Measuring W bosons challenges the Standard Model

After a decade of work, a team of more than 400 scientists has provided an incredibly precise measurement of a subatomic particle. To some dismay, the measurement is not quite what the researchers expected: it is “in tension” with the Standard Model of particle physics.

The Standard Model is (currently) our best way to explain things smaller than atoms. The subatomic particles described in the Standard Model — like the Higgs boson, protons, and neutrinos — are the fundamental building blocks of everything in nature.

This new measurement, explained in an article in Science, concerns one of these subatomic particles: the W boson. The W boson, along with another particle called the Z boson, mediates one of the four fundamental forces in nature (the weak force).

The place of the W boson in the Standard Model. It is responsible for the nuclear processes that make the sun shine and particles decay. Photo credit: Fermilab

The researchers compiled 4.2 million observations of candidate W bosons to determine its mass. These observations are all from the Tevatron particle accelerator at Fermi National Accelerator Laboratory (Fermilab), USA. They are performed by the Collider Detector of the Fermilab (CDF) Collaboration.

The result – “our most stable measurement to date,” according to project spokesperson Giorgio Chiarelli, research director at Italy’s National Institute for Nuclear Physics – is well above what the Standard Model predicts.

“Many collider experiments over the past 40 years have led to measurements of the W boson mass,” says Chiarelli. “These are demanding, complicated measurements that are becoming more and more precise. It took us many years to go through all the details and the required checks.”

The measurements were carried out at Fermilab over almost a decade, from 2002 to 2011.

“The CDF measurement was carried out for many years, hiding the measured value from the analyzers until the methods were fully scrutinized,” says CDF project member Chris Hays, a physicist at the University of Oxford, UK.

“When we discovered the value, it was a surprise.”

The final figure for the mass of the W boson is 80,433.5 MeV/c2 – plus or minus 9.4. That’s about 1.4 × 10-31 Kilogram; With numbers like this, it is not surprising that physicists calculate mega-electron volts per square speed of light (MeV/c2) to describe it.

Diagram showing various w boson mass measurements with error bars and standard model in all
The mass of a W boson is about 80 times that of a proton. The CDF value has an accuracy of 0.01 percent and agrees with many W boson mass measurements – but not with the Standard Model (SM). Source: CDF collaboration

Although the particle is tiny, this number is still much larger than the expected value (80,357 ± 6 MeV/c2). While this could be bad news for the Standard Model, the researchers would like to point out that their results need to be replicated before they can be sure.

“While this is an intriguing result, the measurement needs to be confirmed by another experiment before it can be fully interpreted,” warns Joe Lykken, associate director of Fermilab.

“It is now up to the theoretical physics and other experimental community to investigate and shed light on this mystery,” said CDF co-speaker David Toback, a researcher at Texas A&M, USA.

“If the difference between the experimental and the expected value is due to some kind of new particle or subatomic interaction, which is one possibility, there is a good chance that this could be discovered in future experiments.”

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