Most precise measurement to date of the mass of the W boson

The result is twice as precise as the previous best measurement.


W bosons mediate the weak interaction, one of the fundamental forces in physics. The mass of the W boson is tightly constrained by the symmetries of the standard model of particle physics. Hence, measuring the mass of W boson put the Standard Model to the test.

After a decade of careful analysis and scrutiny, scientists of the CDF collaboration at the U.S. Department of Energy’s Fermi National Accelerator Laboratory have measured the mass of the W boson precisely. This is the most precise measurement of data ever to date. The measurements allowed scientists to test standard models.

Scientists determined the particle’s mass with a precision of 0.01%. This is twice as precise as the previous best measurement. The new mass value shows tension with the value scientists obtain using experimental and theoretical inputs in the context of the Standard Model.

The mass of a W boson is about 80 times the mass of a proton or approximately 80,000 MeV/c2. CDF researchers have worked on achieving increasingly more precise measurements of the W boson mass for more than 20 years. Their latest mass measurement’s central value and uncertainty are 80,433 +/- 9 MeV/c2.

Ashutosh V. Kotwal of Duke University, who led this analysis and is one of the 400 scientists in the CDF collaboration, said, “The number of improvements and extra checking that went into our result is enormous. We took into account our improved understanding of our particle detector and 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.”

Fermilab Deputy Director Joe Lykken said, “The new value agrees with many previous W boson mass measurements, but there are also some disagreements. Future measurements will be needed to shed more light on the result.”

“While this is an intriguing result, the measurement needs to be confirmed by another experiment before it can be interpreted fully.”

CDF co-spokesperson David Toback, Texas A&M University, stated“The result is an important contribution to testing the accuracy of the Standard Model. It’s now up to the theoretical physics community and other experiments to follow up on this and shed light on this mystery. Suppose the difference between the experimental and expected value is due to some new particle or subatomic interaction, which is one of the possibilities. In that case, there’s a good chance it’s something that could be discovered in future experiments.”

Journal Reference:

  1. CDF Collaboration, T. Aaltonen wt al. High-precision measurement of the W boson mass with the CDF II detector. DOI: 10.1126/science.abk1781
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