A collaboration of hundreds of scientists have precisely measured the mass of the W boson, an elementary particle responsible for the weak nuclear force. The researchers found, to their surprise, that the boson is more massive than predicted by the Standard Model of particle physics, the working theory that describes several of the fundamental forces in the universe.
The new value was extracted from 10 years of experiments and calculations by 400 researchers at 54 different institutions around the world, a breathtaking effort. All the data was collected from experiments at the four-story-tall, 4,082 T Collider Detector (CDF-II for short) at Fermilab’s Tevatron accelerator near Chicago, Illinois.
The CDF Collaboration found the W boson’s mass to be 80,433 +/- 9 MeV/c^2, a figure that is roughly twice as precise as the previous measurement of its mass. For a sense of scale, new measurement puts the W boson at about 80 times the mass of a proton. The team’s results are published today in Science.
“The truth is, what happened here is how often most things happen in science. We took a look at the number, and we said, ‘Huh, that’s funny,’” said David Toback, a physicist at Texas A&M University and a spokesperson for the CDF Collaboration, in a video call. “You could see it just washing over people. It was quiet. We didn’t know what to make of it.”
“We were very pleasantly surprised [with the result],” wrote Ashutosh Kotwal, a physicist at Duke University and a member of the CDF collaboration, in an email. “We were so focused on the precision and robustness of our analysis that the value itself was like a wonderful shock.”
The W boson is associated with the weak nuclear force, a fundamental interaction that is responsible for one type of radioactive decay and the nuclear fusion that occurs in stars. Don’t worry — the boson having a very different mass than expected doesn’t mean we’ve completely misunderstood things like nuclear fusion — but it does mean there’s a lot we still don’t understand about the particles that make up our universe and how they interact.
“The Standard Model is the best we’ve got for particle physics. It’s amazingly good. The problem is, we know we’re wrong,” Toback said. “So from the scientist’s perspective, the experimentalists are trying to say, ‘Gee, can we find something that the Standard Model doesn’t predict correctly, which might give us a clue to what’s more true?’”
The Standard Model predicts a value for the W boson mass, a value the team sought to challenge by assessing 4 million W boson candidates generated by collisions between protons and antiprotons at Fermilab. Their result was higher than the Standard Model’s prediction by a whopping seven standard deviations. Kotwal, who’s published five increasingly precise measurements of the particle’s mass over the last 28 years, said that “the odds of the 7 standard deviation increase being a statistical fluke are less than 1 in a billion.”
Toback likened the measurement to measuring the weight of an 363 kg gorilla to within an ounce of its true weight. As is the case with many science experiments — especially in particle physics, where masses are so slight — the researchers blinded their results, to ensure that the calculations weren’t affected by any expectations or hopes of the research team.
But now, with an extraordinarily precise measurement so different from previous, lower estimates, physicists have the unenviable task of figuring out what the Standard Model doesn’t account for. It’s certainly not the first time that subatomic physics has proven different in reality from humanity’s best guesses. Last April, the Muon g-2 Collaboration found further evidence that properties of the muon (another subatomic particle) may not agree with the predictions of the Standard Model. And two of the most important facts of our universe — gravity and dark matter — are famously not explained by the model.
“In order to figure out what the more fundamental theory could be, it is important to find phenomena that cannot be explained by the [Standard Model],” emailed Claudio Campagnari, a physicist at the University of California – Santa Barbara who is unaffiliated with the recent study. “In other words, phenomena where the [Standard Model] approximation breaks down.” Campagmari co-authored a Perspectives article about the new measurement.
There are experiments set to do just that; they will probe the implications of today’s finding with different collision experiments. Results are still forthcoming from ATLAS and the Compact Muon Solenoid (CMS), two detectors at CERN’s Large Hadron Collider (the two detectors responsible for the discovery of the Higgs boson 10 years ago). And the High-Luminosity Large Hadron Collider — an upgrade that will increase the number of collisions possible by a factor of 10 — will also boost the chances seeing compelling new particles when it’s completed in 2027.
The CDF’s collisions were between protons and antiprotons, whereas the Large Hadron Collider produces proton-proton collisions. Kotwal said if humans ever built an electron-positron collider, it would allow precise measurements and searches for rare processes the Large Hadron Collider cannot produce.
As Martijn Mulders, a physicist at CERN who co-wrote the Perspectives article, said in an email, physicists will take a two-pronged approach to testing the model: measuring known particles (like the W boson) with increasing precision, as well as discovering entirely new particles. New particles are often found through ‘bump’ hunting: sifting through the noise of the subatomic mosh pits to see what was unexpectedly created.
The Tevatron accelerator shut down in 2011, just after the collaboration finished its experimental run. So today’s result is something of a life after death for the storied instrument, a massive W for the team and particle physics as a whole.