Understanding Light: Why we need a terascale photon collider
  We do not understand light. I argue that a terascale photon collider is necessary to determine the structure of the photon at 100 GeV. Uncertainties in photon parton distribution functions lead to cross section predictions that vary by a factor of 5. This limits our ability to predict how well we can perform precision measurements, e.g., extracting the width of Higgs into two photons. These uncertainties will only be resolved by measuring the gluonic structure of the photon in situ. 
Zack Sullivan