Recent observations made by the LHC experiments in proton-lead and high multiplicity proton-proton events are reminiscent of the collective, hydrodynamic-like behaviour observed in lead-lead collisions, but results have not been conclusive. A recent study by the ALICE experiment provides a window on such collective behaviour through a deeper insight into the size of the final state at “freeze-out” – that is, when the hadrons involved stop interacting. The measurements, which examine pion correlations, are based on a technique invented by Robert Hanbury Brown and Richard Twiss in 1956 to measure the angular dimensions of stars.

Read more: "Heavy-ion collisions: where size matters" – CERN Courier