The directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p ¼ 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 pT between 1.2 and 1:7 GeV=c. Combined with the observation of a vanishing rapidity-even pT shift along the spectator deflection this is strong evidence for dipolelike initial density fluctuations in the overlap zone of the nuclei. Similar trends in the rapidity-even directed flow and the estimate from two-particle correlations at midrapidity, which is larger by about a factor of 40, indicate a weak correlation between fluctuating participant and spectator symmetry planes. These observations open new possibilities for investigation of the initial conditions in heavy-ion collisions with spectator nucleons.
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|Titolo:||Directed flow of charged particles at mid-rapidity relative to the spectator plane in Pb-Pb collisions at √s_NN= 2.76 TeV|
|Data di pubblicazione:||2013|
|Digital Object Identifier (DOI):||10.1103/PhysRevLett.111.232302|
|Appare nelle tipologie:||1.1 Articolo in rivista|