High-precision measurement of the W boson mass deviates from Standard Model predictions, is surprisingly high

A high-precision measurement of the W boson mass, roughly twice as precise as the previous best measurement, indicates that the particle is far heavier than predicted by the standard model (SM) of particle physics – the theoretical framework that describes nature at its most fundamental level. If confirmed, the findings from the Collider Detector at Fermilab (CDF) Collaboration et al. could highlight areas where the SM needs to be improved or expanded, or even provide a first glimpse of physics beyond the SM. The existence of the W boson – a key building block of the SM – was predicted in the 1960s and first confirmed in the early 1980s. It is responsible for “weak nuclear force,” which is one of the fundamental forces in physics. Because of this, the mass of the W boson is a parameter within the theoretical framework of the SM – one that is constrained by other observable parameters, such as electron charge and the masses of other particles. 

Thus, accurate measurement of W boson mass can provide a stringent evaluation of the consistency of SM predictions. After nearly a decade of analysis of data collected from the Tevatron particle accelerator, the CDF Collaboration et al. – an international team of 400 researchers – report the most precise measurement of the W boson mass to date. According to the authors, the findings show that the mass of the W boson is higher than the SM predicts by 7 standard deviations. “The surprisingly high value of the W boson mass reported by the CDF collaboration et al., directly challenges a fundamental element at the heart of the SM, where both experimental observables and theoretical predictions were thought to have been firmly established and well understood,” write Claudio Campagnari and Martijn Mulders in a related Perspective. “Because extraordinary claims require extraordinary evidence,” the Perspective authors add, “the claim by the CDF Collaboration et al. will require additional experiments to provide an independent confirmation.