Constraints on SMEFT operators from $Z \to μμbb$ decay

TL;DR

This paper investigates how the decay Z → μμbb can constrain physics beyond the Standard Model within the Standard Model Effective Field Theory (SMEFT). It employs a full Monte Carlo framework, including detector effects, and uses an SMEFT reweighting approach in the M_W scheme to capture the impact of six flavor-resolved dimension-six operators, treating the observable yields as quadratic in the Wilson coefficients. The study finds that this mixed leptonic-hadronic channel provides process-specific limits on lepton-bottom four-fermion operators and on Z-fermion couplings, with sensitivity patterns highlighting interference-dominated and quadratic-dominated directions. HL-LHC projections indicate notable improvements, underscoring the channel's complementarity to existing SMEFT analyses and motivating its inclusion in future global fits.

Abstract

The Standard Model Effective Field Theory (SMEFT) provides a systematic framework to probe indirect effects of heavy new physics via precision measurements. While SMEFT constraints have been extensively studied using purely leptonic $Z$ decays and inclusive $Z$ production, mixed leptonic-hadronic modes remain largely unexplored. In this work, we analyze $Z \to μμbb$ decays within the SMEFT framework, deriving constraints on dimension-six operators that affect four-fermion interactions between leptons and bottom quarks, as well as $Z$-fermion couplings. Signal and background events are simulated with state-of-the-art Monte Carlo tools, including detector effects such as $b$-tagging, and limits on the relevant Wilson coefficients are extracted using kinematic distributions and a profile likelihood approach. Our results provide complementary constraints to existing SMEFT studies and yield the first process-specific limits on flavor-resolved four-fermion operators involving muons and bottom quarks from $Z$ decays.

Figures (4)

• Figure 1: Representative Feynman diagram contributing to the process $Z \to \mu\mu bb$ in the Standard Model, where the four-body final state arises from final-state radiation of an off-shell $Z^\ast/\gamma^\ast$.
• Figure 2: Detector-level kinematic distributions for the process $Z \to \mu \mu bb$ obtained from Delphes simulation.
• Figure 3: Detector-level invariant mass distributions for $\mu\mu bb$ from various processes, obtained from Delphes simulation. The first panel shows the full mass spectrum with the signal scaled for better visibility, while the second panel zooms into the signal region.
• Figure 4: Quadratic SMEFT fits for the six operators under study. Each panel shows the fitted yield as a function of the WCs for the corresponding operator, with points representing simulated events and the red line indicating the quadratic fit. The legend shows the operator name following the SMEFTsim 3.0 conventions.