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An ultraprecise measurement of the mass of a subatomic particle called the W boson may diverge from the Standard Model, a long-reigning framework that governs the strange world of quantum physics.
After 10 years of collaboration using an atom smasher at Fermilab in Illinois, scientists announced this new measurement, which is so precise that they likened it to finding the weight of an 800-pound (363 kilograms) gorilla to a precision of 1.5 ounces (42.5 grams). Their result puts the W boson, a carrier of the weak nuclear force, at a mass seven standard deviations higher than the Standard Model predicts. That’s a very high level of certainty, representing only an incredibly small probability that this result occurred by pure chance.
“While this is an intriguing result, the measurement needs to be confirmed by another experiment before it can be interpreted fully,” Joe Lykken, Fermilab’s deputy director of research, said in a statement (opens in new tab).
The new result also disagrees with older experimental measurements of the W boson’s mass. It remains to be seen if this measurement is an experimental fluke or the first opening of a crack in the Standard Model. If the result does stand up to scrutiny and can be replicated, it could mean that we need to revise or extend the Standard Model with possibly new particles and forces.
The strength of the weak nuclear force
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The weak nuclear force is perhaps the strangest of the four fundamental forces of nature. It’s propagated by three force carriers, known as bosons. There is the single Z boson, which has a neutral electric charge, and the W+ and W- bosons, which have positive and negative electric charges, respectively.
Because those three bosons have mass, they travel more slowly than the speed of light and eventually decay into other particles, giving the weak nuclear force a relatively limited range. Despite those limitations, the weak force is responsible for radioactive decay, and it is the only force (besides gravity) to interact directly with neutrinos, the mysterious, ghost-like particles that flood the universe.
Pinning down the masses of the weak force carriers is a crucial test of the Standard Model, the theory of physics that combines quantum mechanics, special relativity and symmetries of nature to explain and predict the behavior of the electromagnetic, strong nuclear and weak nuclear forces. (Yes, gravity is the “elephant in the room” that the model cannot explain.) The Standard Model is the most accurate theory ever developed in physics, and one of its crowning achievements was the successful prediction of the existence of the Higgs boson, a particle whose quantum mechanical field gives rise to mass in many other particles, including the W boson.
According to the Standard Model, at high energies the electromagnetic and weak nuclear forces combine into a single, unified force called the electroweak interaction. But at low energies (or the typical energies of everyday life), the Higgs boson butts in, driving a wedge between the two forces. Through that same process, the Higgs also gives mass to the weak force carriers.
If you know the mass of the Higgs boson, then you can calculate the mass of the W boson, and vice versa. For the Standard Model to be a coherent theory of subatomic physics, it must be consistent with itself. If you measure the Higgs boson and use that measurement to predict the W boson’s mass, it should agree with an independent, direct measurement of the W boson’s mass.
A flood of data
Using the Collider Detector at Fermilab (CDF), which is inside the giant Tevatron particle accelerator, a collaboration of more than 400 scientists examined years of data from over 4 million independent collisions of protons with antiprotons to study the mass of the W boson. During those super-energetic collisions, the W boson decays into either a muon or an electron (along with a neutrino). The energies of those emitted particles are directly connected to the underlying mass of the W boson.
“The number of improvements and extra checking that went into our result is enormous,” said Ashutosh V. Kotwal, a particle physicist at Duke University who led the analysis. “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 CDF collaboration measured the value of the W boson to be 80,433 ± 9 MeV/c2, which is about 80 times heavier than the proton and about 0.1% heavier than expected. The uncertainty in the measurement comes from both statistical uncertainty (just like the uncertainty you get from taking a poll in an election) and systematic uncertainty (which is produced when your experimental apparatus doesn’t always behave in the way you designed it to act). Achieving that level of precision — of an astounding 0.01% — is itself an enormous task, like knowing your own weight down to less than a quarter of an ounce.
“Many collider experiments have produced measurements of the W boson mass over the last 40 years,” CDF co-spokesperson Giorgio Chiarelli, a research director at the Italian National Institute for Nuclear Physics, said in the statement. “These are challenging, complicated measurements, and they have achieved ever more precision. It took us many years to go through all the details and the needed checks.”
Big result, small difference
The result differed from the Standard Model prediction of the W boson’s mass, which is 80,357 ± 6 MeV/c2. The uncertainties in that calculation (the “±”) come from uncertainties in the measurement of the Higgs boson and other particles, which must be inserted into the calculation, and from the calculation itself, which relies on several approximation techniques.
The differences between the results aren’t very large in an absolute sense. Because of the high precision, however, they are separated by seven standard deviations, indicating the presence of a major discrepancy.
The new result also disagrees with previous measurements from other collider experiments, which have been largely consistent with the Standard Model prediction. It’s not clear yet if this result is caused by some unknown bias within the experiment or if it’s the first sign of new physics.
If the CDF result holds up and other experiments can verify it, it could be a sign that there’s more to the W boson mass than its interaction with the Higgs. Perhaps a previously unknown particle or field, or maybe even dark matter, is interacting with the W boson in a way the Standard Model currently doesn’t predict.
Nonetheless, the result is an important step in testing the accuracy of the Standard Model, said CDF co-spokesperson David Toback, a professor of physics and astronomy at Texas A&M University. “It’s now up to the theoretical physics community and other experiments to follow up on this and shed light on this mystery,” he said.
The researchers described their results April 7 in the journal Science (opens in new tab).
Originally published on Live Science.