Above is a diagram showing a possible decay process. The protons collide and produce a pair of B mesons (labelled Bs). These decay to charged particles (p,K) which can be detected by the VELO. The decay positions of the Bs can be inferred from tracking back the traces of these particles to vertices
On the right one of 42 VELO modules is being inspected. The LHC beam passes through the semicircular cutout at the top of the module.