Directed Flow of Charged Particles at Midrapidity Relative to the Spectator Plane in Pb-Pb Collisions at root s(NN)=2.76TeV

TY - JOUR

T1 - Directed Flow of Charged Particles at Midrapidity Relative to the Spectator Plane in Pb-Pb Collisions at root s(NN)=2.76TeV

AU - Abelev, B.

AU - Adam, J.

AU - Adamova, D.

AU - Adare, A.M.

AU - Aggarwal, M.M.

AU - Bearden, Ian

AU - Bøggild, Hans

AU - Christensen, Christian Holm

AU - Bilandzic, Ante

AU - Dalsgaard, Hans Hjersing

AU - Gaardhøje, Jens Jørgen

AU - Gulbrandsen, Kristjan Herlache

AU - Hansen, Alexander Colliander

AU - Nielsen, Børge Svane

AU - Nygaard, Casper

AU - Søgaard, Carsten

PY - 2013/12/6

Y1 - 2013/12/6

N2 - The directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at √sNN=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.

AB - The directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at √sNN=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.

U2 - 10.1103/PhysRevLett.111.232302

DO - 10.1103/PhysRevLett.111.232302

M3 - Journal article

C2 - 24476260

SN - 0031-9007

VL - 111

JO - Physical Review Letters

JF - Physical Review Letters

IS - 23

M1 - 232302

ER -