W-boson mass anomaly from a general SU(2) L scalar multiplet* *Supported in part by the National Key Research and Development Program of China (2021YFC2203003, 2020YFC2201501) and the National Natural Science Foundation of China (NSFC) (12005254, 12147103)

Abstract

We explain the W-boson mass anomaly by introducing an scalar multiplet with general isospin and hypercharge {in the case without its vacuum expectation value}. It is shown that the dominant contribution from the scalar multiplet to the W-boson mass arises at the one-loop level, which can be expressed in terms of the electroweak (EW) oblique parameters T and S at leading order. We first rederive the general formulae of T and S induced by a scalar multiplet of EW charges, confirming the results in literature. We then study several specific examples of great phenomenological interest by applying these general expressions. As a result, it is found that the model with a scalar multiplet in an real representation with cannot generate the required correction because it leads to vanishing values of T and S. However, the cases with scalars in a complex representation under with a general hypercharge can explain the excess observed by CDF-II owing to nonzero T and S. We further consider the strong constraints from the perturbativity and EW global fit of the precision data and vary the isospin representation and hypercharge of the additional scalar multiplet to assess the extent of the model to solve the W-boson mass anomaly. It turns out that these constraints play important roles in setting limits on the model parameter space. We also briefly describe the collider signatures of the extra scalar multiplet, especially when it contains long-lived, heavy, highly charged states.