Oblique lessons from the W-mass measurement at CDF II

The CDF Collaboration recently reported a new precise measurement of the W-boson mass MW with a central value significantly larger than the SM prediction. We explore the effects of including this new measurement on a fit of the Standard Model (SM) to electroweak precision data. We characterize the tension of this new measurement with the SM and explore potential beyond the SM phenomena within the electroweak sector in terms of the oblique parameters S, T and U. We show that the large MW value can be accommodated in the fit by a large, nonzero value of U, which is difficult to construct in explicit models. Assuming U = 0, the electroweak fit strongly prefers large, positive values of T. Finally, we study how the preferred values of the oblique parameters may be generated in the context of models affecting the electroweak sector at tree and loop level. In particular, we demonstrate that the preferred values of T and S can be generated with a real SU(2)L triplet scalar, the humble swino, which can be heavy enough to evade current collider constraints, or by (multiple) species of singlet-doublet fermion pairs. We highlight challenges in constructing other simple models for explaining a large MW value and several directions for further study.

Automated summary

The CDF Collaboration has reported a new measurement of the W-boson mass MW, which is significantly higher than the prediction of the Standard Model. The study investigates the tension between this new measurement and the Standard Model, and explores possible beyond the Standard Model phenomena in the electroweak sector. The analysis shows that the large MW value can be accommodated by a large, nonzero value of the oblique parameter U, and that the preferred value for the oblique parameter T is large and positive assuming U = 0.