Observables for longitudinal flow correlations in heavy-ion collisions

We propose several new observables/correlators, based on correlations between two or more subevents separated in pseudorapidity η, to study the longitudinal flow fluctuations. We show that these observables are sensitive to the event-by-event fluctuations, as a function of η, of the initial condition as well as the nonlinear mode-mixing effects. Experimental measurement of these observables shall provide important new constraints on the boost-variant event-by-event initial conditions required by all 3+1-dimensional viscous hydrodynamics models.