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

The rapidity-even dipolar flow $v_1$ associated with dipole asymmetry of in the initial geometry is measured over broad ranges in $p_T$ ($0.5-9$ GeV) and centrality (0-50%) in Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=2.76$ TeV, recorded by the ATLAS experiment at the LHC. The $v_1$ coefficient is obtained via a two-component fit of the first order Fourier coefficient $v_{1,1}=\langle\cos \Delta\phi \rangle$ of two particle correlation in relative azimuthal angle $\Delta\phi=\phi_{\mathrm{a}}-\phi_{\mathrm{b}}$ as a function of $p_T^{\mathrm a}$ and $p_T^{\mathrm b}$. This fit is motivated by the finding that the $p_T$ dependence of the $v_{1,1}(p_T^{\mathrm a},p_T^{\mathrm b})$ data are consistent with the combined contributions from a rapidity-even $v_1$ and global momentum conservation. The extracted $v_1$ is observed to cross zero at $p_T\approx1.0$ GeV, reaches a maximum at 4--5 GeV with a value comparable to that for $v_3$, and decreases at higher $p_T$. Interestingly, the magnitude of $v_1$ at high $p_T$ exceeds the value of the $v_3$ in all centrality interval and exceeds the value of $v_2$ in central collisions. This behavior suggest 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 $v_1$ an value probe for studying the jet quenching phenomena.