1. M. Sands, Conf. Proc. 257–411 (1969)
2. A. Abada, M. Abbrescia, S. S. AbdusSalam, et al., M. Benedikt et al. (eds.), Eur. Phys. J. Spec. Top. 228 (2019)
3. The CEPC Study Group, Institute of High Energy Physics, Chinese Academy of Sciences, arXiv:1809.00285 [physics.acc-ph] (2018)
4. G. Aad et al., ATLAS Collaboration, CMS Collaboration. Phys. Rev. Lett. 114 (2018)
5. K. Yokoya, KEK Accelerator Seminar 15, 2012 (2012)