PROGRESS OF CONCEPTUAL STUDY FOR THE ACCELERATORS OF A 2-7 G E V SUPER TAU CHARM FACILITY AT CHINA

This paper shows the progress of the conceptual study for the accelerators of a super tau charm facility in China. Since the BEPCII will finish its historical mission in 5~10 years and its upgrade plan will only achieve a luminosity gain of 3~5 times as it is now, a new next generation taucharm collider will play an irreplaceable role in future high energy physics study. The luminosity of this successor is about 5×1034 cm−2s−1 pilot and 1×1035 cm−2s−1 nominal, 100 times as BEPCII, also with the electron beam longitudinally polarized at the IP. The general scheme of the accelerators and the beam parameters are shown. INTRODUCTIONS Bejing Electron Positron Collider II (BEPC II), the most successful tau-charm factory of the world in operation, reached its design goal of luminosity of 1×1033cm-2s-1 in the year 2016. Although IHEP is now planning to upgrade the collider to large crossing angle with crabbed waist collision, the luminosity enhancement of the future upgrade project will be 3~5 times as it is now, due to the limited final focus section space and small total circumference. We believe that BEPCII would finish its historical mission in the next decade. The very ambitious CEPC-SPPC proposal, which will cost a price of several orders higher than a tau charm factory and a study and construction period of more than 15 years, will be a long-term plan that requires global cooperation. As a transitional choice before the construction of CEPC, a new tau charm collider facility was proposed [1] as BEPCII’s successor. It would also be a good backup plan if the CEPC-SPPC construction cannot begin on time as planned. The new super tau charm facility was first named as High Intensity Electron Positron Accelerator (HIEPA) due to the intention to combine the collider and a 3rd /4th generation synchrotron radiation light source together [2]. Then the accelerator physicists realized it is very hard to achieve a perfect performance for different users respectively, because of the strong nonlinear effect and limited lifetime of the beam. After that we make a decision that the new facility will be a collider only. This new collider, operating in the range of center-of-mass energies from 2 to 7 GeV, will have a luminosity of 5×1034 cm−2s−1 for pilot operation, and a luminosity of 1×1035cm-2s-1 with longitudinal polarized electron beam at the collision point for nominal operation. The conceptual study of the STCF is now listed in future plan of the Hefei Comprehensive National Science Center, supported by the Collaborative Innovation Center for Particles and Interactions (CICPI, USTC) and the accelerator division of the National Synchrotron Radiation Laboratory (NSRL, USTC). At this stage, many common technologies which are useful for both CEPC-SPPC and tau charm factory will be developed and a strong team of scientists will be trained. THE EVOLUTION OF THE GENERAL SCHEME AND BEAM PARAMETERS The first STCF idea in the year 2015 was a dual-purpose facility that compatible with a 3rd/4th generation light source [1]. But this idea is not feasible for three reasons. First, the nonlinear effects from the final focus section of the colliders and the arc sections of the ring light sources are both very strong, therefore the dynamic aperture will be too small. Second, the small bunches of the 4th generation light source will induce ultra-strong collective effects, and then degrade the whole performance of the facility. Finally, the beam lifetime of the super tau charm factory is about the order of magnitude of 1000 seconds, unable to meet the need of stability that the light source users requires. Figure 1: General sketch of the STCF accelerators. Figure 1 shows the sketch of the STCF accelerators. The new facility will be a dual-ring collider with symmetric and flat beams. In Fig. 1 the two rings are not identical only because we want to show there should be snakes in the electron ring. Last year we reported that the whole construction of the Chinese STCF will be divided into three stages: the pilot, ____________________________________________ * Work supported by National Natural Science Foundation of China U1832169 and the Double Fist-Class University Project Foundation of USTC † Corresponding Author Email address: luoqing@ustc.edu.cn 10th Int. Partile Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOPRB031 MC1: Circular and Linear Colliders A24 Accelerators and Storage Rings, Other MOPRB031 643 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I