Beam Crossing Angle

The layout of the interaction region is shown in Fig.   [4]. The beam crossing angle of $\pm$11 mr allows us to fill all RF buckets with the beam and still avoid parasitic collisions, thus permitting higher luminosity. Another important merit of the large crossing-angle scheme is that it eliminates the need for the separation-bend magnets, significantly reducing beam-related backgrounds in the detector. The risk associated with this choice of a non-zero crossing angle is the possibility of luminosity loss caused by the excitation of synchro-beta resonances [5].

Figure: Layout of the interaction region for the beam crossing angle of $\pm$ 11 mr.

The low energy beam line ($e^+$) is aligned with the axis of the detector solenoid since the lower-momentum beam particles would suffer more bending in the solenoid field if they were off-axis. This results in a 22 mr angle between the high energy beam line ($e^-$) and the solenoid axis.