Speaker
Description
The strongly intensive quantity $\Sigma$ is a new observable, introduced recently to the domain of heavy-ion physics. In superposition models which assume independent particle production from statistically identical sources, $\Sigma$ is insensitive to the number of sources and its fluctuations, contrary to the standard forward-backward correlation coefficient ($b_{\rm corr}$). Therefore, it provides direct information on the multiplicity correlations and fluctuations from a single source.
This talk presents new results on forward-backward correlations studied with the quantity $\Sigma$, measured by ALICE at the LHC in Xe--Xe reactions at $\sqrt{s_{\rm NN}}=5.44$~TeV and in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ and $5.02$ TeV.
The data are shown as a function of the gap between forward and backward pseudorapidity ($\eta$) intervals and the centrality of the collision. The study is made with two independent centrality estimators. An opposite trend of the values of $\Sigma$ as a function of centrality class in Pb--Pb and Xe--Xe collisions is observed for the experimental data and MC HIJING simulations. This nontrivial discrepancy implies that the physical mechanism of particle production differs from that predicted by the models.
