# Initial Stages 2021

Jan 10 – 15, 2021
Weizmann Institute of Science
Asia/Jerusalem timezone
See you at IS2023 in Copenhagen in June 2023

## Probing the nucleus with linearly polarized photons

Jan 13, 2021, 7:25 PM
20m
Andrea's room 3 (vDLCC)

### Andrea's room 3

#### vDLCC

oral Partonic structure of protons and nuclei

### Speaker

Dr James Brandenburg (Brookhaven National Laboratory)

### Description

The intense electromagnetic fields produced by ultra-relativistic heavy nuclei have been proposed as a source of quasi-real photons i.e. in the Weizsacker-Williams equivalent photon method. A photon from one nucleus can fluctuate into a quark antiquark pair and interact directly with the other nucleus to produce a vector meson (e.g. $\rho^0$). Recent STAR measurements of the Breit-Wheeler pair poduction process - $\gamma\gamma \rightarrow e^+e^-$ have demonstrated that the interacting photons are linearly polarized, and that photon polarization induces angular modulations in the final state particle distribution.

In this talk we present STAR measurements of 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. We measure azimuthal angular distributions of the final state $\pi^+\pi^-$ pairs and observe $\cos 2\Delta\phi$ and $\cos 4\Delta\phi$ modulations (where $\Delta\phi = \Delta\phi[ (\pi^++\pi^-), (\pi^+-\pi^-)]$. Theoretical predictions suggest that such modulations may provide new insight into nuclear structure and may shed light on the transverse momentum dependent (TMD) distributions of gluons within large nuclei - a topic of great interest both at existing experiments and at a future Electron Ion Collider.

[1] C. Weizsäcker, Zeitschrift für Physik 88 (1934): 612–25.
[2] H. Xing, C. Zhang, J. Zhou, and Y.-J. Zhou, ArXiv:2006.06206 [Hep-Ph] (2020).
[3] A. Metz, and J. Zhou. Physical Review D 84 5 (2011).

### Primary author

Dr James Brandenburg (Brookhaven National Laboratory)

### Presentation materials

 jdb_initialstages_final.pdf Recording