The Zero Degree Calorimeter (ZDC) is part of the CMS detector. The ZDC system sits 140m down the beam line and is able to catch particles very close to the beam axis. This event display shows an 80 GeV photon candidate from a glancing blow between two colliding protons. The energy activated the electromagetic section of the ZDC which recorded its absorption as a deposit of a small amount of charge. Note the angle (theta-x): it is expressed in microradians, which is pretty close to zero degrees.