Directed flow of charged particles at midrapidity relative to the spectator plane in Pb-Pb collisions at root s(NN)=2.76TeV
Alme, Johan; Erdal, Hege Austrheim; Helstrup, Håvard; Hetland, Kristin Fanebust; Kileng, Bjarte; Altinpinar, Sedat; Djuvsland, Øystein; Fehlker, Dominik; Haaland, Øystein Senneset; Huang, Meidana; Langøy, Rune; Lien, Jørgen; Lønne, Per-Ivar; Nystrand, Joakim; Rehman, Attiq ur; Røed, Ketil; Røhrich, Dieter; Skjerdal, Kyrre; Ullaland, Kjetil; Wagner, Boris; Yang, Shiming; Bätzing, Paul Christoph; Dordic, Olja; Eyyubova, Gyulnara; Kværnø, Henning; Lindal, Svein; Løvhøiden, Gunnar; Milosevic, Jovan; Nilsson, Mads Stormo; Qvigstad, Henrik; Richter, Matthias; Skaali, Toralf Bernhard; Tveter, Trine Spedstad; Wikne, Jon Christopher; Abelev, Betty; Adam, Jaroslav; Adamová, Dagmar; Adare, Andrew Marshall; Aggarwal, Madan M.; Aglieri Rinella, Gianluca; Agocs, Andreas Gabor; Agostinelli, Andrea; Ahammed, Zubayer; Ahmad, Nazeer; Ahmad Masoodi, Arshar; Ahn, Sang Un; Ajaz, Muhammad; Akindinov, Alexander; Aleksandrov, Dimitry; Alessandro, Bruno; ALICE, Collaboration
Journal article, Peer reviewed
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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.