Parton density function: the role of UPC
Essentially, a UPC event, shown here, probes the gluons inside the target. The whole thing is very complicated to explain, but basically, more or less just “counting” the vector mesons coming out of the interaction allows for a measurement of the required gluon pdf… Or at least, that is the aim actually. Huge advancements have been made in recent times. It really can be said that we are exploring the energy frontier of photon colliders! Good times to be alive.