Posing a hefty problem for physicists, a fundamental particle weighs in heavier than expected

A new measurement of the W boson suggests the Standard Model is wrong. Yet there still isn't a "smoking gun"

By Nicole Karlis - Keith A. Spencer

Published April 14, 2022 6:52PM (EDT)

A part of complex Large Hadron Collider (LHC) is seen underground during the Open Days at the CERN particle physics research facility on September 14, 2019 in Meyrin, Switzerland. The 27km-long Large Hadron Collider is currently shut down for maintenance, which has created an opportunity to offer access to the public.  (Ronald Patrick/Getty Images)
A part of complex Large Hadron Collider (LHC) is seen underground during the Open Days at the CERN particle physics research facility on September 14, 2019 in Meyrin, Switzerland. The 27km-long Large Hadron Collider is currently shut down for maintenance, which has created an opportunity to offer access to the public. (Ronald Patrick/Getty Images)

The model we have for understanding the universe's fundamental particles is a bit like a gearbox: one tiny change to any one single particles' properties throws off the mechanics of the other particles, too. 

So when a paper comes out that finds that the mass of one fundamental particle is off by a tiny bit from what was previously accepted, it does more than merely raise eyebrows in the physics world. If true, such a finding would mean that fundamental physics is "wrong" in some as-yet-undetermined way, and would shake up particle physics for decades to come.

Our understanding of the fundamental particles, something known as the Standard Model of particle physics, is one of the most towering human achievements of the past 150 years. It took thousands of physicists and engineers working over a century to put together all the pieces, starting 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, scientists at the Collider Detector at Fermilab ( CDF) announced that they have made the most precise measurement of the mass of the W boson. After millions of trials and observations, their mass measurement came out to 1.43385738 × 10-22 grams. (That sounds light, but it's heavier than it should be.)

The precision in the measurement of one of nature's force-carrying particles is remarkable: scientists say the particle's revised mass has a precision of 0.01%—twice as precise as the previous best measurement. Results were published in the journal Science.

RELATED: Why some physicists are skeptical about the muon experiment that hints at "new physics"

But there's one big problem: this measurement conflicts with the value scientists use in theoretical inputs for the Standard Model. In other words, if true, the mass measurement suggest 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 like quarks, electrons and photons, the W boson isn't a particle one typically learns about in grade school science. Yet just as those particles, it is fundamental to the makeup 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 gravitation. While the electromagnetic force and gravity are quotidian to human interactions and everyday life, and the strong force is what binds atomic nuclei together, the weak interaction is not as overtly visible. Yet the weak force is implicated in the radioactive decay of atoms, and is just as indispensable as the other forces to the way that our universe looks today as any of the other three forces. And the weak interaction can't occur without help from a W boson.

In order to make the new measurement of the W boson's mass, researchers used collision data from the Fermi National Accelerator Laboratory, a now out-of-service particle accelerator in Illinois. Fermilab's particle accelerator fires protons and anti-protons into each other at near-light speed and closely observes the explosion of energetic particles resulting in the aftermath, then extrapolates their characteristics. 

During its run, the accelerator managed to synthesize four million W boson candidates, whose properties were measured again and again. Through extensive calculations, scientists landed on their measurement, which is precise to seven standard deviations — far above the five standard deviations that yields 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 unveiled the result, we found that it differed from the Standard Model prediction."

"The number of improvements and extra checking 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, saidin 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 unveiled the result, we found that it differed from the Standard Model prediction."

The difference? The new measurements put the W boson at about one-tenth of one percent more massive than previously predicted and accepted. That 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 was "missing a smoking gun."

"The fact that the measured mass of the W boson doesn't match the predicted mass within the Standard Model could mean three things. Either the math is wrong, the measurement is wrong or there is something missing from the Standard Model," writes high-energy particle physicist John Conway in The Conversation.

In other words, making any changes to the Standard Model wouldn't merely affect the Standard Model —  it could shake up all of physics and our understanding of the universe. 

"It's now up to the theoretical physics community and other experiments to follow up on this and shed light on this mystery," CDF co-spokesperson David Toback said in a press statement. "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's a good chance it's something that could be discovered in future experiments."

The Standard Model has proven incredibly successful at predicting the properties of its constituent particles, and even the properties of previously unseen particles. Because of its remarkable prophetic nature, physicists are eager to try to poke holes, which could yield new discoveries and new physics. Indeed, as Salon reported in 2021, Fermilab's Muon g-2 experiment produced bizarre results that were slightly different from what the Standard Model projected — though those results did not quite surpass the 5-standard-deviation "gold standard" that would make them definitive. 


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But when it comes to making measurements so precise and with such a small margin of error, some physicists say that it is equally likely that the experiment has flaws, rather than the Standard Model.

"Precision is the size of the uncertainty and accuracy is the size of potential mistake," Schumm said. "You can have something that's very, very precise, but way wrong." 

"You can ask, 'Could that be an experimental effect, experimental mistake, and could the calibration be the source of that? Well, it's one of a couple of possibilities," Bruce Schumm, a professor of physics at the University of California–Santa Cruz, and the author of a popular book on particle physics, told Salon. "If the difference [in mass] is a mistake, perhaps yes, the calibration of the detector is a very likely source of that error, of that mistake."

Schumm said that it is important to distinguish between accuracy and precision, noting that one might make an inaccurate measurement very precisely. 

"Precision is the size of the uncertainty and accuracy is the size of potential mistake," Schumm said. "You can have something that's very, very precise, but way wrong."

Schumm said the new measurement of the W boson mass from the CDF was "missing a smoking gun" — specifically, a clearly identified reason that other measurements from different experiments disagree with the CDF's result for the W boson mass.

"It's conceivable that all the other measurements are missing something and the CDF measurement has done it more carefully and is getting the right answer," Schumm said. "But I think in all likelihood, either the CDF result is wrong, or the body of other results is wrong." 

Previously, Schumm told Salon it's "an over-dramatization" 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's called an 'effective theory,'" Schumm said. He likened the Standard Model to the "tip of an iceberg," in which the tip is observed and well-understood even if we do not know entirely what lies beneath the water. "I would bet any amount of money [the Standard Model] will never be toppled, as a representation of that tip of the iceberg," he mused.

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Nicole Karlis

Nicole Karlis is a senior writer at Salon. Tweet her @nicolekarlis.

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Keith A. Spencer

Keith A. Spencer is a senior editor at Salon who edits Salon's science/health vertical. His book, "A People's History of Silicon Valley: How the Tech Industry Exploits Workers, Erodes Privacy and Undermines Democracy," was released in 2018. Follow him on Twitter at @keithspencer, or on Facebook here.

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