New W Boson Mass Measurement of CMS
Prof. Dr. Lorenzo Bianchini (University of Pisa, Italy)

The mass of the W boson, the mediator of the charged weak interaction, can be predicted with a relative precision of about 80 ppm within the Standard Model (SM) of particle physics. The existence of new physics could however affect the W boson mass via quantum loops and shift it with respect to the SM prediction. Thus, a direct measurement of the W mass can be both a sensitive test of consistency of the theory as well as a window to new physics. In this respect, great interest, together with confusion, was raised by the measurement delivered by the CDF Collaboration in 2022 which, besides being the most precise measurement to date, is in disagreement with the SM and also barely consistent with previous measurements. Up until recently, the CMS experiment was the last missing contributor to the W mass effort. The new result by CMS which I will present in this seminar is based on a partial sample of LHC proton-proton collision data collected during the 2016 data-taking period. The W boson mass is extracted using single-muon events via a highly granular maximum likelihood fit of the muon kinematics split by charge and by relying on state-of-the-art tools for the modeling of W boson production and decay. This novel approach enables significant in-situ constraints of experimental and theoretical uncertainties. The CMS result has an uncertainty comparable to the CDF measurement and agrees with the SM. It represents a crucial step in solving the W boson mass puzzle.