Directed flow at midrapidity in sqrt[S(NN)] = 2.76 TeV Pb + Pb collisions

Area of Science:

• Nuclear Physics
• High-Energy Physics
• Quantum Chromodynamics

Background:

• Directed flow (v(1)) is a key observable in heavy-ion collisions, sensitive to the initial state geometry and system dynamics.
• Previous analyses focused on higher-order flow harmonics, leaving v(1) less explored.
• The ALICE Collaboration has provided crucial data for studying these phenomena.

Purpose of the Study:

• To perform the first extraction of the rapidity-even directed flow observable v(1).
• To investigate the sensitivity of directed flow to the viscosity of the system.
• To constrain the non-equilibrium dynamics of the early-time system in heavy-ion collisions.

Main Methods:

• Analysis of published data from the ALICE Collaboration.
• Accounting for transverse momentum conservation to isolate the directed flow correlation.
• First viscous hydrodynamic calculations of directed flow.

Main Results:

• Successful extraction of the rapidity-even directed flow observable v(1).
• Demonstration that directed flow is less sensitive to viscosity than higher harmonics.
• Direct extraction of the dipole asymmetry of the initial state.

Conclusions:

• The extracted v(1) provides a reliable measurement of directed flow.
• Directed flow offers a unique probe of the initial state dipole asymmetry.
• This measurement imposes a strict constraint on the non-equilibrium dynamics of the early-time system.