Momentum Anisotropy of Leptons from Two-Photon Processes in Heavy-Ion Collisions

We investigate the azimuthal angular correlation between the lepton transverse momentum $P_{\perp }$ and the impact parameter $b_{\perp }$ in noncentral heavy-ion collisions, where the leptons are produced through two-photon scattering. Among the Fourier harmonic coefficients, a significant $v_4$ asymmetry is found for the typical kinematics at RHIC and LHC with a mild dependence on the $P_{\perp }$, whereas $v_2$ is power suppressed by the lepton mass over $P_{\perp }$. This unique prediction, if confirmed from the experiments, shall provide crucial information on the production mechanism for the dilepton in two-photon processes.

Figure 1

Illustration of the polarized photon flux associated with a relativistic heavy nucleus moving to the right. The physical polarization of the photon propagating to the right is along the direction of ${\overrightarrow{b}}_{1\perp }$ with respect to the center of the nucleus in the transverse plane.

Figure 2

Estimates of the $\cos 4\varphi$ asymmetry as the function of $b_{\perp }$ in Au-Au collisions at $\sqrt{s}=200\mathrm{GeV}$ (upper plot) and in Pb-Pb collisions at $\sqrt{s}=5.02\mathrm{TeV}$ (lower plot). The dilepton rapidities are integrated over the regions $[-1,1]$.

Figure 3

The asymmetries are plotted as the function of the lepton’s transverse momentum.