Forward-backward eccentricity and participant-plane angle fluctuations and their influences on longitudinal dynamics of collective flow

We argue that the transverse shape of the fireball created in the heavy-ion collision could be strongly influenced by event-by-event fluctuations of the eccentricity vectors for the forward-going and backward-going wounded nucleons: ε - nF≡εnFeinΦn∗F and ε - nB≡εnBeinΦn∗B. Due to the asymmetric energy deposition of each wounded nucleon along its direction of motion, the eccentricity vector of the produced fireball is expected to interpolate between ε - nF and ε - nB along the pseudorapidity, and hence exhibits sizable forward-backward (FB) asymmetry (εnB≠εnF) and/or FB twist (Φn∗F≠Φn∗B). A transport model calculation shows that these initial-state longitudinal fluctuations for n=2 and 3 survive the collective expansion, and result in similar FB asymmetry and/or a twist in the final-state event-plane angles. These novel event-by-event longitudinal flow fluctuations should be accessible at RHIC and the LHC using the event-shape selection technique proposed in earlier papers. If these effects are observed experimentally, it could improve our understanding of the initial-state fluctuations, particle production, and collective expansion dynamics of the heavy-ion collision.