  We calculate the effect of the emergent photon on threshold production of
spinons in $U(1)$ Coulomb spin liquids such as quantum spin ice. The emergent
Coulomb interaction modifies the threshold production cross-section
dramatically, changing the weak turn-on expected from the density of states to
an abrupt onset reflecting the basic coupling parameters. The slow photon
typical in existing lattice models and materials suppresses the intensity at
finite momentum and allows profuse Cerenkov radiation beyond a critical
momentum. These features are broadly consistent with recent numerical and
experimental results.
