What is an "Event"?
In particle physics, an event is a single collision of two particles or a decay of a single particle. (The chance that three or more particles will meet at exactly the same place and time is so tiny that it can usually be ignored.)
A collision means any process which results in a deflection in the path of the original particles, or their annihilation.
Physicists use the term "scattering" rather than collision because for particles in quantum theory, there is no sense in which we can say the particles actually touched one another. The particles interact through long-range fields, such as electromagnetic fields, that can extend far beyond their matter size. (In fact, as far as we know, quarks and leptons have no matter size!)
Annihilation is the process in which a particle and a related antiparticle disappear. Their energy appears as some virtual force field carrier particle, which then rapidly produces a new particle/antiparticle pair.
The produced system of fundamental particles evolves according to the interaction laws of The Standard Model, so the particles seen in a detector are always some collection of hadrons and leptons, not isolated quarks.
Event Pictures
In a high-energy collision, there are typically many particles eventually produced. An event picture is a pictorial representation of the record of these produced particles in a detector.
