The Sill distribution for all partial waves

Abstract

In high-energy physics, short-lived resonances appearing in scattering processes are described by energy-dependent spectral functions. The relativistic Breit--Wigner distribution (rBW), obtained as a relativistic extension of the standard Breit--Wigner form, is commonly employed owing to its simplicity. However, when the lower-energy threshold lies sufficiently close to the resonance peak, sizable deviations from the rBW line shape are expected. The Sill distribution provides a simple way to incorporate the physical threshold while preserving exact normalization, but in its original formulation it is inherently restricted to $s$-wave decays. Here, we extend the Sill distribution to arbitrary partial waves. The correct $\ell$-wave threshold behaviour is implemented while exact normalization is retained through a simple modification of the mass term of the propagator. As a benchmark, the $p$-wave Sill provides a very good description of the $Δ(1232)^{++}$ line shape up to 1.35 GeV and simultaneously reproduces its on-shell mass and width and its complex pole position, in agreement with independent determinations compiled by the PDG.