Workers with the collider-detector at Fermilab, Illinois. Photo: Fermilab/US Department of Energy
- A new analysis of data collected at an experiment in the US before 2011 has revealed the mass of the W boson to be higher than expected according to theory.
- The W boson is an elementary particle involved in radioactive decay and nuclear fusion.
- The new measurement disagrees with the Standard Model of particle physics, which describes the properties and behaviour of elementary particles.
There’s a framework of rules that describes how the elementary particles of our universe look[footnote]Figuratively speaking[/footnote] and behave that physicists have spent decades putting together, called the Standard Model. And now there’s new evidence that it’s wrong about the mass of one particle.
Bosons are particles that carry force. There are four fundamental forces in the universe; one of them is the weak nuclear force: it’s involved in nuclear fusion reactions and for radioactive decay. This force is mediated by two particles: the two W bosons and the Z boson. When two other particles exchange the W and/or Z bosons, they’re said to be acted on by the weak nuclear force.
According to a new measurement, the mass of W bosons appears to be higher than that predicted by the Standard Model. And far from being an embarrassment to physicists, who used the Standard Model to predict the W boson’s mass, they’re delighted.
The Standard Model is famously broken but physicists don’t know how. The Model can’t explain gravity and dark matter. It also can’t explain why the Higgs boson is so heavy, why the universe has more matter than antimatter, why gravity is so weak or why the size of the proton is what it is.
So when a Standard Model prediction is found to be wrong in an experiment, physicists can study the experiment more closely to understand where the value might have deviated. These deviations are broadly called ‘new physics’: they’re what physicists can use to fix the Standars Model. And they’re exceedingly rare.
Last year, physicists running experiments in New York reported that another elementary particle called a muon behaved differently in a magnetic field than the Model predicted. This was a potential sign of new physics that physicists are working on. Now we have the W boson mass anomaly (published on April 7).
At the Fermilab facility in Illinois, physicists smashed beams of protons and antiprotons into each other and recorded the fallout using a detector called CDF II. All this happened before 2011, when the instruments in question shut down.
In the dataset the new team analysed, CDF II had recorded approximately four million W bosons. Using this they calculated the particle’s mass to be 80.4 GeV (give or take a little).
(By the mass -energy equivalence, GeV – units of energy – can be converted to mass by dividing by c2, where c is the speed of light in a vacuum.)
The Standard Model predicts the W boson’s mass to be 80.3 GeV. Quantitatively, the new measurement is a small difference – but calculations based on the Standard Model are notorious as much for their complexity as for their precision. Even a slight deviation could be a sign of new physics.
Ashutosh Kotwal, a physicist at Duke University, a member of the CDF II team and lead author of the study, told Gizmodo, “We were very pleasantly surprised. We were so focused on the precision and robustness of our analysis that the value itself was like a wonderful shock.”
Discoveries like this often generate a lot of hype – only for subsequent attempts to reproduce the findings to come up short. In a new infamous incident in 2011, a detector in Italy reported that it had spotted an elementary particle travelling at faster than the speed of light. If it had been true, it would have been the brightest bit of new physics. But subsequent tests confirmed that there was a glitch.
The new measurement at Fermilab is similarly susceptible to being wrong. According to ScienceNews, the measurement is “about twice as precise as the previous record”. And according to Kotwal, the odds of their measurement being a fluke are “1 in one billion”. This is considered very low, and notably lower than the threshold at which a particle physics data point is considered to be evidence.
However, as theoretical physicist Matt Strassler wrote on his blog on April 8:
I’d say the chance that [the measurement is] wrong is substantial. Why? This measurement, which took several many years of work, is probably among the most difficult ever performed in particle physics. Only first-rate physicists with complete dedication to the task could attempt it, carry it out, convince their many colleagues on the CDF experiment that they’d done it right, and get it through external peer review into Science. But even first-rate physicists can get a measurement like this one wrong. The tiniest of subtle mistakes will undo it.
And that mistake, if there is one, might not even be their own, in a sense. Any measurement like this has to rely on other measurements, on simulation software, and on calculations involving other processes, and even though they’ve all been checked, perhaps they need to be rechecked.
Indeed, Martijn Mulders, an experimental physicist at CERN in Europe, told Scientific American that the discovery was “very unexpected”. “You almost feel betrayed because suddenly they’re sawing off one of the legs that really support the whole structure of particle physics,” he added.
Speaking to The Guardian, Harry Cliff, a particle physicist at Cambridge University, said that while the Standard Model is “probably the most successful theory and scientific theory that has ever been written down,” it can’t just be fine-tuned to account for the new finding. “It’s a like a house of cards, you pull on one bit of it too much, the whole thing comes crashing down.”
Effectively, the result encourages physicists to get back to the blackboard – where they must figure out either where the Standard Model went wrong or where the measurement went wrong, why, and if that might be the key to unlocking answers to all the other unsolved problems.
Note: This article was updated at 11:29 am on April 9, 2022, to include Matt Strassler’s comment.