Effect of the Drift Chamber Outer Cylinder On the DIRC Resolution

It has been proposed that the outer cylinder of the drift chamber be changed from 1.6 mm carbon fiber (0.6% X0) to 2.5 mm aluminum (2.5% X0). While such a change would have only a slight effect on the barrel CsI calorimeter performance, the question has been raised as to how it might affect the DIRC. Multiple scattering in the outer cylinder would cause as uncertainty in the entrance angle and position of the particle at a DIRC bar. However, the DIRC is remarkably robust against uncertainties of this type. A detailed Monte Carlo simulation indicates that even 1 X0 of aluminum (10 cm) would cause only a 15% reduction in the significance of π/K separation at 4 GeV and cos θ = 0.85. In fact, if the drift chamber were solid aluminum the resolution of the DIRC would deteriorate only by a factor of two.