Speaker
Description
Recently, theoretical and experimental studies of flow and multiplicity correlations in the longitudinal direction in heavy ion collision have revealed a rich dynamics not probed by traditional measurements that focused only on the transverse direction. Event-by-event longitudinal fluctuations in the initial conditions are expected to result in a strong asymmetry in particle multiplicities at forward and backward (FB) rapidities. In this talk, FB multiplicity correlations are measured in $pp$, $p$+Pb and Pb+Pb collisions, with a data-driven method utilized to separate long-range FB correlations (LRC) and short-range correlations (SRC). The magnitude of the LRC reveals a significant FB multiplicity asymmetry, quantified by its slope in pseudorapidity $a_1$, which decreases with increasing multiplicity and is similar in magnitude across the three collision systems. The measured correlation in $pp$ collisions is compared in detail with expectations from the PYTHIA 8 and EPOS-LHC models. Both models significantly underpredict the measured $a_1$ values (by a factor of two), and PYTHIA 8 is found to significantly overpredict the magnitude of the SRC, even though both of these models were tuned to reproduce the total charged particle multiplicity distributions out to $N_{ch}$~120. These findings imply that the measurement of FB multiplicity correlations provides supplementary constraints on multi-particle production mechanisms, including restricting the number of independent sources in $pp$, $p$+Pb and Pb+Pb collisions.
| Preferred Track | Correlations and Fluctuations |
|---|---|
| Collaboration | ATLAS |
