Longitudinal fluctuations of the fireball density in heavy-ion collisions

We show that fluctuations of the fireball shape in the longitudinal direction generate nontrivial rapidity correlations that depend not only on the rapidity difference, y₁-y₂, but also on the rapidity sum, y₁+y₂. This is explicitly demonstrated in a simple wounded nucleon model, and the general case is also discussed. It is argued that rapidity fluctuations are large compared to normal statistical fluctuations. We show how to extract different components of the fluctuating fireball shape from the measured two-particle rapidity correlation function. The experimental possibility of studying the longitudinal initial conditions in heavy-ion and proton-proton collisions is emphasized.