  Understanding the sources controlling the longitudinal distribution of
produced particles in heavy-ion collisions is crucial to characterize the shape
of the Quark-Gluon Plasma. This study proposes a novel correlator between the
forward-backward asymmetry of spectator numbers and final-state multiplicities
to quantify the contribution from preferential emission to the longitudinal
distribution of produced particles. Using the AMPT model, we demonstrate that
the correlator peaks at large $\eta$, indicating an increased contribution of
preferential emission to particle production towards forward pseudorapidities.
Thus, this correlator provides a direct constraint on the initial state sources
governing rapidity-odd directed flow and flow decorrelation. The correlator
exhibits sensitivity to string fragmentation parameters and spectator matter
dynamics such as fragmentation and evaporation. Our findings motivate an
experimental measurement of this correlator to constrain initial-state sources
determining the longitudinal evolution of the Quark-Gluon Plasma, including
complex dynamics of spectators in heavy-ion collisions.
