Physics PAN Seminar by Dr. Zhangbu Xu (Brookhaven National Laboratory, USA)

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Date: October 1, 2021
Location: Virtual event
Category: Seminar


Title: How STAR discovered the Breit-Wheeler Process and Vacuum Birefringence

Abstract: In 1934, Breit and Wheeler predicted that collisions of photons could create matter/antimatter, even suggesting doing so by accelerating heavy ions as the only viable approach. Heisenberg, Euler (1936) and Toll (1952) predicted vacuum polarization in magnetic field, and the refractive index depending on the photon polarization. At relativistic velocity, the spiraling magnetic and perpendicular electric fields generated by ultra-relativistic heavy ions, are of equal strength—which is the definition of a photon, a quantized “particle” of light. It provides an ideal physical laboratory for testing strong field QED.

The STAR collaboration investigated if collisions of photons surrounding RHIC’s ions could be uniquely identified as the Breit-Wheeler process. We studied 6085 e+e- pairs produced in UPC collisions. All available kinematic variables of the electron-positron pairs are studied with this high-statistics data sample. By correlating the photons’ momentum, spatial location, and polarization with experimental observables, we assessed that the transverse momentum vectors of photons was driven by the local electromagnetic field, not by Heisenberg’s uncertainty principle randomly determined by the nuclear size. Following a newly proposed method, we discovered a striking fourth-order azimuthal distribution of (16.8+-2.5)%. The high-precision data were consistent with particles being generated by real photon interactions, rather than from highly virtual photons. We experimentally investigated the spatial distribution of the electromagnetic field and the dependence of photon interaction cross section on the polarization angle relative to the magnetic field. The experimental results have implications for vacuum birefringence and for mapping the magnetic field which is important for emergent QCD phenomena.


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Debojit Sarkar


October 2021