Glueball spectral densities from the lattice

Understand

The propagator of a physical degree of freedom ought to obey a K\"{a}ll\'{e}n-Lehmann spectral representation, with positive spectral density.

• The latter quantity is directly related to a cross section based on the optical theorem.
• The spectral density is a crucial ingredient of a quantum field theory with elementary and bound states, with a direct experimental connection as the masses of the excitations reflect themselves into (continuum) $\delta$-singularities.
• In usual lattice simulational approaches to the QCD spectrum the spectral density itself is not accessed.