Are we facing “a new physics” as anticipated, with an excess of pardonable enthusiasm, by a previous communication from the Chicago FermiLab about a year ago? What we can say today, with greater caution, is that we are dealing with the “most accurate measurement of the mass of the W boson ever made”
as communicated by the National Institute of Nuclear Physics which coordinated the participation in the Italian project: “It is the result of a ten-year analysis carried out by the Collaboration of the CDF experiment (Collider Detector at Fermilab, that is the particle accelerator ), which has been in business for over 25 years at Fermi National Accelerator Laboratory’s Tevatron Accelerator in the United States. The value obtained from scientific collaboration through this scrupulous analysis differs from that predicted by the theory of the Standard Model which describes the world of elementary particles and fundamental forces ». A technical language to say that it was not easy to make these statements.
“We did everything possible to be rigorous before opening the ‘box’ of results,” he tells al Courier Giorgio Chiarelli, researcher of the Infn Pisa Section and co-responsible for the CDF scientific collaboration. «For ten years we have checked the correctness of the method without knowing the result. This is in order not to be influenced: we are all men and women … sometimes we risk deluding ourselves that we will find exactly what we are looking for. For this reason we have waited a long time. I still remember when we opened the “box” in November 2000. It was a very special moment. We were all on Zoom. But even at that moment there was a very calm discussion ».«As the sociologist Robert Merton recalled – adds Chiarelli – one of the pillars of science is organized skepticism. Now I expect some excitement, but also organized skepticism. We are aware that the absolute truth of science does not exist. The truth is only of nature “
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“Over the last forty years, many accelerator experiments have measured the mass of the W boson: these are complicated measurements but over time ever increasing levels of precision have been achieved” commented Chiarelli. «It took us many years to evaluate all the various aspects to be taken into consideration to the extent and to carry out all the necessary checks and verifications. To date, this is our strongest measure, and the discrepancy between the expected and the measured value remains, ”he concluded.
The measurement of the mass of the mediators W and Z is particularly important, because unlike that of the other particles of the Standard Model, their value is predicted by theory. Thanks to this new CDF measurement, the researchers determined the mass of the W particle with an accuracy of 0.01%, which is two times greater than the best of the previous measurements, and this allows them to test the current theoretical framework describing the nature at the level of its fundamental constituents. The result was based on the observation and analysis of 4.2 million candidate W particles, quadruple those of the analysis published by the same collaboration in 2012. The new value of the W particle is consistent with many previous measurements, but there are also others who disagree. Future steps will therefore be needed to clarify this aspect.
In short, either the measurement is wrong or the theory is incomplete: «It is the first crack that opens in the standard model» added Chiarelli. But the ball, or rather the particle, now passes to CERN which will have to repeat the measurement. For now, the yellow of physics is destined to remain so: it will take years.
“If confirmed, the discrepancy between the measurement we have obtained and the one envisaged by the Standard Model would be very important,” added Chiarelli. A conclusion also shared by another great expert in particle physics, the scientist Fernando Ferroni: «In case of confirmation we should conclude that in the theory of the Standard Model there is at least one hidden effect that we have not yet focused on. We should question the global picture ». «The Standard Model – continues Chiarelli – describes the material we are made of, but we have known for some time that it cannot be the final theory. Small discrepancies were observed here and there, not as significant as the one our group observed. If our result were confirmed, it would give us the direction to look in. ” It is now a question of verifying the data by repeating the experiment, and this will be possible at CERN. “The answer will take years,” he notes. “To date, this is our most solid measure, and the discrepancy between the expected and the measured value remains,” says Chiarelli. It is a measurement «accurate to 0.01%, difficult to reproduce. It’s a challenge”.
Matter can wait: the universe is 13.7 billion years old. Homo sapiens 100-200 thousand years. The scientific method 400 years. Particles, patience and prudence must have something in common.
April 7, 2022 (change April 8, 2022 | 18:01)
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