Electroweak Precision Constraints on the Littlest Higgs Model with T Parity

TL;DR

The paper assesses electroweak precision constraints on the Littlest Higgs model with T parity (LHT) by computing leading one-loop corrections to W/Z properties and performing a global fit to precision data. It demonstrates that a broad region of parameter space is consistent with measurements, allowing a symmetry-breaking scale f around 0.5 TeV and even lighter values in restricted regimes, while also enabling a heavy Higgs (up to ~800 GeV) due to partial cancellations with new physics contributions. Importantly, there is overlap between EW-fit–allowed regions and the dark matter relic density region provided by the lightest T-odd particle, highlighting LHT’s dark matter and collider phenomenology relevance. Overall, the results support the viability of TeV-scale LHT as a natural extension of the Standard Model with testable predictions at current and future colliders.

Abstract

We compute the leading corrections to the properties of W and Z bosons induced at the one-loop level in the SU(5)/SO(5) Littlest Higgs model with T parity, and perform a global fit to precision electroweak data to determine the constraints on the model parameters. We find that a large part of the model parameter space is consistent with data. Values of the symmetry breaking scale as low as 500 GeV are allowed, indicating that no significant fine tuning in the Higgs potential is required. We identify a region within the allowed parameter space in which the lightest T-odd particle, the partner of the hypercharge gauge boson, has the correct relic abundance to play the role of dark matter. In addition, we find that a consistent fit to data can be obtained for large values of the Higgs mass, up to 800 GeV, due to the possibility of a partial cancellation between the contributions to the T parameter from Higgs loops and new physics.

Figures (9)

• Figure 1: The one-loop diagrams contributing to the oblique corrections from the top sector of the LH model with T parity.
• Figure 2: The box diagrams which provide the leading contribution to the four-fermion operators in the limit $\kappa \gg g$.
• Figure 3: The dominant standard model diagrams which contribute to the $Zb\bar{b}$ vertex renormalization at one-loop in the $R_\xi$ gauge.
• Figure 4: The additional "mixing" diagrams contributing to the $Zb\bar{b}$ vertex renormalization at one loop in the LH model with T parity.
• Figure 5: Exclusion contours in terms of the parameter $R=\lambda_1/\lambda_2$ and the symmetry breaking scale $f$. The contribution of the T-odd fermions to the T parameter is neglected. From lightest to darkest, the contours correspond to the 95, 99, and 99.9 confidence level exclusion.