A particle that poses a major problem for particle physics weighs heavier than expected

The model we have for understanding the fundamental particles of the universe is a bit like a gearbox: a small change in the properties of a single particle also rejects the mechanics of the other particles.

So when a paper comes out that finds that the mass of a basic particle is a little bit smaller than what was previously accepted, it does more than just raise eyebrows in the world of physics. If true, such a discovery would mean that fundamental physics is “wrong” in an as yet undetermined way and would shake particle physics for decades to come.

Our understanding of the fundamental particles, somewhat known as the standard model of particle physics, is one of the most towering human achievements of the last 150 years. It took thousands of physicists and engineers who worked over a century to assemble all the pieces, beginning with the discovery of the electron in 1897 and culminating with the discovery of the long-theorized Higgs Boson in 2012.

Earlier this month, after 20 years of analysis, researchers at the Collider Detector at Fermilab (CDF) announced that they have made the most accurate measurement of the mass of the W boson. After millions of experiments and observations, their mass measurement came out to 1.43385738 × 10-22 gram. (It sounds light, but it’s heavier than it should be.)

The precision of the measurement of one of nature’s force-carrying particles is remarkable: Scientists say that the revised mass of the particle has an accuracy of 0.01% – twice as accurate as the previous best measurement. The results were published in the journal Science.

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But there is a big problem: this measurement conflicts with the value that scientists use in theoretical inputs to the standard model. In other words, if true, the mass measurement suggests that the standard model of physics – which is a gold standard theory that explains the four known forces in the universe and all fundamental particles – is on shaky ground.

Unlike other fundamental particles such as quarks, electrons and photons, the W boson is not a particle typically learned about in elementary school science. But like these particles, it is fundamental to the composition of matter in the universe. The W boson is a messenger particle in what is known as the “weak nuclear force”, which forms part of the four known fundamental interactions in particle physics; the others are electromagnetism, the strong interaction and gravity. While the electromagnetic force and gravity are quotidian for human interactions and everyday life, and the strong force is what binds atomic nuclei together, the weak interaction is not so overtly visible. Yet the weak force is implicated in the radioactive decay of atoms and is as indispensable as the other forces for the way our universe looks today, as any of the other three forces. And the weak interaction can not occur without the help of a W-boson.

To make the new measurement of the W boson’s mass, scientists used collision data from the Fermi National Accelerator Laboratory, a now out of service particle accelerator in Illinois. Fermilab’s particle accelerator launches protons and anti-protons into each other at almost light speed, closely observing the explosion of energetic particles resulting in the aftereffects, and then extrapolating their properties.

During its run, the accelerator managed to synthesize four million W-boson candidates whose properties were measured over and over again. Through extensive calculations, the researchers landed on their measurement, which is accurate to seven standard deviations – well above the five standard deviations that give a statistical gold standard finding.

“We took into account our improved understanding of our particle detector as well as advances in the theoretical and experimental understanding of the W boson’s interactions with other particles. When we finally revealed the result, we found that it deviated from the standard model prediction.”

“The number of improvements and extra controls that went into our result is enormous,” Ashutosh V. Kotwal of Duke University, who led the analysis and is one of the 400 scientists in the CDF collaboration, said in a press release. “We took into account our improved understanding of our particle detector as well as advances in the theoretical and experimental understanding of the W boson’s interactions with other particles. When we finally revealed the result, we found that it deviated from the standard model prediction.”

The difference? The new measurements put the W boson at about one-tenth of a percent more massive than previously predicted and accepted. It seems small, but it’s enough to cause a big problem for particle physics – if true.

Schumm said the new measurement of the W boson mass “lacked a smoking gun.”

“The fact that the measured mass of the W boson does not match the predicted mass within the standard model can mean three things. Either the mathematics is wrong, the measurement is wrong, or something is missing in the standard model,” writes aloud. energy particle physicist John Conway in The Conversation.

In other words, changes in the standard model would not only affect the standard model – it could shake the whole of physics and our understanding of the universe.

“It is now up to the theoretical physics community and other experiments to follow up on this and shed light on this mystery,” CDF spokesman David Toback said in a press release. “If the difference between the experimental and expected value is due to some kind of new particle or subatomic interaction, which is one of the possibilities, there is a good chance that it is something that can be discovered in future experiments.”

The standard model has proven to be incredibly successful in predicting the properties of its constituents and even the properties of previously unseen particles. Because of its remarkable prophetic nature, physicists are eager to try to punch holes, which can provide new discoveries and new physics. In fact, as Salon reported in 2021, Fermilab’s Muon g-2 experiment produced bizarre results that were slightly different from what the standard model expected – although these results did not completely exceed the “gold standard” with 5 standard deviations that would make them definitive.


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But when it comes to making measurements so accurate and with such a small margin of error, some physicists say that it is just as likely that the experiment has errors rather than the standard model.

“Accuracy is the magnitude of the uncertainty, and accuracy is the magnitude of potential errors,” Schumm said. “You can have something that is very, very precise, but very wrong.”

“You may ask, ‘Could it be an experimental effect, experimental error, and could the calibration be the source of it? Well, that’s one of a few possibilities,'” Bruce Schumm, professor of physics at the University of California – Santa Cruz , and the author of a popular book on particle physics, Salon said. “If the difference [in mass] is an error, maybe yes, the calibration of the detector is a very likely source of that error, to that error. “

Schumm said it is important to distinguish between accuracy and precisionand notes that one can make an inaccurate measurement very accurately.

“Accuracy is the magnitude of the uncertainty, and accuracy is the magnitude of potential errors,” Schumm said. “You can have something that is very, very precise, but very wrong.”

Schumm said the new measurement of the W-boson mass from the CDF “lacked a smoking gun” – specifically that other measurements from various experiments disagree with the CDF’s result for the W-boson mass.

“It is conceivable that all the other measurements are missing something and the CDF measurement has made it more careful and gets the right answer,” Schumm said. “But I think in all likelihood, either the CDF result is wrong, or else the rest of the other results are wrong.”

In the past, Schumm has told Salon that it is “an overdramatization” to say that the standard model would ever be completely rewritten or undone.

“The standard model has always, since the day it was invented, been known to be what is called an ‘effective theory,'” Schumm said. He compared the standard model to the “top of an iceberg”, where the tip is observed and well understood, although we do not quite know what lies underwater. “I would bet any amount [the Standard Model] will never be toppled, as a representation of the tip of the iceberg, “he pondered.

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