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2022 unexpected measurement of W boson mass

Before 2022, measurements of the W boson mass appeared to be consistent with the Standard Model. For example, in 2021, experimental measurements of the W boson mass were assessed to converge around 80 379 ± 12 MeV.

However, in April 2022, a new analysis of data that was obtained by the Fermilab Tevatron collider before its closure in 2011 determined the mass of the W boson to be 80 433 ± 9 MeV, which is seven standard deviations above that predicted by the Standard Model, meaning that if the model is correct there should only be a one-trillionth chance that such a large mass would arise by non-systematic observational error. According to Ashutosh Kotwal of Duke University and the leader of the Collider Detector at Fermilab collaboration, the lower beam luminosity used reduced the chance that events of interest would be obscured by other collisions and that the use of proton–antiproton collisions simplifies the process of quark–antiquark annihilation, which then decayed to give a lepton and a neutrino. The team deliberately encrypted its data and withheld any preliminary results from themselves until the analysis was complete, to prevent "confirmation bias" bending their interpretation of the data. Kotwal described it as the 'largest crack in this beautiful theory', speculating that it might be the 'first clear evidence' of other forces or particles not accounted for by the Standard Model, and which might be accounted for by theories such as supersymmetry. The Nobel-winning theoretical physicist Frank Wilczek described the result as a 'monumental piece of work'.

Besides being inconsistent with the Standard Model, the new measurement is also inconsistent with previous measurements such as ATLAS. This suggests that either the old or the new measurements, despite all precautions, have an unexpected systematic error, such as an undetected quirk in the equipment. Future experiments with the LHC may help determine which set of measurements, if either, are the correct ones. Fermilab Deputy Director Joseph Lykken reiterated that "... the (new) measurement needs to be confirmed by another experiment before it can be interpreted fully." Matthias Schott, of the University of Mainz, commented that "I do not think we have to discuss which new physics could explain the discrepancy between CDF and the Standard Model – we first have to understand why the CDF measurement is in strong tension with all ".

In 2023, the ATLAS experiment released an improved measurement for the mass of the W boson, 80 360 ± 16 MeV, which aligned with predictions from the Standard Model. The combination of all measurements of the W boson mass, excluding the measurement from CDF, yields a value of (80369.2 ± 13.3) MeV/c², which deviates from the CDF measurement with a significance of 3.6σ.