Forward-backward multiplicity fluctuation and longitudinal harmonics in high-energy nuclear collisions

Forward-backward (FB) multiplicity fluctuation in high-energy nuclear collisions can be quantified by two-particle pseudo-rapidity correlation function and its expansion into Legendre polynomials. The corresponding coefficients represent different fluctuation modes in longitudinal direction. The leading term 〈a₁ ²〉 corresponds to the asymmetry of numbers of the participants from the two colliding nuclei. This method is tested in events generated from AMPT and HIJING model. The a_n signal are found to be strongly dampened in AMPT than in HIJIGN, due to weaker short-range correlaitons and final-state effects in AMPT. Two-particle correlation also reveals an intresting shallow minimum around Δη≈0 in AMPT events, which is absent in HIJING results. The method opens a new avenue to elucidate the particle production mechanism and early time dynamics in heavy-ion collisions.