Measurement of the rapidity-even dipolar flow in Pb-Pb collisions with the ATLAS detector

The rapidity-even dipolar flow v₁ associated with dipole asymmetry in the initial geometry is measured over a broad range in transverse momentum 0.5 GeV < p_T < 9 GeV, and centrality (0-50)% in Pb-Pb collisions at √s_NN = 2.76 TeV, recorded by the ATLAS experiment at the LHC. The v₁ coefficient is determined via a two-component fit of the first order Fourier coefficient v_1,1 = 〈cos Δφ〉 of two-particle correlations in azimuthal angle Δφ = φ_a - φ_b as a function of p^a _T and p^b_T. This fit is motivated by the finding that the p_T dependence of v_1,1(p^a _T,p^b_T) data are consistent with the combined contributions from a rapidity-even v₁ and global momentum conservation. The magnitude of the extracted momentum conservation component suggests that the system conserving momentum involves only a subset of the event (spanning about 3 units in η in central collisions). The extracted v ₁ is observed to cross zero at p_T ≈ 1.0 GeV, reaches a maximum at 4-5 GeV with a value comparable to that for v₃ and decreases at higher p_T. Interestingly, the magnitude of v₁ at high p_T exceeds the value of the v₃ in all centrality interval and exceeds the value of v₂ in central collisions. This behavior suggests that the path-length dependence of energy loss and initial dipole asymmetry from fluctuations corroborate to produce a large dipolar anisotropy for high p_T hadrons, making the v₁ a valuable probe for studying the jet quenching phenomena.