A decade ago, physicists wondered whether the discovery of the Higgs boson at Europe’s Large Hadron Collider would point to a new frontier beyond the Standard Model of subatomic particles. So far, that’s not been the case — but a new measurement of a different kind of boson at a different particle collider might do the trick.
That’s the upshot of fresh findings from the Collider Detector at Fermilab, or CDF, one of the main experiments that made use of the Tevatron particle collider at the U.S. Department of Energy’s Fermilab in Illinois. It’s not yet time to throw out the physics textbooks, but scientists around the world are scratching their heads over the CDF team’s newly reported value for the mass of the W boson.
Bosons are force-carrying particles that transfer discrete amounts of energy between particles of matter. For example, the electromagnetic force is carried by bosons known as photons, while the Higgs boson is responsible for transferring the force that endows particles with mass.
The W boson plays a role in the weak nuclear force, which comes into play in radioactive decay as well as nuclear fusion — the process that makes the sun shine. The particle was discovered decades ago at Europe’s CERN research center, which is now home to the Large Hadron Collider, and its mass has been the subject of study ever since.
Roughly speaking, the W boson about 80 times heavier than a proton. But for physicists, “roughly speaking” isn’t good enough.
Knowing the precise weight of the W boson is a big deal because that value is factored into the finely tuned equations that are woven into the Standard Model, one of the most successful theories in science. The theory explains how atoms are put together — and its predictions, including the prediction of the existence of the Higgs boson, have been repeatedly confirmed.
And yet, there’s a lot the Standard Model doesn’t explain. A couple of the biggies have to do with the nature of dark matter and dark energy, which together make up more than 95% of the universe’s content. If there’s some measurement that runs counter to the Standard Model, that may point to an opening for revising the theory.
This is where the CDF findings, published in this week’s issue of the journal Science, come in: Physicists analyzed huge amounts of data collected at the Tevatron between 1985 and 2011, and came up with a mass measurement that carries a precision of 0.01%. That’s twice as precise as the best previous measurement. Fermilab says it’s like measuring the weight of an 800-pound gorilla to within 1.5 ounces.
The only problem is, the 800-pound gorilla appears to tip the scales at three-quarters of a pound overweight. The expected value for the W boson’s mass was 80,357 mega electron volts, or MeV, plus or minus 6 MeV. The CDF’s value is 80,433 MeV, plus or minus 9 MeV.
“It was a surprise,” the University of Oxford’s Chris Hays, a member of the CDF team, said in a news release.
The CDF researchers say their findings carry a confidence level of 7 sigma, which translates to a 1-in-390 billion chance that they could be explained away as a statistical fluke.
If the findings hold up, theoretical physicists will have to turn their firepower toward figuring out how to explain the discrepancy. There could be all sorts of hand-waving to link the too-bulky boson to weird phenomena ranging from dark matter and dark energy to supersymmetry and new arrays of as-yet-undiscovered particles.
But it’s too early for that. Although the statistical analysis sounds impressive, there’s still a chance that something threw off the measurement. That was the case for the claim in 2011 that neutrinos could travel faster than light. When those findings were first announced, researchers claimed a confidence level nearly as high as what the CDF team is claiming now. But upon review, the researchers found glitches in their experimental setup, including a fiber-optic cable that was misattached. Those faster-than-light neutrinos actually weren’t.
Fermilab’s deputy director, Joe Lykken, said the CDF findings alone aren’t enough to force a full rethinking of the Standard Model. “While this is an intriguing result, the measurement needs to be confirmed by another experiment before it can be interpreted fully,” he said.
CDF co-spokesperson David Toback, a physicist at Texas A&M University, said the newly reported findings represent a valuable check on the Standard Model, whether or not they end up being confirmed.
“It’s now up to the theoretical physics community and other experiments to follow up on this and shed light on this mystery,” he said. “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.”
Check out this Twitter thread for informed speculation from Durham University physicist Martin Bauer about what an overweight W boson could mean for the Standard Model:
Great question! Let us assume the effect is real and not due to some systemic error in the analysis (or the SM prediction). What would that actually mean in terms of new physics?
(speculation warning) 1/17 🧵 https://t.co/u6axpiHFsg
— Martin Bauer (@martinmbauer) April 8, 2022
The 400 researchers of the CDF Collaboration are listed as co-authors of the Science paper, “High-Precision Measurement of the W Boson Mass with the CDF II Detector.” For what it’s worth, the CDF experiment was decommissioned in 2011 and disassembled.