CMS experiment announces high-precision measurement of the W boson mass

The mass of the W boson, which along with the Z boson mediates the weak force, is a key parameter in the standard model, and is uniquely related to the coupling strengths of the electromagnetic and weak interactions. Its value is precisely predicted by this theory and the measurements of other parameters, such as the Higgs boson mass, to be mW = 80 355 ± 6 MeV. The presence of physics beyond the standard model, such as the existence of new heavier particles, could modify the measured mass, thus strongly motivating experimental measurements with the highest possible precision. The CMS measurement announced today achieves the highest precision to date at the LHC: mW 80360.2 ± 9.9 MeV, in strong agreement with the standard model value.

Although the W boson was discovered over 40 years ago at CERN, measuring its mass remains a formidable experimental challenge. The W boson mass has previously been measured by the LEP and Tevatron experiments, and the ATLAS and LHCb experiments at the LHC. The recent measurement from the CDF Collaboration, with an uncertainty of 9.4 MeV, is in strong tension with both the standard model expectation and all other measurements. The CMS result, with comparable precision, has reinforced the electroweak prediction.

The main challenge of this analysis is that the invariant mass of the W boson cannot be reconstructed directly in its leptonic decay channels, owing to the presence of a neutrino, which necessitates a more indirect approach. The CMS result is built on a fine-grained measurement of the muon transverse momentum and pseudorapidity distributions, relying on a precise muon reconstruction and calibration, and state of the art theoretical predictions. CMS members at LLR have been deeply involved in overseeing the extensive, multi-year, development and review of this analysis - the most sophisticated of its kind ever performed at CMS.
Andrew Gilbert, over the past two years, has provided invaluable leadership in his role as CMS Standard Model convener, tirelessly guiding the review process of the analysis. Additionally, Roberto Salerno is currently the CMS Physics coordinator.

Contact : Andrew Gilbert, Roberto Salerno