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.
Geometry
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.
