The particle surface of spinning test particles Yong Song, Yiting Cen, Leilei Tang, Jiabao Hu, Kai Diao, Xiaofeng Zhao and Shunping Shi Eur. Phys. J. C, 83 9 (2023) 833 Published online: 21 September 2023 DOI: 10.1140/epjc/s10052-023-11970-5