Decade of science and trillions of collisions show the W boson is bigger than expected — a physicist on the team explains what it means for the Standard Model

“You can do it quickly, you can do it inexpensively, or you can do it right. We did it right.” These were some of the opening remarks from David Toback, the leader of the collider detector at Fermilab, as he announced the results of a ten-year experiment to measure the mass of a particle called the W boson.

I’m a high-energy particle physicist, and I’m part of the team of hundreds of scientists who built and operated the Collider detector at Fermilab in Illinois – known as CDF.

After trillions of collisions and years of data collection and number-crunching, the CDF team found that the W boson has a slightly greater mass than expected. Although the discrepancy is slight, the results, described in a research paper published in Science on April 7, 2022, made a particle physicist electrified. If the measurement is correct, this is another strong indication that there are pieces missing in the physical puzzle of how the universe works.

The Standard Model of particle physics describes the particles that make up the mass and forces of the universe.
MissMJ / Wikimedia Commons

a particle that carries the weak force

The Standard Model of particle physics is the current best scientific framework for the fundamental laws of the universe and describes three fundamental forces: the electromagnetic force, the weak force, and the strong force.

Strong force holds atomic nuclei together. But some nuclei are unstable and undergo radioactive decay, slowly releasing energy by emission of particles. This process is driven by the weak force, and since the early 1900s, physicists have sought to explain why and how atoms decay.

According to the Standard Model, forces are transmitted by particles. In the 1960s, a series of theoretical and experimental discoveries suggested that the weak force is transmitted by particles called W and Z bosons. He also postulated that a third particle, the Higgs boson, was what gives all other particles—including the W and Z bosons—mass.

Since the advent of the Standard Model in the 1960s, scientists have been working their way down the list of predicted and yet to be discovered particles and measuring their properties. In 1983, two experiments at CERN in Geneva, Switzerland, yielded the first evidence of a W boson. It appears to contain roughly the mass of a medium-sized atom like bromine.

By 2000, there was one piece missing to complete the Standard Model and tie everything together: the Higgs boson. I helped search for the Higgs boson in three successive experiments, and we finally discovered it in 2012 at the Large Hadron Collider at CERN.

The Standard Model was complete, and all of the measurements we made hung beautifully with the predictions.

A large yellow tube surrounded by the electronics.
The collider detector at Fermilab collected data from trillions of collisions that produced millions of W bosons.
Bodhita / Wikimedia Commons, CC BY-SA

Measurement of the W bosons

Testing the Standard Form is fun – you just smash particles together with very high energy. These collisions produce heavier particles for a while and then decay again into lighter particles. Physicists are using huge and highly sensitive detectors at places like Fermilab and CERN to measure the properties and interactions of the particles produced by these collisions.

In CDF, W bosons are produced about one in 10 million times when a proton and antiproton collide. Antiprotons are the antimatter version of protons, having exactly the same mass but opposite charge. Protons are made of smaller fundamental particles called quarks, and antiprotons are made of antiquarks. It is the collision between quarks and antiquarks that produces the W bosons. W bosons decay too quickly to be measured directly. So physicists track the energy from their decay to measure the mass of the W bosons.

In the 40 years since scientists first discovered evidence of the W boson, successive experiments have achieved more accurate measurements of its mass. But only since the Higgs boson has been measured — since it gives mass to all other particles — researchers have been able to check the measured mass of the W boson against the mass predicted by the Standard Model. Prediction and experiments have always been identical – until now.

Diagram showing two circles near a line.
The new measurement of the W boson (the red circle) is much further from the mass predicted by the Standard Model (the purple line) and also larger than the initial measurement from the experiment.
CDF Collaboration via Science Magazine, CC BY

Unexpectedly heavy

Fermilab’s CDF detector is excellent at accurately measuring W bosons. From 2001 to 2011, the accelerator collided protons with antiprotons trillions of times, producing millions of W bosons and recording as much data as possible from each collision.

The Fermilab team published preliminary results using a fraction of the data in 2012. We found the cluster to be a bit off, but it’s close to prediction. The team then spent a decade painstakingly analyzing the entire data set. The process involved many internal checks and required years of computer simulation. To avoid any bias creeping into the analysis, no one was able to see any results until the entire computation was completed.

When the physicist finally saw the result on April 7, 2022, we were all surprised. Physicists measure elementary particle masses in units of millions of electron volts – shortened to MeV. The mass of the 80,433 megaelectronvolt-70 megaelectronvolt W boson was higher than what the standard model predicts. This may seem like a slight excess, but the measurement is accurate to within 9 meV. This is a deviation of approximately eight times the margin of error. When my colleagues and I saw the result, we reacted with a resounding “Awesome!”

What does this mean for the standard model

The fact that the measured mass of the W boson does not match the mass expected in the Standard Model can mean three things. Either the math is wrong, the measurement is wrong, or something is missing from the Standard Model.

First, the math. In order to calculate the mass of the W boson, physicists use the mass of the Higgs boson. CERN experiments have allowed physicists to measure the mass of the Higgs boson in the order of a quarter of a percent. In addition, theoretical physicists have been working on calculations of the mass of the W boson for decades. While the math is complex, the prediction is powerful and unlikely to change.

The next possibility is a defect in the experiment or analysis. Physicists around the world are already revising the result to try to punch holes in it. In addition, future experiments at CERN may eventually achieve a more accurate result that will either confirm or disprove Fermilab’s block. But in my opinion, experience is as good a measure as is currently possible.

This leaves the last option: there are unexplained particles or forces causing the upward shift in the mass of the W boson. Even before this measurement, some theorists had suggested possible new particles or forces that would lead to the observed aberration. In the coming months and years, I expect a slew of new papers that seek to explain the puzzling mass of W bosons.

As a particle physicist, I’m confident to say there must be more physics waiting to be discovered beyond the Standard Model. If this new result holds, it will be the latest in a string of results that show that the Standard Model and real-world measurements often don’t match up exactly. It is these mysteries that give physicists new clues and new reasons to continue the search for a fuller understanding of matter, energy, space and time.

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