Directed flow of charged particles at mid-rapidity relative to the spectator plane in Pb-Pb collisions at $\sqrt{s_\rm{NN}}$=2.76 TeV

Understand

The directed flow of charged particles at mid-rapidity is measured in Pb-Pb collisions at $\sqrt{s_\rm{NN}}=2.76$ TeV relative to the collision symmetry plane defined by the spectator nucleons.

• A negative slope of the rapidity-odd directed flow component with approximately 3 times smaller magnitude than found at the highest RHIC energy is observed.
• This suggests a smaller longitudinal tilt of the initial system and disfavors the strong fireball rotation predicted for the LHC energies.
• The rapidity-even directed flow component is measured for the first time with spectators and found to be independent of pseudorapidity with a sign change at transverse momenta $p_{\rm T}$ between $1.2$ and $1.7$ GeV/$c$.