PrimEx experiment (π0 lifetime measurement)
Collision of photons over nuclei, detection of π0's at small angle.
The cross section is directly proportional to Γπ0->γγ
(It also goes like Z2, E4, sin2θπ0)
Other processes can give the same final state, but don't have the Z dependence (so it's good to run with different targets) and/or different angular behaviour.