Forward–backward rapidity correlations in a two-step scenario

We argue that two-step models, like String and Glasma models, with creation first of sources extended in rapidity that after locally decay into particles, lead to long range forward–backward correlations due to fluctuations in the number or the colour of the sources, and to a similar expression for the F–BF–B correlation parameter b. In the simplest String percolation scenario, b increases with centrality or the number of participating nucleons. However, asymptotically it decreases with energy. This is different from the Glasma model where b increases with both, centrality and energy.