b-quark/antiquark pairs are the heaviest particles that can be produced in the Belle II detector. They decay basically immediately and because of their high energy typically in a lot of particles. Therefore it’s characteristic for these events to have many tracks. If an event has twelve or more tracks it’s most likely a b-/anti-b-quark event.

The energy of the SuperKEKB accelerator is actually chosen precisely so that the collision energy of the accelerated electron and positron are almost exactly equal to the energy required to produce the first kinds of mesons containing b-quarks (“B mesons”). As a result there’s almost no energy left for movement after the production – the B mesons are almost at rest. Therefore in their following decay there’s no preferred direction for the decay products. Therefore they are distributed evenly “in all directions” and the events are on average less straight.

Other than that there’s a lot more energy available after the production of light quark/antiquark pairs, which is converted into kinetic energy. Accordingly the light quark and antiquark are produced with considerable boost into opposite directions, which is passed on to further decay products. This leads to those events being comparatively straight.

So for events with five to eleven tracks one can differentiate b-/anti-b-quark events those with light quarks, because the former usually has a very low straightness.