T. Horn
August, 2006
A stand-alone Monte Carlo has been used to investigate the properties of the SHMS design. The Monte Carlo was run with the spectrometer at 90 degrees and 10cm, 20cm, and 30cm target lengths. Electrons were generated uniformly over the phase space and sent through the spectrometer with a central momentum set to 8 GeV/c. Events were generated over the δ range of -10% to +22%, horizontal angle (YPtar=dy/dz) range of +- 80mrad and vertical angle (XPtar=dx/dz) range of +- 80mrad relative to the spectrometer axis. Studies were done with the SHMS tune including the horizontal bend magnet.
Figure 1 shows the δ dependence of the acceptance after the quadrupoles, and the Ytar, YPtar and XPtar acceptance. Events in the δ region shown fall inside all of the nominal detector apertures. The red lines denote the acceptance determined by the nominal detector configuration. The acceptance in Ytar is relatively flat. The vertical lines indicate different target lengths.
The effective solid angle was calculated by generating 100k events over a range of the horizontal angle of XPtar=+-0.08, vertical angle YPtar=+-0.08, -10% < δ < +22%, and -15cm < Ytar < +15cm. The effective solid angle, Ω, was calculated as
Ω = (N_acc/N_gen) * XPtar * YPtar
The solid angle was determined for each target length. The solid angle for the SHMS including the HB is > 4.5 msr.
Figure 1: SHMS acceptance including the HB. Black=50-cm target, blue=30-cm target, green=20cm-target.
In order to test the influence of the calorimeter, the size of the calorimeter was changed by +- 10cm. The effect on the solid angle is small (~0.6%). Changing the vertical location (and size) of the calorimeter, affects the momentum acceptance is clearly affected, but there is no gain in solid angle. The current size and location of the calorimeter have already been optimized.
Figure 2: SHMS acceptance with a shifted calorimeter.
