CMS experiment confirms W boson mass aligns with Standard Model
A decade-long effort to precisely measure the mass of a fundamental particle has produced a result that bolsters physicists' leading theory of the universe. The CMS collaboration at CERN reports in Nature that the W boson has a mass of 80,360.2 ± 9.9 megaelectronvolts, fully consistent with Standard Model predictions. This finding resolves a major anomaly that emerged in 2022.
The prior discrepancy arose from the CDF experiment at Fermilab's Tevatron, which measured the mass at 80,433.5 ± 9.4 MeV—seven standard deviations above the predicted 80,353 ± 6 MeV. That outlier sparked speculation about unknown particles or forces. CMS's new measurement, achieved with similar precision, aligns closely with the model and diverges sharply from CDF's value.
Researchers analyzed over a billion proton-proton collisions recorded by the Compact Muon Solenoid detector in 2016. They identified about 100 million events where a W boson decayed into a muon and a neutrino. Since neutrinos evade detection, the team reconstructed the mass from the muon alone, using 4 billion simulated collisions and years of calibration.
The result joins consistent findings from CERN's ATLAS and others, isolating CDF as an outlier. The Particle Data Group in 2024 excluded CDF from the world average, setting the mass at 80,369.2 ± 13.3 MeV. CDF's 2025 reanalysis upheld its claim, but CMS data strengthens the consensus.
More than 3,000 physicists contributed, with a core team of 30 from 10 institutions led by MIT researchers. Future plans include analyzing additional LHC data for greater precision. Kenneth Long, lead author from MIT, called it a major relief that reaffirms trust in the Standard Model. Co-author Christoph Paus aims to refine techniques further.
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