The CMS collaboration at CERN has measured the mass of the W boson at 80,360.2 plus or minus 9.9 MeV — a result that agrees with the Standard Model's prediction of 80,353 plus or minus 6 MeV and directly contradicts the anomalous measurement that shook particle physics in 2022. The result, published in Nature on April 8, drew on roughly one billion proton-proton collision events and approximately 100 million W boson events. [1] [2]

The CDF collaboration at Fermilab had reported a W boson mass of 80,433.5 plus or minus 9.4 MeV in 2022 — a value so far from the Standard Model prediction that it implied new physics was required. That measurement consumed thousands of hours of theoretical attention. The CMS result now joins earlier measurements from ATLAS and LHCb in disagreeing with CDF. [1]

Kenneth Long of MIT, a CMS collaborator, called the result "a huge relief." The Standard Model, the most successful theory in physics, had appeared to have a crack. The crack, it turns out, was in the measurement, not the model. [1]

On X, physicists treat the CDF outlier as a lesson in how hard precision measurements are. MSM covers the CMS result as vindication. The deeper story is what did not happen: the new physics that CDF's number seemed to demand does not exist — at least not here.

-- KENJI NAKAMURA, Tokyo