A decade of science and trillions of collisions show the W boson is more massive than expected – a physicist on the team explains what it means for the Standard Model

The Conversation

You can do it quickly, you can do it cheaply, or you can do it right. David Toback, leader of the collider detector at Fermilab, said that they did it right as he announced the results of a decadelong experiment to measure the mass of a particle.

I am a high energy particle physicist and I am part of the team of hundreds of scientists that built and ran the CDF.

After trillions of collisions and years of data collection and number crunching, the CDF team found that the W boson has slightly more mass than expected. The results described in a paper published in Science on April 7, 2022, have changed the world of particle physics. The measurement is a strong signal that there are missing pieces to the physics puzzle of how the universe works.

A particle that carries the weak force

The Standard Model of particle physics is the best framework for the basic laws of the universe and describes three basic forces.

The force holds the nuclei together. Some nuclei are unstable and radioactive, releasing energy by emitting particles. Physicists have sought an explanation for why and how atoms decay since the early 1900s.

The Standard Model states that forces are transmitted by particles. The weak force is transmitted by particles called W and Z bosons, which were proposed in the 1960s. The theory says that the third particle, the Higgs boson, is what gives all other particles mass.

Scientists have been working their way down the list of predicted yet undiscovered particles and measuring their properties since the advent of the Standard Model. The first evidence of the existence of the W boson was captured in two experiments in 1983. It looked like it had the mass of a medium-sized atom.

By the 2000s, the Standard Model was complete and the only missing piece was the Higgs boson. I helped search for the boson on three separate occasions, and at last we found it in 2012 at the Large Hadron collider.

The Standard Model was complete, and all the measurements we made hung together beautifully.

Measuring W bosons

It's fun to test the Standard Model by smashing particles together. The heavier particles are produced by the collisions and decay back into lighter ones. Physicists use huge and very sensitive detectors to measure the properties and interactions of the particles produced in the collider.

When a protons and an antiproton collide, W bosons are produced about one out of every 10 million times. The antimatter version of protons has the same mass but a different charge. Antiprotons are made of antiquark particles. There is a collision between quarks and antiquarks. It is impossible to measure W bosons directly. Physicists use the energy produced from their decay to measure the mass of W bosons.

In the 40 years since the first evidence of the W boson was found, scientists have made more precise measurement of its mass. Since the mass of all particles has been measured, researchers can check against the mass predicted by the Standard Model. The prediction and the experiments always matched up.

Unexpectedly heavy

The CDF detector is very good at measuring W bosons. The data from each collision of the protons and antiprotons was recorded and millions of W bosons were produced.

The initial results were published using a fraction of the data. The mass was close to the prediction. The team spent a decade analyzing the full data set. The process required years of computer simulations. Nobody could see the results until the full calculation was done.

We were all surprised when the result was finally seen. Physicists measure particle densities in units of millions of electron volts. The mass of the W boson was 70 MeV higher than the Standard Model predicts. The measurement is accurate to within 9 MeV. The deviation is eight times the margin of error. When we saw the result, our reaction was a resounding "WOW!"

What this means for the Standard Model

The measured mass of the W boson does not match the predicted mass within the Standard Model. Either the math is wrong, the measurement is wrong or there is something missing from the Standard Model.

The first thing is the math. Physicists use the mass of the Higgs boson to calculate the W boson's mass. Physicists have been able to measure the mass within a quarter-percent. Physicists have been working on mass calculations for decades. The prediction is solid and not likely to change.

A flaw in the experiment or analysis is the next possibility. Physicists all over the world are looking at the result to see if they can poke holes in it. Future experiments at CERN may eventually achieve a more precise result that will either confirm or refute the Fermilab mass. In my opinion, the experiment is a good measurement.

The last option is if there are unexplained particles or forces causing the upward shift in the mass. Some theorists had proposed new particles or forces that would cause the observed deviation. I expect a lot of new papers in the coming months and years to explain the mass of W bosons.

There must be more physics discovered beyond the Standard Model if I am to be believed. The Standard Model and real-world measurements often don't match up, and if this new result holds up, it will be the latest in a series of findings. These mysteries give physicists new clues and new reasons to keep searching for more understanding of matter, energy, space and time.

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The Conversation is a news site dedicated to sharing ideas from academic experts. It was written by a University of California student.

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