Speaker
Description
Ultra-relativistic heavy-ion collisions are expected to produce some of the strongest magnetic fields ($10^{13}$ $-$ $10^{16}$ Tesla) in the Universe. The initial strong electromagnetic fields have been proposed as a source of linearly-polarized, quasi-real photons that can interact via the Breit-Wheeler process to produce $e^{+}e^{-}$ pairs.
In this talk, we will present latest STAR measurements of $e^{+}e^{-}$ pair production in ultra-peripheral and peripheral Au+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV. A comprehensive study of the pair kinematics is presented to distinguish the $\gamma\gamma\rightarrow e^{+}e^{-}$ process from other possible production mechanisms. Furthermore, we will present and discuss the observation of a 4th-order azimuthal modulation of $e^{+}e^{-}$ pairs produced in the Breit-Wheeler process. The striking 4th-order angular modulation is a direct result of vacuum birefringence, a phenomenon predicted in 1936 that empty space can split light according to its polarization components when subjected to a strong magnetic field. Their implications for the properties of the magnetic filed produced in heavy-ion collisions will be discussed.
| speaker affiliation | for STAR Collaboration |
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