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

Submission Date: 
18/06/2013
Article Information
Submission Form
System: 
Pb-Pb
Energy: 
2.76 TeV
Abstract Plain Text: 

The directed flow of charged particles at mid-rapidity was measured in Pb-Pb collisions at sNN= 2.76 TeV relative to the collision plane defined by the spectator nucleons. The observed negative slope of the rapidity-odd directed flow component with about a three times smaller magnitude than observed at the highest RHIC energy suggests a smaller longitudinal tilt of initial system and disfavors a picture of strong fireball rotation predicted at the LHC energies. Measured for a first time with spectators, the rapidity-even directed flow component is found to be independent of pseudorapidity and change sign at transverse momenta around 1.2-1.7 GeV/c. Combined with an observation of a vanishing rapidity-even transverse momentum shift along the spectator deflection
this forms a strong evidence for dipole-like initial density fluctuations in the overlap zone of the nuclei. Similar trends and a factor of forty smaller magnitude of the rapidity-even directed flow measured relative to the spectator plane and that previously estimated from correlation of two particles emitted at midrapidity
indicates a weak correlation between fluctuating participant and spectator collision symmetry planes. These observations open a new direction for experimental probes of the initial conditions in heavy-ion collision with spectator nucleons.