Global polarization of Λ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at √ s N N = 200 GeV, with no significant difference between Λ andΛ.These new results reveal the collision energy dependence of the global polarization together with the results previously observed at √ s N N = 7.7 -62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from the AMPT (A Multi-Phase Transport) model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity η H within |η H | < 1. An indication of the polarization dependence on the event-by-event charge asymmetry 3 is observed at the 2σ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field. PACS numbers: 25.75.-q, 25.75.Ld

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We calculate the cross section and transverse-momentum (P ⊥ ) distribution of the Breit-Wheeler process in relativistic heavy-ion collisions and their dependence on collision impact parameter (b). To accomplish this, the Equivalent Photon Approximation (EPA) was generalized in a more differential way compared to the approach traditionally used for inclusive collisions. In addition, a lowest-order QED calculation with straightline assumption was performed as a standard baseline for comparison. The cross section as a function of b is consistent with previous calculations using the equivalent one-photon distribution function. Most importantly, the P ⊥ shape from this model is strongly dependent on impact parameter and can quantitatively explain the P ⊥ broadening observed recently by RHIC and LHC experiments. This broadening effect from the initial QED field strength should be considered in studying possible trapped magnetic field and multiple scattering in a Quark-Gluon Plasma (QGP). The impact-parameter sensitive observable also provides a controllable tool for studying extreme electromagnetic fields.