Correlation between the sum and the difference of times recorded by the neutron ZDCs on either side (ZNA and ZNC) in Pb-Pb collisions. The large cluster in the middle corresponds to collisions between ions in the nominal RF bucket on both sides, while the small clusters along the diagonals (spaced by 2.5 ns in the time difference) correspond to collisions in which one of the ions is displaced by one or more RF buckets.