Why Scientists Are Worried About the W Boson: 'Something Is Amiss'

And that something could totally change one of the universe's most fundamental frameworks.

Monisha Ravisetti Former Science Writer
Monisha Ravisetti was a science writer at CNET. She covered climate change, space rockets, mathematical puzzles, dinosaur bones, black holes, supernovas, and sometimes, the drama of philosophical thought experiments. Previously, she was a science reporter with a startup publication called The Academic Times, and before that, was an immunology researcher at Weill Cornell Medical Center in New York. She graduated from New York University in 2018 with a B.A. in philosophy, physics and chemistry. When she's not at her desk, she's trying (and failing) to raise her online chess rating. Her favorite movies are Dunkirk and Marcel the Shell with Shoes On.
Monisha Ravisetti
5 min read

The particle detector inside the collision hall.


You've probably heard of protons, positive specks anchoring atoms. You've likely come across electrons, negative blips roaming around those protons. You may have even pondered photons, the stuff coming out of lightbulbs in your room. 

But right now, we need to worry about an odd little particle that usually escapes the limelight: the W boson.

Along with its partner-in-crime, the Z boson, the W boson dictates what's called the "weak force." I'm going to save you from the rabbit hole of how the weak force works because it involves physics that'll explode our minds. Trust me. Just know that without the weak force, the sun would basically stop burning. 

Anyway, there's drama with the W boson. According to a paper published Thursday in the journal Science, 10 years of unimaginably precise data suggest the particle is more massive than our physics predicts. Unless you're a physicist, at first glance, this might sound trivial. But it's actually a major problem for…kind of everything. 

More specifically, it spurs a paradox for the Standard Model of particle physics, a well-established, evolving theory that explains how all the universe's particles behave -- protons, electrons, photons, and even those we don't really hear about like gluons, muons, I could go on. The W boson is in there, too. 

"It is one of the cornerstones of the Standard Model," said Giorgio Chiarelli, research director of the Istituto Nazionale di Fisica Nucleare in Italy, and co-author of the study.

But here's the crux of the Standard Model. It's like a symbiotic particle world. Think of each particle in the model as a string, perfectly organized to tie everything together. If one string is too tight, stuff starts getting wonky -- it doesn't matter which string. As such, the Standard Model predicts a few parameters for each "string," or particle, and a very important one is the W boson mass.

Simply put, if this particle doesn't equal that mass, the rest of the model wouldn't quite work out. And if that were true, we'd have to change the model -- we'd have to change our understanding of how all the particles in the universe work. 

Well, remember the new paper? We're pretty much entering that worst case scenario. 


An image of the particles in the Standard Model.


A decade of calculations, measurements, cross-checking, head-scratching and deep breathing from about 400 international researchers concluded that the W boson is slightly heavier than the Standard Model predicts it should be. 

"It's not a big difference, but we can really see clearly that it is different," said David Toback, a particle physicist from Texas A&M University and co-author of the study. "Something is amiss."

You might be wondering if we're sure about this. The science community had the same reaction, which is why researchers are now laser-focused on confirming that this greater W boson mass is really the case. 

"It could be we got it wrong," Toback said. But he quickly added, "We don't think so." 

It's because, as Toback explains, the team "measured this tiny difference with such incredible precision that it sticks out like a sore thumb." And fascinatingly, these measurements sort of resemble crime-scene-style deduction. 

Watching for what's missing

To get a W boson in the first place, you literally have to smash two protons together. 

That produces an array of other Standard Model particles, and scientists just have to hope one of those is the one they want to examine. (In this case, that's the W boson).

For the new measurements, the researchers used collision data from a now-out-of-service particle accelerator at the Fermi National Accelerator Laboratory in Illinois. Thankfully, it did make some W bosons, and in fact, held enough W boson data to procure about four times the amount as used in previous measurements. Jackpot.

But there's a complication. The W boson is fleeting. It rapidly splits into two smaller particles, so you can't measure it directly. One of those is either an electron or muon, which can be measured directly, but the other is arguably even stranger than the W boson itself: A neutrino. 

Neutrinos are aptly called "ghost particles," because they don't touch anything. They're even zooming through you right now, but you can't tell because they don't touch the atoms that make up your body. Eerie, I know. 

This ghostly hurdle means scientists have to get creative. Enter, the art of deduction.

Once neutrinos vanish, they leave behind a sort of hole. "The footprint of the neutrino is missing energy," Chiarelli said. "This tells us where the neutrino went and how much energy was carried away." 

It's kind of the same concept as an X-ray. "The X-ray goes through, but for the point in which you have some piece of metal, you can see the shape," Chiarelli said. The "shape" is the "missing energy."


An aerial view of the collider from 1999.


Then, after decoding the neutrino, the scientists used a bunch of complex equations to add it up with the electron or muon data. That led to the overall W boson mass. This measurement was done many, many times to make sure everything was as precise as possible. Plus, all the data was bolstered by theoretical calculations that have matured since the last time the W boson was measured.

Yet… there's another complication. 

As with all scientific pursuits, there is no right or wrong answer. There is only theanswer. But as with all human thought, there's the possibility of bias, and the team did not want to fall victim to such personal error. Toback quotes Sherlock Holmes to explain the team's mentality: "One must find theories to suit facts, and not facts to suit theories."

"Is it more stressful?" he remarked. "Yes, but nature doesn't care about my stress. What we want is to know the answer."

Therefore, not only did the team double, triple, quadruple check their data, they did so while fully blinded to the final answer. When the box with the W boson mass result was opened, everyone would be looking at it for the first time. 

Fast-forward to the year 2020, when tensions are high, the box is finally opened and the W boson mass is in clear contention with the Standard Model's prediction.

"It was not a Eureka moment," Chiarelli said. "It was a rather sobering moment. We were skeptical. Science is organized in skepticism." 

But over time, even that skepticism faded and here we are. 

This all seems very solid. Now what?

In a sense, this information has been a long time coming.  "We've known since the beginning that the Standard Model cannot be the ultimate theory," Chiarelli said. 

For instance, the Standard Model famously cannot explain gravity, dark matter, and many other elusive aspects of our universe

One idea is that this new information about the W boson mass might mean we need to add some particles to the Standard Model to account for the change. This could, in turn, impact what we know about the famous Higgs Boson, or "god particle," which was finally detected in 2012 and met with world-shattering applause. 

"But we're not there," Toback said. "That would be pure speculation."

And, rather than speculate, Toback and Chiarelli agree that we just have to follow the facts, even if we know the facts will one day lead us to a new fundamental theory of particle physics.

"It's like moving in the dark," Chiarelli said. "You know that there is one way which is correct, but you don't know where…maybe our measurement can give us the right direction to move."