A record-setting experiment finds "significant tension" with the most important equation in physics

The newly calculated mass of the W boson doesn't square with the Standard Model
Grant Currin

If you listen very closely, you might be able to hear the sound of physicists scratching their heads after hearing the latest news out of Fermilab in Chicago.

Researchers there set a new record by measuring the size of a fundamental particle — the W boson — far more precisely than ever before.

If a W boson were the size of an 800-pound gorilla, the new measurement would be accurate to “within a couple of ounces,” project leader Ashutosh V. Kotwal tells IE. But the accuracy of the measurement isn’t the big surprise. The finding itself is what's sure to send theoretical physicists rushing to their chalkboards.

Kotwal says the result is “in significant tension” with the Standard Model of particle physics. That means the most successful scientific theory ever might be wrong.

Of course, science requires confirmation. “While this is an intriguing result, the measurement needs to be confirmed by another experiment before it can be interpreted fully,” says Fermilab Deputy Director Joe Lykken.

The W boson is key to the Universe as we know it

Nearly all physicists are convinced that all matter is made of some combination of 17 subatomic particles. The W boson is one of the four subatomic particles that together carry three of the four fundamental forces (the fourth is gravity, which is not explained by the standard model). These forces are responsible for controlling how the rest of the subatomic particles interact with one another.

The W boson is partly responsible for a force called the weak interaction, which transforms protons into neutrons. The weak interaction allows the Sun to shine by fusing hydrogen atoms into helium. Throughout cosmic history, that same basic process has created all of the heavier atoms in the universe, including the carbon and oxygen that humans are made of.

“Nothing about the Universe that we see around us — all the richness, all the complexity — could’ve happened without the weak nuclear force,” Kotwal says. “One of the crucial, fundamental things responsible for so much around us... is explained by the exchange of the W boson.”

What does the finding mean for the Standard Model?

It was just over one hundred years ago that physicists started to realize that matter acts very differently at infinitesimal scales. Since those initial discoveries (such as the double-slit experiment), physicists have been constructing and testing theories to explain what matter is at its most fundamental level.

“At some point, the name Standard Model evolved. It became a proper noun,” Kotwal says. The equation predicted subatomic particles — notably the Higgs boson and the top quark — long before experimentalists could observe them directly.

But the Standard Model isn’t a perfect theory of everything. It doesn’t explain gravity or dark matter. Its conclusions about anti-matter don’t square with the data, and “there are certain aspects of the Higgs which are not explained,” Kotwal says.

This new finding about the size of the W boson joins those anomalies and a handful of others that aren’t explained by the Standard Model, at least in its current form.

In a short essay published alongside these findings, physicists Claudio Campagnari and Martijn Mulders write that "[w]ith more and more precise measurements of physical quantities... fissures between [Standard Model] predictions and reality may have begun to show.”

“When not in agreement with the theoretical predictions, such measurements can provide a first glimpse of physics beyond the [Standard Model],” they say.

What does that future look like? Kotwal says it’s up to theoretical physicists to make sense of the findings. Some will look for creative ways to fit this finding into the Standard Model as it currently exists while others will look for ways to extend the equation so that it can account for the new findings.

“We make the measurements, we do the fact finding,” he says. “Our community is going to look at it hard and think about this calculation versus the other calculation [and] see which one works better. That's one of the things that drives us.”

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