Posing a hefty problem for particle physics, a particle weighs in heavier than

The model we have for understanding the universe’s fundamental particles is a bit like a gearbox: one tiny change to any one single particles’ properties throws off the mechanics of the other particles, too. 

So when a paper comes out that finds that the mass of one fundamental particle is off by a tiny bit from what was previously accepted, it does more than merely raise eyebrows in the physics world. If true, such a finding would mean that fundamental physics is “wrong” in some as-yet-undetermined way, and would shake up particle physics for decades to come.

Our understanding of the fundamental particles, something known as the Standard Model of particle physics, is one of the most towering human achievements of the past 150 years. It took thousands of physicists and engineers working over a century to put together all the pieces, starting with the discovery of the electron in 1897 and culminating with the discovery of the long-theorized Higgs Boson in 2012.

Earlier this month, after 20 years of analysis, scientists at the Collider Detector at Fermilab ( CDF) announced that they have made the most precise measurement of the mass of the W boson. After millions of trials and observations, their mass measurement came out to 1.43385738 × 10^-22 grams. (That sounds light, but it’s heavier than it should be.)

The precision in the measurement of one of nature’s force-carrying particles is remarkable: scientists say the particle’s revised mass has a precision of 0.01%—twice as precise as the previous best measurement. Results were published in the journal Science.

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But there’s one big problem: this measurement conflicts with the value scientists use in theoretical inputs for the Standard Model. In other words, if true, the mass measurement suggest the Standard Model of physics — which is a gold standard theory that explains the four known forces in the universe and all fundamental particles — is on shaky ground. 

Unlike other fundamental particles like quarks, electrons and photons, the W boson isn’t a particle one typically learns about in grade school science. Yet just as those particles, it is fundamental to the makeup of matter in the universe. The W boson is a messenger particle in what is known as the “weak nuclear force,” which forms part of the four known fundamental interactions in particle physics; the others are electromagnetism, the strong interaction, and gravitation. While the electromagnetic force and gravity are quotidian to human interactions and everyday life, and the strong force is what binds atomic nuclei together, the weak interaction is not as overtly visible. Yet the weak force is implicated in the radioactive decay of atoms, and is just as indispensable as the other forces to the way that our universe looks today as any of the other three forces. And the weak interaction can’t occur without help from a W boson.

In order to make the new measurement of the W boson’s mass, researchers used collision data from the Fermi National Accelerator Laboratory, a now out-of-service particle accelerator in Illinois. Fermilab’s particle accelerator fires protons and anti-protons into each other at near-light speed and closely observes the explosion of energetic particles resulting in the aftermath, then extrapolates their characteristics. 

During its run, the accelerator managed to synthesize four million W boson candidates, whose properties were measured again and again. Through extensive calculations, scientists landed on their measurement, which is precise to seven standard deviations — far above the five standard deviations that yields a statistical gold-standard finding. 

“The number of improvements and extra checking that went into our result is enormous,” Ashutosh V. Kotwal of Duke University, who led the analysis and is one of the 400 scientists in the CDF collaboration, saidin a press release. “We took into account our improved understanding of our particle detector as well as advances in the theoretical and experimental understanding of the W boson’s interactions with other particles. When we finally unveiled the result, we found that it differed from the Standard Model prediction.”

The difference? The new measurements put the W boson at about one-tenth of one percent more massive than previously predicted and accepted. That seems small, but it’s enough to cause a big problem for particle physics — if true.

“The fact that the measured mass of the W boson doesn’t match the predicted mass…