Constraining SMEFT BSM scenarios with EWPO and $\Delta_{CKM}$

TL;DR

This paper develops a comprehensive framework to constrain BSM scenarios using electroweak precision observables and $ abla_{CKM}$ measurements, improving bounds on SMEFT Wilson coefficients and testing minimal extensions like vectorlike leptons.

Contribution

It introduces a unified fitting approach combining EWPO and $ abla_{CKM}$ data to tighten constraints on SMEFT operators and BSM models, including the impact of additional observables.

Findings

Inclusion of $ abla_{CKM}$ strengthens bounds on SMEFT Wilson coefficients.

Minimal vectorlike lepton extension is in tension with $A_b^{FB}$ and $M_W$ measurements.

Adding LEP-II $WW$ observables helps resolve fit degeneracies.

Abstract

Precision observables are well known for constraining most of the Beyond Standard Model (BSM) scenarios tightly. We present here a simple and comprehensive fitting framework for various BSM scenarios to these observables. We start with the fit of $S$, $T$ and $V$ parameter and their correlations using the Electroweak Precision Observables (EWPO) including the recent $m_W$ measurement from CDF-II. Utilizing these observables, we also fit various New Physics (NP) scenarios consisting of different subsets of dimension-6 Standard Model Effective Field Theory (SMEFT) operators in the Warsaw basis out of a total of 10 appearing at tree level in EWPO. To further constrain these scenarios, we augment these observables with $\Delta_{CKM}$ measurement using 1-loop matching of the Low Energy Effective Field Theory (LEFT) to SMEFT operators at the Z-pole. We show that the inclusion of $\Delta_{CKM}$ constraint indeed results in stronger bounds on the SMEFT Wilson Coefficients. We also constrain the UV parameters of BSM extensions like Vectorlike leptons (VLL) and find out that such a minimal extension is in tension with the forward-backward asymmetry in $b$-sector ($A_b^{FB}$) and the recent measurement of $M_W$. In order to lift the two blind directions, which one encounters while fitting all the 10 SMEFT WCs at tree-level, we also include the LEP-II observables pertaining to the $WW$ production and present the results for the fits with and without $\Delta_{CKM}$ constraint.