Toponium: the smallest bound state and simplest hadron in quantum mechanics

We explore toponium, the smallest known quantum bound state of a top quark and its antiparticle, bound by the strong force. With a Bohr radius of $8\times {10}^{-18}$~m and a lifetime of $2.5\times {10}^{-25}$~s, toponium uniquely probes microphysics. Unlike all other hadrons, it is governed by ultraviolet freedom. This distinction offers novel insights into quantum chromodynamics. Our analysis reveals a toponium signal exceeding $5\sigma$ in the distribution of the cross section ratio between $e^{+}{e}^{-}\to b\bar{b}$ and $e^{+}{e}^{-}\to q\bar{q}$ ($q=b,c,s,d,u$), based on 400~fb${}^{-1}$ of data collected at $\sqrt{s}\approx 341\mathrm{G}\mathrm{e}\mathrm{V}$. This discovery enables a top quark mass measurement with an uncertainty reduced by a factor of ten compared to current precision levels. Moreover, this method improves the systematic uncertainty by at least a factor of 2.7 compared to any other possible methods.