The ATLAS experiment at CERN has presented a better estimate of the mass of one of the cornerstones of the Standard Model. The W boson turns out to be a fraction lighter than thought.
The electrically charged W+ and W- bosons are among the force particles involved in the transmission of the weak nuclear force according to the Standard Model. The particle was found at CERN forty years ago this year, as was the closely related uncharged Z boson. Over the years, the mass of these bosons has been mapped more and more precisely.
At the Moriond winter conference in Italy this week, the results of a new mass determination of the boson by ATLAS were presented. This used existing measurement data from collisions at the LHC accelerator, but in an even more careful analysis in which especially systematic errors were eliminated.
The result of the new measurement is a fraction smaller than the value from the last analysis in 2017. Officially, according to ATLAS, the mass of the W boson is now 80360 MeV, some 10 MeV lower than measurements showed in 2017. W-bosons are some 80 times heavier than a proton.
Physicists are especially excited about the new margin of error of 16 MeV on this result, nearly 20 percent smaller than at the time. “An enormous effort,” said physics coordinator at ATLAS and Nikhef researcher Pamela Ferrari in commenting on the improvements.
ATLAS physicist Ivo van Vulpen of Nikhef calls the analysis “insanely clever”. Nikhef has traditionally been a partner in ATLAS, the world’s largest particle detector, but had not been directly involved in this new W-mass analysis.
The reanalysis of the W mass was prompted in part by a measurement of another experiment, the CDF collider at Fermilab in the US. That experiment last year published a W mass far above the values expected in the Standard Model and previously measured. There was much discussion about that, but also a need to revisit other results.
The new determination of the W mass is in full agreement with the Standard Model.
