The most precise measurement of the W boson mass to date deviates from the prediction of the Standard Model

The most precise measurement of the W boson mass to date shows strains with the Standard Model.

After 10 years of careful analysis and scrutiny, scientists from the CDF Collaboration at the US Department of Energy’s Fermi National Accelerator Laboratory announced on April 7, 2022 that they had achieved the most precise measurement yet of the mass of the W boson, one of the force-bearing particles of the Nature. Using data collected by Fermilab’s Collider Detector or CDF, scientists have now determined the mass of the particle with an accuracy of 0.01% – twice as accurate as the previous best measurement. It is equivalent to measuring the weight of an 800-pound gorilla to 1.5 ounces.

The new precision measurement, published in the magazine Scienceallows scientists to test the Standard Model of Particle Physics, the theoretical framework that describes nature at its most basic level. The result: the new mass value is in tension with the value that scientists get from experimental and theoretical inputs in the context of the Standard Model.

Fermilab Collider Detector

The Collider Detector at Fermilab recorded high-energy particle collisions produced by the Tevatron Collider from 1985 to 2011. About 400 scientists at 54 institutions in 23 countries are still working on the wealth of data collected by the experiment. Photo credit: Fermilab

“The number of improvements and additional checks that went into our result is tremendous,” said Ashutosh V. Kotwal of Duke University, who led this analysis and is one of the 400 scientists in the CDF collaboration. “We have taken into account our improved understanding of our particle detector as well as advances in theoretical and experimental understanding of the interactions of the W boson with other particles. When we finally revealed the result, we found that it deviated from the Standard Model’s prediction.”

If this measurement is confirmed, it indicates the potential need for improvements to the standard model computation or extensions to the model.

Scientists have now determined the mass of the W boson with an accuracy of 0.01%. This is twice as accurate as the previous best measurement and shows tension with the standard model.

The new value agrees with many previous W boson mass measurements, but there are also some inconsistencies. Future measurements will be necessary 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 fully interpreted,” said Joe Lykken, associate director of Fermilab.

The W boson is a messenger particle of the weak nuclear force. It is responsible for the nuclear processes that make the sun glow and cause particles to decay. Using high-energy particle collisions generated by the Tevatron collider at Fermilab, the CDF collaboration collected vast amounts of data containing W bosons from 1985 to 2011.

Standard Model of Elementary Particles

The W boson is the messenger particle of the weak nuclear force. It is responsible for the nuclear processes that make the sun glow and cause particles to decay. CDF scientists are studying the properties of the W boson using data collected at the Tevatron Collider at Fermilab. Photo credit: Fermi National Accelerator Laboratory

CDF physicist Chris Hays from the[{” attribute=””>University of Oxford said, “The CDF measurement was performed over the course of many years, with the measured value hidden from the analyzers until the procedures were fully scrutinized. When we uncovered the value, it was a surprise.”

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. The central value and uncertainty of their latest mass measurement is 80,433 +/- 9 MeV/c2. This result uses the entire dataset collected from the Tevatron collider at Fermilab. 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.

W Boson Mass Comparison

The mass of a W boson is about 80 times the mass of a proton, or approximately 80,000 MeV/c2. Scientists of the Collider Detector at Fermilab collaboration have achieved the world’s most precise measurement. The CDF value has a precision of 0.01 percent and is in agreement with many W boson mass measurements. It shows tension with the value expected based on the Standard Model of particle physics. The horizontal bars indicate the uncertainty of the measurements achieved by various experiments. The LHCb result was published after this paper was submitted and is 80354+- 32 MeV/c2. Credit: CDF collaboration

“Many collider experiments have produced measurements of the W boson mass over the last 40 years,” said CDF co-spokesperson Giorgio Chiarelli, Italian National Institute for Nuclear Physics (INFN-Pisa). “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 collaboration 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, discovered at the Tevatron collider at Fermilab in 1995, and the Higgs boson, discovered at the Large Hadron Collider at DOI: 10.1126/science.abk1781

The CDF collaboration comprises 400 scientists at 54 institutions in 23 countries.

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