Lighting up the Photon Wigner Distribution via Dilepton Productions

Abstract

We present a systematic investigation of lepton pair production through photon-photon fusion processes in heavy-ion collisions. It is demonstrated that the dilepton production at a given impact parameter ($b_{\perp }$) with a fixed transverse momentum imbalance ($q_{\perp }$) can be factorized into a unified formula in terms of the Wigner photon distribution of heavy nuclei. We show that this framework provides a comprehensive description of all the relevant data from RHIC to the LHC, with a strong evidence that the quasi-real photon can be radiated not only from the nucleus as a whole, standing for the coherent contribution, but also from the sub-structures inside the nucleus, representing the incoherent contribution. Further predictions are made for the anisotropies in the correlations between $q_{\perp }$, $b_{\perp }$, and the dilepton transverse momentum ($P_{\perp }$). This will help us to constrain the photon Wigner distribution which plays a crucial role to study the gluonic matter of nucleus at small-$x$ through the diffractive photoproduction processes in heavy ion collision.

9 pages (main text) plus 18 pages (supplemental material), 25 figures; Version to be published in PLB