Physicists Precisely Measure Mass of W Boson

Physicists from the Collider Detector at Fermilab (CDF) Collaboration have determined the mass of the W boson, a mediator of the weak force between elementary particles, with a precision of 0.01% — twice as precise as the previous best measurement. The new value shows tension with the value scientists obtain using experimental and theoretical inputs in the context of the Standard Model of elementary particle physics.

The W boson is the messenger particle of the weak nuclear force. It is responsible for the nuclear processes that make the sun shine and particles decay. Image credit: Fermilab.

The observation of the Higgs boson at CERN’s Large Hadron Collider has validated the last missing piece of the Standard Model.

This model, which incorporates quantum mechanics, special relativity, gauge symmetry, and group theory, currently describes most particle physics measurements with high accuracy.

It postulates a number of experimentally established symmetries among particle properties, which tightly constrain the parameters of the model from experimental data.

Given the current experimental precision and the predictive power of the Standard Model, global fits of the model to the data render precise estimates of fundamental parameters, such as the mass of the W boson.

As one of the mediators of the weak nuclear force, this particle is a key component of the Standard Model framework.

“The number of improvements and extra checking that went into our result is enormous,” said Dr. Ashutosh Kotwal, a physicist at Duke University and a member of the CDF Collaboration.

“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.”

“If confirmed, this measurement suggests the potential need for improvements to the Standard Model calculation or extensions to the model.”

The CDF physicists have worked on achieving increasingly more precise measurements of the W boson mass for more than 20 years.

The central value and uncertainty of their latest mass measurement is 80,433.5 ± 9.4 MeV/c2.

This result uses the complete dataset collected by the CDF II detector at the Fermilab Tevatron. It is based on the observation of 4.2 million W boson candidates, about four times the number used in the analysis the collaboration published in 2012.

“Many collider experiments have produced measurements of the W boson mass over the last 40 years,” said Dr. Giorgio Chiarelli, a physicist at the Italian National Institute for Nuclear Physics and co-spokesperson of the CDF Collaboration.

“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.”

“It is our most robust measurement to date, and the discrepancy between the measured and expected values ​​persists.”

The CDF researchers also compared their result to the best value expected for the W boson mass using the Standard Model, which is 80.357 ± 6 MeV/c2.

This value is based on complex Standard Model calculations that intricately link the mass of the W boson to the measurements of the masses of two other particles: the top quark and the Higgs boson.

“It’s now up to the theoretical physics community and other experiments to follow up on this and shed light on this mystery,” said Dr. David Toback, a physicist at Texas A&M University and co-spokesperson of the CDF Collaboration.

“The result is an important contribution to testing the accuracy of the Standard Model.”

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

Tea results appear in the journal Science.


T. Aaltonen et al. (CDF Collaboration). High-precision measurement of the W boson mass with the CDF II detector. Science 376 (6589): 170-176; doi:10.1126/science.abk1781

Leave a Comment