Measurement of the effective weak mixing angle at the CEPC

Abstract

We present a study of the measurement of the effective weak mixing angle parameter ($\sin^2\theta^{\ell}_\text{eff}$) at the Circular Electron Positron Collider (CEPC). As a fundamental physics parameter, $\sin^2\theta^{\ell}_\text{eff}$ plays a key role not only in the global test of the standard model electroweak sector, but also in constraining the potential beyond standard model new physics at high energy frontier. CEPC proposes a two year running period around the Z boson mass pole at high instataneous luminosity, providing a large data sample with $4\times 10^{12}$ $Z$ candidates generated in total. It allows a high precision measurement of $\sin^2\theta^{\ell}_\text{eff}$ both in the lepton and quark final states, of which the uncertainty can be one order of magnitude lower than any previous measurement at the LEP, SLC, Tevatron and LHC. It will not only improve the overall precision of the $\sin^2\theta^\ell_\text{eff}$ experimental determination to be comparable to the preicision of the theoretical calculation with two-loop radiative corrections, but also provide direct comparisons between different final states. In this paper, we also study the measurement of $\sin^2\theta^{\ell}_\text{eff}$ at high mass region. With one month data taken, the precision of $\sin^2\theta^{\ell}_\text{eff}$ measured at 130 GeV from $b$ quark final state is 0.00010, which will be an important experimental observation on the energy-running effect of $\sin^2\theta^{\ell}_\text{eff}$.