Archivio Istituzionale della RicercaThe directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at root s(NN) p 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 p(T) between 1.2 and 1: 7 GeV/c. Combined with the observation of a vanishing rapidity-even p(T) 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.
Directed Flow of Charged Particles at Midrapidity Relative to the Spectator Plane in Pb-Pb Collisions at root s(NN)=2.76TeV / Abelev, B.; Adam, J.; Adamova, D.; Adare, A. M.; Aggarwal, M. M.; Aglieri, G.; Agnello, Michelangelo; Agocs, A. G.; Agostinelli, A.; Ahammed, Z.; Ahmad, N.; Ahmad, A.; Ahmed, I.; Ahn, S. A.; Ahn, S. U.; Aimo, Ilaria; Ajaz, M.; Akindinov, A.; Aleksandrov, D.; Alessandro, B.; Alexandre, D.; Alici, A.; Alkin, A.; Alme, J.; Alt, T.; Altini, V.; Altinpinar, S.; Altsybeev, I.; Andrei, C.; Andronic, A.; Aguelov, V.; Anielski, J.; Anson, C.; Antičić, T.; Antinori, F.; Antonioli, P.; Bufalino, Stefania. - In: PHYSICAL REVIEW LETTERS. - ISSN 0031-9007. - STAMPA. - 111:23(2013), pp. 1-11. [10.1103/PhysRevLett.111.232302]
Directed Flow of Charged Particles at Midrapidity Relative to the Spectator Plane in Pb-Pb Collisions at root s(NN)=2.76TeV
AGNELLO, MICHELANGELO;AIMO, ILARIA;BUFALINO, STEFANIA
2013
Abstract
The directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at root s(NN) p 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 p(T) between 1.2 and 1: 7 GeV/c. Combined with the observation of a vanishing rapidity-even p(T) 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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https://hdl.handle.net/11583/2534493
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