In particle physics, an event is a single collision of two particles or a decay of a single particle. A collision means any process which results in a deflection in the path of the original particles, or their annihilation . The particles interact through long-range fields, such as electromagnetic fields, that can extend far beyond their matter size.
Annihilation is the process in which a particle and a related antiparticle disappear.
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 . 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.