Fiducial Volume cut for photons:

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We have several cases which need to be distinguished

  • MC and DATA
  • photons starting there shower in the Pre-shower or in Calorimeter


For all the cases above the photon acceptance is limited by the front and rear field clamps, which define the opening of the spectrometer magnet. Here are some [picture] of the field clamp.
General philosophy the most important is that the photon

  • is not a conversion photon from the magnet edges.
  • to take the extended target into account use the zvx position from the lepton. Yes, it is a simplification but there are very few photons, which come from particles, which have a decay length
  • For the Pre-shower the photon needs to be only in a volume, which guarantees the first requirement.
  • For the calorimeter the photon must come from a volume, which guarantees the first requirement, but in addition at least on block must be available in addition to guarantee the shower is contained.

--> there for the cuts need to be applied to the surface of the detectors.

MC Case :

The MC case is easy, because it has a perfect geometry and we can take it from the geometry file.

Front field clamp: 

    z= 172.0 cm
   
           <         63.0cm          >
           --------------------------- 32.2 cm
           |                         |
           |                         |
           |                         |
           --------------------------- 6.8 cm
                        O e+


Rear Field clamp: 

   z= 383.0 cm
    
           <         201.0cm          >
           --------------------------- 54.5 cm
           |                         |
           |                         |
           |                         |
           --------------------------- 5.5 cm
                        O e+
  • Calorimeter z-position in MC: 720 cm; Calorimeter y-dist from beam line in MC: 24cm, height: 90cm; total width in x: 378cm
    • lower acceptable y=position: 31cm; upper acceptable y-position: 105cm; +/- x-cut: <180cm
  • Pre-Shower in the MC: the scintillator is at 708.8cm with a thickness of 1cm; the lead is 1.38 cm thick and it's surface is 702.77cm the distance lead-scintillator is 4.65cm; total height: 91cm; Preshower y-dist from beam line in MC: 22.15cm total width in x: 369.288 cm
Front clamp limits on photons showering in calorimeter:
ZVX lower y-cut upper y-cut +/-x-cut
-20 cm 26.21 124.10 121.21
0 cm 28.47 134.79 131.88
+20 cm 31.32 148.29 145.07
Rear clamp limits on photons showering in calorimeter:
ZVX lower y-cut upper y-cut +/-x-cut
-20 cm 10.10 100.07 184.54
0 cm 10.34 102.45 188.93
+20 cm 10.61 105.10 193.80
Front clamp limits on photons showering in Pre-Shower:
ZVX lower y-cut upper y-cut +/-x-cut
-20 cm 25.60 121.21 118.58
0 cm 27.78 131.57 128.70
+20 cm 30.54 144.64 141.50
Rear clamp limits on photons showering in Pre-Shower:
ZVX lower y-cut upper y-cut +/-x-cut
-20 cm 9.86 97.74 180.24
0 cm 10.09 100.00 184.41
+20 cm 10.35 102.51 189.03


This leads to the following fiducial volume cuts:

Pre-Shower:
ZVX lower y-cut upper y-cut +/-x-cut
-20 cm 31.00 97.74 118.58
0 cm 31.00 100.00 128.70
+20 cm 31.00 102.51 141.50
Calorimeter:
ZVX lower y-cut upper y-cut +/-x-cut
-20 cm 31.00 100.07 121.41
0 cm 31.00 102.45 131.86
+20 cm 31.00 105.10 145.07

Detector Case :

We can make the following assumptions:

  • the front and rear field clams are like in the MC
  • we assume the Pre-Shower is the same
  • so what remains is the calo.

Information from Calorimeter survey:
Average size of a block: 90.910x91.020mm^2 for the upper and 91.030x91.020mm^2 for the lower volumes; indeed this ~1mm extra comes from ~500um spacers + wrapping material + the fact that blocks are not tightly fit in X-direction.
Following positions of the center of the block (first upper row - last lower row) have been measured in surveys and by hand:

Calorimeter:
survey nominal (=MC) 06d-Production
Upper pXC1 -1864.4 -1845.0 -1867.1
pYC1 286.1 285.0 281.7
Lower pXC1 -1869.9 -1845.0 -1868.4
pYC1 -1104.8 -1095.0 -1103.3

The calo surface is at 4695mm+2774mm-250mm=7219mm.
This numbers are: block center offset wrt the dipole yoke; yoke offset wrt the nominal IP; half-length of a calo block) --> such we have a 2cm difference to the MC in the z-position. The difference in x of 3 cm plays no role for photons. In lower y-there is no difference in in upper y we have also a 2cm difference.

As the differences to the MC are tiny, we can use for data and MC the same fiducial volume cuts.

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