Scaling Behavior of Circular Colliders Dominated by Synchrotron Radiation

Abstract

The quite low Higgs particle mass makes it natural for the next high energy facility to be a circular e+e- Higgs factory and, after that, a next-generation p,p collider in the same tunnel. Surveying the luminosity-limiting phenomena of synchrotron radiation power loss, beam-beam interaction limitations, and beamstrahlung, scaling laws are established that fix all parameters of the Higgs factory, as functions of assumed radius $r$, and RF power $P$. at least to a first approximation. Historically the accelerator formalisms of electron and hadron rings have been distinguished largely by the importance of synchrotron radiation for electrons, and its unimportance for protons. While electron beams equilibrate within seconds, proton beam distributions have survived largely intact for extended periods. For future hadron colliders, this distinction will no longer be valid. This will have a large impact on the design of the future FCC-pp proton collider whose parameters can be extrapolated using formulas previously applicable only to electron rings. The most important scaling law has the luminosity depending on $r$ and $P_{\rm rf}$ only as a function of their product $rP$. This relation simplifies choosing a tunnel radius that is optimal for both Higgs factory and p,p collider while (almost) minimizing the initial cost---by increasing $r$ and decreasing $P$ proportionally, the tunnel circumference can be increased (as required for p,p) without significant increase in Higgs factory cost.