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[1].
These intense electromagnetic fields have been proposed as a source of linearly-polarized, quasi-real photons[2] that can interact via the Breit-Wheeler process to produce $e^+e^-$ pairs[3]. Demonstration that these photons are linearly polarized provides a precision tool for the study of open questions in Quantum Chromodynamics.
In this talk we present STAR measurements of $e^+e^-$ pair production and diffractive photo-production of the $\rho^0$-meson (and direct $\pi^+\pi^-$ pairs) in ultra-peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The pairs produced in the $\gamma\gamma \rightarrow e^+e^-$ process display a striking 4th-order azimuthal modulation which is a direct result of vacuum birefringence[4,5].
Using the same technique we present measurements of azimuthal modulations in $\pi^+\pi^-$ pairs from diffractive photo-production of the $\rho^0$ and of direct $\pi^+\pi^-$ pairs.
The measured $\pi^+\pi^-$ pairs reveal a similar 4th-order azimuthal modulation. We will discuss the implications of these measurements for the study of gluon transverse momentum dependent (TMD) distributions within nuclei[6,7] at existing experiments and at a future Electron Ion Collider.
[1] V. Skokov, A. Illarionov, and V. Toneev. International Journal of Modern Physics A 24 (2009): 5925–32.
[2] C. Weizsäcker, Zeitschrift für Physik 88 (1934): 612–25.
[3] G. Breit and J. A. Wheeler. Physical Review 46 (1934): 1087
[4] L. Cong, J. Zhou, and Y. Zhou. (2019). arxiv:1903.10084v1
[5] Heisenberg, W., and H. Euler. Zeitschrift für Physik, (1936) arXiv: physics/0605038
[6] J. Collins, and D. Soper. Nuclear Physics B 194 3 (1982): 445–92.
[7] A. Metz, and J. Zhou. Physical Review D 84 5 (2011).
