Event 10056: a high-energy cosmic ray?

This event triggered an ACD (the black rectangle shown above the detector tower), so it probably represents a charged particle, not a gamma ray. The straight-line passage through the detector means that the particle was moving quite fast; the daughter particles which it creates on its way through the instrument are all aligned with the parent particle's direction of motion. (A slower parent particle tends to produce daughters moving in different directions.)

At the end of its trip through the detector, the particle shower deposits its energy in the calorimeter, again along a fairly straight path.