Towards a Precision Measurement of the Muon Pair Asymmetry in e+e− Annihilation at Belle and Belle II

Abstract

The Standard Model predicts interference effects between γ and Z boson exchange in fermion pair production which cause a forward-backward asymmetry A F B even for energies well below the Z-pole. This energy-dependent asymmetry (A F B ≈ 10 −2 at 10.58 GeV) is proportional to the SM ρ parameter and can be measured at the Belle and Belle II experiments. The sensitivity to New Physics as well as theoretical and experimental challenges are described. The process e + e − → µ + µ − is among the simplest reactions of the Standard Model (SM) where both quantum electrodynamics (QED) and electroweak (EW) predictions can be tested. The distribution of the polar angle θ * of the outgoing leptons in the center of mass system, defined as the angle between the e + and the µ + , is expected to be asymmetric in the SM at Born level, caused by the interference of γ and Z exchange even at energies well below the Z pole, whereas lowest-order QED predicts a symmetric angular distribution. The forward-backward asymmetry is defined as A

The ZFITTER project

“…A completely different application of ZFITTER is presently planned for the analysis of muon pair production data at Belle and Belle 2 (with about 10 9 muon pairs) at a center-of-mass energy √ s = 10.58 GeV. One may study there, besides QED properties, the axial vector coupling of leptons [73,74].…”