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# Directed flow of charged particles at mid-rapidity relative to the spectator plane in Pb-Pb collisions at $$\sqrt{s_\rm{NN}}$$=2.76 TeV

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### Abstract

The directed flow of charged particles at mid-rapidity is measured in Pb-Pb collisions at $$\sqrt{s_\rm{NN}}=2.76$$ 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 $$p_{\rm T}$$ between $$1.2$$ and $$1.7$$ GeV/$$c$$. Combined with the observation of a vanishing rapidity-even $$p_{\rm T}$$ 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.

### Most cited references37

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### The Chiral Magnetic Effect

Topological charge changing transitions can induce chirality in the quark-gluon plasma by the axial anomaly. We study the equilibrium response of the quark-gluon plasma in such a situation to an external magnetic field. To mimic the effect of the topological charge changing transitions we will introduce a chiral chemical potential. We will show that an electromagnetic current is generated along the magnetic field. This is the Chiral Magnetic Effect. We compute the magnitude of this current as a function of magnetic field, chirality, temperature, and baryon chemical potential.
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### KMCLib A general framework for lattice kinetic Monte Carlo (KMC) simulations

(2014)
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### COLLECTIVE FLOW IN HEAVY-ION COLLISIONS

(1997)
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### Author and article information

###### Journal
2013-06-18
2015-02-02
1306.4145
10.1103/PhysRevLett.111.232302

CERN-PH-EP-2013-100
Phys. Rev. Lett. 111 (2013) 232302
17 pages, 4 captioned figures, authors from page 12, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/463
nucl-ex
Alice Collaboration

Nuclear physics