Beam-beam interaction with a horizontal crossing angle

Abstract

We report measurements of the dependence of luminosity and beam-beam tune-shift parameter on horizontal crossing angle at a single interaction point in the Cornell Electron Storage Ring. The report is based on data collected between September 1991 and January 1992 at CESR. For head-on collisions (zero crossing angle) the achieved tune-shift parameter is ξ v = 0.03 ± 0.002 at 11 mA/bunch. For a crossing half-angle of θ c = ± 2.4 mrad, we achieve ξ v = 0.024 ± 0.002 at similar bunch currents. The data suggest some degradation of performance if the trajectory through the interaction region is distorted magnetically even while head-on collisions are preserved. Therefore at least some of the observed dependence of tune-shift parameter on crossing angle may be due to the associated large displacement of the beam trajectories in the interaction region optics. Furthermore, with the introduction of the crossing angle, the algorithm for optimizing luminosity is significantly complicated due to linear optical errors and the solenoid compensation. We interpret the measured tune-shift parameter at θ c = 2.4 mrad as a lower limit to what can ultimately be achieved.