Polarization issues in tau-charm factories

This study explored several options for organizing the longitudinal spin orientation of a polarized electron beam at its collision point with positrons in the currently accepted geometry of super-tau charm factory of the University of Science and Technology of China (USTC) in Hefei. Modifications to the colliders optics were considered, using either one or three spin rotators of Siberian snake type. A variant with two partial Siberian snakes with a half-strength longitudinal magnetic field was also studied. The best polarization lifetime, achieved by injecting a linearly accelerated beam of polarized electrons from a gun with a polarized photocathode with an initial polarization degree greater than 90% into the collider ring, is achieved with a variant with three Siberian snakes. In the range from 1 to 2.5 GeV, this time, taking into account the depolarizing effect of damping wigglers, smoothly decreases from 1600 to 300 seconds. However, even with shorter polarization lifetimes, it is possible to maintain a high average polarization level, say above 80%, by frequently completely replacing spent bunches with fresh ones with a high initial degree of polarization. This approach is envisioned in the STCF project. Technically less complex and cheaper options with a single Siberian snake or two partial snakes provide acceptable polarization lifetimes in the energy range from 1 to 1.7 GeV. The paper also discusses the technical possibility of frequent particle spin flips, thereby creating equal background conditions when collecting statistics with a change in the sign of the spin projection onto the collision axis.