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Generating Ultra-Peripheral Collisions

There are a few things to consider while writing a user program for simulation of ultra peripheral hadron-hadron collisions (UPC):

• In Sartre, the beam particle emitting the photon is always the beam particle coming from the right, i.e. with negative momentum in the z direction. For UPC events with symmetric beams one then needs to reflect the entire event in the xy-plane in half of the events, preferably chosen at random.
• When running UPC for symmetric beams, the cross-section given by calling Sartre::totalCrossSection() has to be multiplied by a factor 2.
• When running with asymmetric beams, e.g. proton-gold, one has to run events with A=1 and UPCA=197, as well as events with A=197 and UPCA=1. These events then has to be weighted by respective cross-section. This can be avoided if one can make experimental cuts such that one of the processes dominates the events.