Oblique Lessons from the $W$ Mass Measurement at CDF II

TL;DR

The paper analyzes the implications of the new $W$ boson mass measurement from CDF II on the Standard Model fit, exploring potential beyond the SM explanations through oblique parameters and specific new physics models.

Contribution

It demonstrates how the large $M_W$ value can be explained by nonzero oblique parameter $U$ or large positive $T$, and proposes models like a real SU(2)$_L$ triplet scalar or singlet-doublet fermions to generate these effects.

Findings

Large $M_W$ can be accommodated with a nonzero $U$ parameter.

Electroweak fit favors large, positive $T$ when $U=0$.

Models like a triplet scalar or fermion pairs can produce the required oblique parameters.

Abstract

The CDF collaboration recently reported a new precise measurement of the $W$ boson mass $M_W$ 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 $M_W$ 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 a singlet-doublet fermion pair. We highlight challenges in constructing other simple models, such as a dark photon, for explaining a large $M_W$ value, and several directions for further study.