What’s up with the W boson mass?

Duke physics professor Ashutosh Kotwal always tells his students to work through problem sets independently before consulting with classmates.

If you discuss too much with each other right from the beginning, you might end up reproducing someone else’s thoughts,” he says. “Take your best shot and see how far you can go. Once you get to the end, then—and only then—get together with your colleagues and compare

As researchers, Kotwal and his colleagues apply this same methodology. Independent teams of scientists tackle the same fundamental problems using distinct detectors and techniques. Only after making their measurement do the physicists compare their methodology and results with those from other experiments.

But unlike for Kotwal’s students, the right answer is not listed in the back of a book.

“We’re searching for the facts with as much rigor as we can,” Kotwal says. “I think that the excitement of figuring something out is almost as much as what the answer will be.”

Kotwal works on the CDF experiment at the US Department of Energy’s Fermi National Accelerator Laboratory. For the last 27 years, he and his colleagues have worked on measuring the mass of the W boson, a fundamental particle responsible for nuclear fusion and decay. They published their most recent measurement in the journal Science in April. 

Since the W boson was discovered in 1983, physicists on eight different experiments have measured its mass a total of 10 times. Every other time, the measurement has fit within the range predicted by the Standard Model. But this time, to the surprise of everyone, it did not; it came in higher than expected.

“We knew that CDF was working on this,” says Mika Vesterinen, a researcher at the University of Warwick. “We could extrapolate from their previous measurement in 2012 and guess what the uncertainty was going to be. But the central value was a complete shock. We were not expecting that.”

Theorists immediately published dozens of papers speculating what new physics could be bolstering the W boson’s mass. “It’s not in agreement with the theory, so something must be wrong,” says Matthias Schott, an experimental physicist and professor at Johannes Gutenberg University Mainz. “Either our theory is wrong, or the measurement is wrong.”

A balancing act

In astronomy, the masses and motions of different celestial bodies affect one another. These connections allow scientists to use the measurements they know to solve for the ones they don’t. Perturbations in Uranus’ orbit, for example, allowed scientists to infer the existence of Neptune long before they were able to see it with a telescope.

Fundamental particles participate in a similar balancing act, the rules of which are outlined by the Standard Model, the best description scientists have of the subatomic world. The better scientists understand one particle, the better they can estimate the properties of others. 

Schott remembers applying this principle to his work as an ATLAS fellow at CERN shortly before the start-up of the Large Hadron Collider. “Before the Higgs boson discovery, we used the W boson mass to restrict the range where the Higgs boson could be,” Schott says.

After the Higgs boson was discovered, scientists ran the equations in reverse to predict the mass of the W boson more precisely. “When we discovered the Higgs in 2012, it changed the game,” he says. “The Standard Model can predict the W boson mass with amazing precision. You measure several quantities—one of which is the Higgs boson—put them into a big formula, and you get back the predicted W boson mass.”

According to Schott, if the new CDF measurement is correct, then it means something must be missing from the theory. “It would mean we forgot something in the prediction,” he says. “There would need to be new physics.”

However, scientists still have more to investigate before they assume that’s the case. While many measurements in physics line up, it’s not uncommon for the results of different experiments to disagree. 

The mass of the W boson has been particularly enigmatic because W bosons are notoriously difficult to measure.

Reference: Fermi Lab

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