# Observables for longitudinal flow correlations in heavy-ion collisions

## TL;DR

This paper introduces new observables based on subevent correlations in pseudorapidity to study longitudinal flow fluctuations in heavy-ion collisions, providing constraints for initial condition models in 3+1D hydrodynamics.

## Contribution

It proposes novel correlation observables sensitive to longitudinal flow fluctuations and mode-mixing, aiding the understanding of initial conditions in heavy-ion collision modeling.

## Key findings

- Observables are sensitive to event-by-event fluctuations in $ ext{η}$.
- Measurement constrains boost-variant initial conditions.
- Enhances understanding of non-linear mode-mixing effects.

## Abstract

We propose several new observables/correlators, based on correlations between two or more subevents separated in pseudorapidity $\eta$, to study the longitudinal flow fluctuations. We show that these observables are sensitive to the event-by-event fluctuations, as a function of $\eta$, of the initial condition as well as the non-linear 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.