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

TL;DR

This paper measures the rapidity-even dipolar flow v1 in Pb-Pb collisions at the LHC, revealing its pT dependence and its potential as a probe for jet quenching and initial geometry fluctuations.

Contribution

It provides the first detailed measurement of v1 over a broad pT range, including its behavior at high pT and its relation to momentum conservation and initial geometry.

Findings

v1 crosses zero at ~1 GeV and peaks at 4-5 GeV

High pT v1 exceeds v3 and v2 in central collisions

Momentum conservation involves a subset of the event spanning ~3 units in η

Abstract

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<pT<9 GeV, and centrality (0-50)% in Pb-Pb collisions at sqrt(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 \Delta\phi, of two-particle correlations in azimuthal angle \Delta\phi=\phi_a-\phi_b as a function of pT^a and pT^b. This fit is motivated by the finding that the pT dependence of v_{1,1}(pT^a,pT^b) 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 \eta in central collisions). The extracted v1 is observed to cross zero at pT~1.0 GeV, reaches a maximum at 4-5 GeV with a value comparable to that for v3, and decreases at higher pT. Interestingly, the magnitude of v1 at high pT 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 pT hadrons, making the v1 a valuable probe for studying the jet quenching phenomena.