Directed flow of charged particles at mid-rapidity relative to the spectator plane in Pb–Pb collisions at √ sNN = 2.76 TeV

Abelev, B. and Adam, J. and Adamová, D. and Adare, A. M. and Aggarwal, M. M. and Aglieri, Rinella G. and Agnello, M. and Agócs, András and Agostinelli, A. and Ahammed, Z. and Ahmad, N. and Ahmad, Masoodi A. and Ahmed, I. and Ahn, S. A. and Ahn, S. U. and Aimo, I. and Ajaz, M. and Akindinov, A. and Aleksandrov, D. and Alessandro, B. and Alexandre, D. and Alici, A. and Alkin, A. and Alme, J. and Alt, T. and Altini, V. and Altinpinar, S. and Altsybeev, I. and Andrei, C. and Andronic, A. and Barnaföldi, Gergely Gábor and Bencédi, Gyula and Berényi, Dániel and Boldizsár, László and Dénes, Ervin and Hamar, Gergő and Kiss, Gábor and Lévai, Péter and Molnár, Levente and Oláh, László and Pochybova, Sona (2013) Directed flow of charged particles at mid-rapidity relative to the spectator plane in Pb–Pb collisions at √ sNN = 2.76 TeV. PHYSICAL REVIEW LETTERS, 111 (23). ISSN 0031-9007

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Abstract

The directed flow of charged particles at mid-rapidity 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 dipole-like 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 mid-rapidity, 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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