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Brookhaven National Laboratory, Upton, New York 11973.
AGH University of Science and Technology, FPACS, Cracow 30-059, Poland.
Phys Rev Lett. 2021 Jul 30;127(5):052302. doi: 10.1103/PhysRevLett.127.052302.
The Breit-Wheeler process which produces matter and antimatter from photon collisions is experimentally investigated through the observation of 6085 exclusive electron-positron pairs in ultraperipheral Au+Au collisions at sqrt[s_{NN}]=200 GeV. The measurements reveal a large fourth-order angular modulation of cos4Δϕ=(16.8±2.5)% and smooth invariant mass distribution absent of vector mesons (ϕ, ω, and ρ) at the experimental limit of ≤0.2% of the observed yields. The differential cross section as a function of e^{+}e^{-} pair transverse momentum P_{⊥} peaks at low value with sqrt[⟨P_{⊥}^{2}⟩]=38.1±0.9 MeV and displays a significant centrality dependence. These features are consistent with QED calculations for the collision of linearly polarized photons quantized from the extremely strong electromagnetic fields generated by the highly charged Au nuclei at ultrarelativistic speed. The experimental results have implications for vacuum birefringence and for mapping the magnetic field which is important for emergent QCD phenomena.
通过在质心能量(\sqrt{s_{NN}} = 200) GeV的超外围金金碰撞中观测6085对排他性电子 - 正电子对，对由光子碰撞产生物质和反物质的 Breit-Wheeler 过程进行了实验研究。测量结果揭示了(\cos^4\Delta\phi=(16.8\pm2.5)%)的大四阶角调制，并且在实验极限（观测产额的(\leq0.2%)）下，矢量介子（(\phi)、(\omega)和(\rho)）不存在，不变质量分布平滑。作为正电子 - 电子对横向动量(P_{\perp})函数的微分截面在低值处达到峰值，(\sqrt{\langle P_{\perp}^{2}\rangle}=38.1\pm0.9) MeV，并显示出显著的中心度依赖性。这些特征与量子电动力学计算结果一致，该计算针对由超相对论速度下高电荷金核产生的极强电磁场量子化的线偏振光子的碰撞。实验结果对真空双折射以及绘制对新兴量子色动力学现象很重要的磁场具有重要意义。
