Custodial Isospin Violation in the Lee-Wick Standard Model

TL;DR

This paper investigates the balance between naturalness and isospin violation in the Lee-Wick Standard Model by calculating electroweak parameters and Zbb coupling contributions, deriving constraints on LW partner masses from experimental data.

Contribution

It provides a detailed analysis of electroweak constraints on the Lee-Wick Standard Model, highlighting how different sectors contribute to observable parameters and setting bounds on LW partner masses.

Findings

LW gauge bosons must have masses > 2.4 TeV at 95% CL.

LW fermions must have masses > 1.6 TeV for light Higgs, > 4 TeV from Zbb coupling.

Heavy Higgs scenarios tend to exclude the LW SM based on constraints.

Abstract

We analyze the tension between naturalness and isospin violation in the Lee-Wick Standard Model (LW SM), by computing tree-level and fermionic one-loop contributions to the post-LEP electroweak parameters and the Zbb coupling. The model is most natural when the LW partners of the gauge bosons and fermions are light, but small partner masses can lead to large isospin violation. The post-LEP parameters yield a simple picture in the LW SM: the gauge sector contributes to Y and W only, with leading contributions arising at tree-level, while the fermion sector contributes to S-hat and T-hat only, with leading corrections arising at one loop. Hence, W and Y constrain the masses of the LW gauge bosons to satisfy M1, M2 > 2.4 TeV at 95% CL. Likewise, experimental limits on T-hat reveal that the masses of the LW fermions must satisfy Mq, Mt > 1.6 TeV at 95% CL if the Higgs mass is light and tend to exclude the LW SM for any LW fermion masses if the Higgs mass is heavy. Contributions from the top-quark sector to the Zbb coupling can be even more stringent, placing a lower bound of 4 TeV on the LW fermion masses at 95% CL.