New Physics at Tera-$Z$: Precision Renormalised

TL;DR

A Tera-Z run at FCC-ee can indirectly detect or constrain nearly all heavy new physics that influence electroweak precision observables, often reaching sensitivities at the TeV scale or beyond.

Contribution

The paper demonstrates that Tera-Z measurements can universally probe heavy new physics through SMEFT operators, with minimal exceptions, and provides projected sensitivity analyses.

Findings

Most new particles affect EWPOs at tree level or via RG running.

FCC-ee Tera-Z can test new physics up to tens of TeV.

Almost all heavy new physics scenarios are accessible with Tera-Z.

Abstract

We study the power of a Tera-$Z$ run at FCC-ee for indirectly detecting or constraining heavy new physics. Our main finding is that nearly every new particle which matches at tree level to dimension-six operators of the Standard Model Effective Field Theory (SMEFT) affects electroweak precision observables (EWPOs) at either tree level or via one loop renormalisation group (RG) running. This is true almost regardless of the structure of couplings to the Standard Model; just a handful of exceptions are identified which can produce zeroes in the EWPO RG equations. Under simple flavour assumptions, we perform fits of each state to projected FCC-ee $Z$ pole measurements, showing that all scenarios can be tested at the TeV scale or better, with many projected exclusions reaching tens of TeV. Tera-$Z$ is argued to provide an almost inescapable probe of heavy new physics.