This is an event where the Z particle decays into a quark-antiquark pair. In this case the pair is a b-quark and an b-antiquark, the heaviest type of quark that can be produced in Z decays. These quarks and antiquarks typically form short-lived particles that decay close to the collision point. Try zooming in on the event to see the tracks that come from the the decay of the short-lived particles.
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