Why scientists are worried about the W boson: “Something’s wrong”

The particle detector in the collision hall.


You’ve probably heard of protons, positive points that anchor atoms. You’ve probably encountered electrons, negative blips, wandering around these protons. You may have even thought about photons, the stuff that comes out of lightbulbs in your room.

But for now, we have to worry about a strange little particle that usually escapes the limelight: the W boson.

Together with its accomplice, the Z boson, the W boson dictates the so-called “weak force”. I’m going to save you from the rabbit hole of how the weak force works because it involves physics that will blow our minds. Trust me. Just know that without the weak force, the sun would basically stop burning.

Anyway, there is a drama with the W boson. according to a Paper published Thursday in the journal Science, 10 years of impossibly precise data suggest the particle is more massive than our physics predicts. Unless you’re a physicist, this may sound trivial at first. But it’s actually a big problem for… pretty much everything.

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

“It’s one of the cornerstones of the Standard Model,” said Giorgio Chiarelli, research director at the Istituto Nazionale di Fisica Nucleare in Italy and co-author of the study.

But here’s the gist of the Standard Model. It’s like a symbiotic particle world. Think of each particle in the model as a string, perfectly organized to connect everything together. If a string is too tight, stuff gets wobbly – 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 mass of the W boson.

Put simply, if that particle didn’t match that mass, the rest of the model wouldn’t quite work. And if that were true, we would have to change the model – we would have to change our understanding of how all particles in the universe Work.

Do you remember the new paper? We’re pretty much entering the worst-case scenario.


An image of the particles in the Standard Model.


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

“It’s not a huge difference, but we can really clearly see that it’s different,” said David Toback, a particle physicist at Texas A&M University and co-author of the study. “Something is not correct.”

You may be wondering if we’re sure about that. The scientific community had the same reaction, which is why researchers are now focused on confirming that this larger W boson mass is really the case.

“It could be that we were wrong,” Toback said. But he was quick to add, “We don’t think so.”

That’s because, as Toback explains, the team “measured that tiny difference with such incredible precision that it sticks out like a sore thumb.” And intriguingly, these measurements resemble a crime scene-style inference.

Watch what’s missing

To even get a W boson, you literally have to smack two protons together.

That creates a bunch of other Standard Model particles, and scientists just have to hope that one of them is the one they want to study. (In this case that is the W boson).

For the new measurements, the researchers used collision data from a particle accelerator that is now out of service Fermi National Accelerator Laboratory in Illinois. Luckily, it produced some W bosons and actually contained enough W boson data to retrieve about four times the amount used in previous measurements. Jackpot.

But there is a complication. The W boson is volatile. It quickly splits into two smaller particles, so you can’t measure it directly. One of which is either an electron or a muon, that can 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’re not touching the atoms that make up your body. Scary I know.

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

As soon as neutrinos disappear, they leave a kind of hole behind. “The neutrino footprint lacks energy,” Chiarelli said. “That tells us where the neutrino went and how much energy was carried away.”

It’s a bit the same concept as an X-ray. “The X-ray goes through, but for the point where you have a piece of metal, you can see the shape,” Chiarelli said. The “form” is the “missing energy”.

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An aerial view of the collider from 1999.


Then, after decoding the neutrino, scientists used a series of complex equations to add it to the electron or muon data. This led to the total mass of the W boson. This measurement was taken many, many times to ensure everything was as accurate as possible. In addition, all data have been corroborated by theoretical calculations that have matured since the W boson was last measured.

But…there is another complication.

As with all scientific activities, there is no right or wrong answer. There’s only them Answers. But as with all human thoughts, there is a possibility of bias, and the team didn’t want to fall victim to such personal error. Toback quotes Sherlock Holmes as explaining the team’s mentality: “You have to find theories that fit facts, not facts that fit theories.”

“Is it more stressful?” he remarked. “Yes, but nature doesn’t care about my stress. We want to know the answer.”

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

Fast forward to 2020, when tensions are high, the box is finally opening, and the W boson mass is in clear contradiction to Standard Model predictions.

“It wasn’t a eureka moment,” Chiarelli said. “It was a pretty sobering moment. We were skeptical. Science is organized in skepticism.”

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

It all looks very solid. What now?

In a way, this information has been a long time coming. “We knew from the start that the Standard Model couldn’t be the ultimate theory,” Chiarelli said.

For example, the Standard Model is notoriously incapable of explaining gravity, Dark matterand many other elusive ones aspects of our universe.

One idea is that this new information about the mass of the W boson could mean that we need to add some particles to the Standard Model to account for the change. This, in turn, could affect what we know about the famous Higgs boson, or “god particle,” which was finally discovered and struck in 2012 with earth-shattering applause.

“But we’re not there,” Toback said. “That would be pure speculation.”

And instead of speculating, Toback and Chiarelli agree that we simply have to follow the facts, even though we know that the facts will one day lead us to a new fundamental theory of particle physics.

“It’s like walking in the dark,” Chiarelli said. “You know there’s a way that’s right, but you don’t know where to go… maybe our measurement can point us in the right direction.”

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