DVCS calorimeter in g12?

While improving pi0 detection, DVCS calorimeter reduces acceptance for charged tracks due to its shadow on the DC areas at small Theta_lab angle.


Approximating DVCS calorimeter as a non-transparent circle with R=22cm, the conversion between the distance from the target center to the calorimeter in centimeters and the cut-off on the Theta angle in degrees:
Distance, cm 100 103 113 125 139 150 157 175 179 200 209 225 251
Angle, deg 12.4 12.0 11.0 10.0 9.0 8.3 8.0 7.2 7.0 6.3 6.0 5.6 5.0

Monte Carlo study

Monte Carlo events were generated for the reaction   gamma p -> pi+ pi- pi0 p.
3pi mass was generated as a phase space between 1 and 2 GeV.
To be accepted, all 3 charged tracks should be reconstructed.

Distribution of Theta for accepted events:

Acceptance as a function of the minimum allowed angle Theta for charged tracks:
Proposed setup has DVCS calorimeter at +25cm and target at -100cm. This corresponds to the shadowing angle Theta_min of 10 deg, or a drop in the total acceptance from 0.14 to 0.12, or by about 15%.

Target position and magnetic field

Below is acceptance as a function of 3pi mass for different target positions and magnetic fields:
There is a very sharp drop in acceptance at Z_target=-125cm. The primary reason is the loss of recoil poroton detection. Therefore, drop in acceptance due to DVCS shadow cannot be compensated by moving the target further upstream.

Detecting pi0 instead of a charged track

We don't have yet accepted Monte Carlo events with DVCS calorimeter included. However, simple analysis of the raw Monte Carlo events indicates that there are almost no events in which 1 charged track and both photons from pi0 have Theta<10deg. Therefore, it is unlikely that a loss of a charged track hitting DVCS can be compensted by detecting pi0 instead.