Answer the following about ultra-peripheral collisions, or UPCs, which are used to study novel nuclear physics at the RHIC (“rick”) and the LHC, for 10 points each.
[10m] Ultra-peripheral collisions occur when this quantity is large. In scattering theory, this quantity symbolized b measures the perpendicular distance between a scattering particle’s trajectory and the more massive particle.
ANSWER: impact parameter
[10e] During UPCs, photons with this property either interact with each other or with a nucleus. Particles with this property are off the mass-shell and colloquially pop in and out of existence during interactions.
ANSWER: virtual particles [accept virtual photons]
[10h] UPCs are used to study processes like gluon saturation that occur at small values of this scattering parameter. This parameter is the independent variable of a parton distribution function, and equals “capital Q squared over 2 mv.”
ANSWER: x [or Bjorken x; or momentum fraction; or the fraction of the momentum of a parton]
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