Verifying the Resonance Schemes of Unstable Particles at Lepton Colliders

TL;DR

This paper proposes methods to differentiate various resonance schemes of unstable particles at lepton colliders by analyzing the $Z$ lineshape, asymmetries, and threshold scans, highlighting how precision measurements can identify the correct scheme.

Contribution

It introduces practical techniques to distinguish resonance schemes of unstable particles using collider data, focusing on $Z$ lineshape, asymmetries, and threshold scans.

Findings

Energy-dependent scheme can be identified via $Z$ lineshape fitting.

Scheme conversion involves inverse scaling of decay width with $Z$ mass.

Combining $WW$ threshold scans with Fermi constant measurements aids scheme distinction.

Abstract

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. For the $WW$ threshold scan, its combination with the precise Fermi constant provides another way of distinguishing the Breit-Wigner and theoretical schemes.