Understand
We propose practical ways of differentiating the various (Breit-Wigner, theoretical, and energy-dependent) resonance schemes of unstable particles at lepton colliders.
- First, the energy-dependent scheme can be distinguished from the other two by fitting the $Z$ lineshape scan and forward-backward asymmetries at LEP and future lepton colliders with the $Z$ mass $m_Z$, decay width $\Gamma_Z$, and coupling strength as fitting parameters.
- Although the Breit-Wigner and theoretical schemes work equally well, the scheme conversion requires the decay width $\Gamma_Z$ to scale inversely with $m_Z$ rather than the usual linear dependence from theoretical calculation.
- These contradicting behaviors can be used to distinguish the Breit-Wigner and theoretical schemes by the precision $Z$ measurements with single parameter ($m_Z$) fit at future lepton colliders.