Particle physics could be rewritten after shock W boson measurement

By Alex Wilkins

A new measurement of a fundamental particle called the W boson appears to defy the standard model of particle physics, our current understanding of how the basic building blocks of the universe interact. The result, which was a decade in the making, will be heavily scrutinised, but if it holds true, it could lead to entirely new theories of physics.

“It would be the biggest discovery since, well, since the start of the standard model 60 years ago,” says Martijn Mulders at the CERN particle physics laboratory near Geneva, Switzerland, who has written a commentary on the result for the journal Science.

The standard model describes three distinct forces: electromagnetism, the strong force and the weak force. Particles called bosons serve as mediators for these forces between particles of matter. The weak force, which is responsible for radioactive decay, uses the W boson as one of its messengers.

The W boson is so central to the standard model that physicists have tried to measure its mass with ever greater precision since it was first observed in 1983. These measurements have all broadly agreed with each other, an apparent confirmation of the standard model’s validity.

But we know that the standard model is wrong. It has no explanation for gravity, dark matter and the absence of antimatter in our universe, so physicists are constantly on the lookout for deviant measurements that could lead to new theories.

Now, Ashutosh Kotwal at Duke University in North Carolina and his colleagues have announced a new measurement for the W boson’s mass using data from the Tevatron collider in Illinois, putting it at 80.4335 gigaelectronvolts.

The generally accepted mass for the W boson is 80.379 gigaelectronvolts, and while the discrepancy may seem small, the new value is the most precise so far, equivalent to measuring your body weight to within under 10 grams.

More importantly, its difference from the generally accepted value of the W boson mass has a statistical significance of around 5 sigma, corresponding to a probability of about 1 in 3.5 million that a pattern of data like this would show up as a statistical fluke.

Physicists normally use 5 sigma as the level of significance to count something as a “discovery”, but the difference between the new mass measurement and that predicted by the standard model is even higher, at 7 sigma. This corresponds to around a 1 in 780 billion probability of seeing a result like this by chance.

Kotwal and his team are aware that they are making an extraordinary claim that could overturn physics as we know it, but he says they have done all the tests they can think of to confirm it. A small amount of systematic uncertainty – essentially potential errors within the experimental set-up – remains, but now it is time for others to weigh in on the result, he says. “We think the answer is holding up to our own scrutiny,” he says.

Measuring W boson’s mass

The team measured the boson’s mass by smashing beams of protons and antiprotons together and analysing the particles produced in the collision. The analysis was so complex that the result took more than a decade to produce, after the Tevatron shut down in 2011, but its potential implications are huge.

“If the W boson mass is deviating that much from the standard model expectation, and if we understand all the [systematic] uncertainties, then it’s a huge deal,” says Ulrik Egede at Monash University in Australia.

The “if” is the important point for many physicists who, while excited at the result, are cautious about its divergence from previous measurements. “We need first to understand the discrepancy between [this result] and all other experiments before we think about explanations from physics beyond the standard model,” says Matthias Schott at CERN, who worked on a previous W boson measurement using data from the ATLAS experiment gathered at the Large Hadron Collider (LHC) up to its shutdown in 2018.

Figuring out the source of the discrepancy is no easy task. W bosons quickly decay into other particles, either an electron and an electron neutrino, or a heavier muon and muon neutrino. Neutrinos are hard to detect, so Kotwal and his team had to infer where they were from large amounts of data. “[W boson masses] are recognised to be some of the experimentally most difficult measurements to make,” says Egede.

The 2018 ATLAS measurement for the W boson mass is the most recent to date, but it may also not be much help in solving the riddle. ATLAS used two beams of protons, rather than a second one of antiprotons, making the results harder to compare, says Kotwal.

If physicists can’t find a problem with Kotwal and his team’s work, then the next step will be producing another measurement, which could come from three experiments at the LHC. “It’s the only collider with a high enough energy to create W bosons,” says Harry Cliff at the University of Cambridge. The LHC is gearing up for a new run this year after being offline since 2018, but Mulders says data collected for the CMS experiment during the previous run could yield a new W boson measurement by next year.

If the result is borne out, it could join other unexplained anomalies like those from the Muon g-2 experiment and discrepancies picked up at the LHC relating to subatomic particles called bottom quarks, which might require new theories of physics to explain. While there are no clear contenders for such a theory at present, Kotwal says that some variants of supersymmetry, which requires the existence of a whole new set of particles, might accommodate the higher W boson mass.

Despite the result taking 10 years to produce, Kotwal says this is just the beginning for understanding its significance as physicists around the world get their hands on the data. “The science will be investigated and we will continue to think about it,” he says.

Journal reference: Science, DOI: 10.1126/science.abk1781

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