Electroweak precision tests for triplet scalars

Abstract

Electroweak precision observables are fundamentally important for testing the standard model (SM) or its extensions. The influences to observables from new physics within the electroweak sector can be expressed in terms of oblique parameters S, T, U. The recently reported W mass excess anomaly by CDF modifies these parameters in a significant way. By performing the global fit with the CDF new W mass measurement data, we obtain S=0.03±0.03, T=0.06±0.02 and U=0.16±0.03 (or S=0.14±0.03, T=0.24±0.02 with U=0) which is significantly away from zero as SM would predict. The CDF excess strongly indicates the need of new physics beyond SM. We carry out global fits to study the influence of two different cases of simple extensions by a hyper-charge Y=1 triplet scalar Δ (corresponding to the type-II seesaw model) and a Y=0 real triplet scalar Σ, on electroweak precision tests to determine parameter space in these models to solve the W mass anomaly and discuss the implications. We find that these triplets can affect the oblique parameters significantly at the tree and loop levels. For Y=1 case, there are seven new scalars in the model. The tree and scalar loop effects on oblique parameters can be expressed in terms of three parameters, the doubly-charged mass mH++, potential parameter λ4 and triplet vev vΔ. Our global fit obtains mH++=103.02±9.84 GeV, λ4=1.16±0.07 and vΔ=0.09±0.09 GeV. This parameter correspondingly results in mass difference Δm=mH+−mH++=64.78±2.39 GeV. We find that the doubly-charged mass mH++ satisfies the current LHC constraints. We also further adopt the phenomenological analysis, such as vacuum stability and perturbative unitarity, Higgs data, triple Higgs self-coupling and lepton colliders analysis. For Y=0 case, there are four new scalars in the model. The tree and loop effects on the oblique parameters can be parameterized by the singly charged mass mH+, the mass difference Δm=mH+−mH0 and triplet vev vΣ with the fit values mH+=199.45±39.73 GeV, Δm=−2.32±1.99 GeV and vΣ=3.86±0.27 GeV, respectively. However, these strongly violate the perturbative unitarity of the potential parameter b4, which can be satisfied within 1σ errors.