Trigger fix

[peter-mckeown]

(Copied from Original Repo MR! 25
As spotted by Anatolii, there are (rare) edge cases close to a geometrical region excluded from fast sim where secondaries created in a full sim shower can cross the trigger boundary and cause unwanted fast sim triggers inside the calorimeter. This MR proposes a fix for this with two important steps:

1. Adding a trigger plane to the compact file right at the front of the calorimeter face (and of course adjusting the model.RegionName in scripts/ddsim_steer.py ).
   E.g. for the ECAL Barrel of ILD_l5_o1_v02.xml this would mean creating a local copy of k4geo and modifying SEcal06_hybrid_Barrel.xml accordingly:

diff --git a/ILD/compact/ILD_common_v02/SEcal06_hybrid_Barrel.xml b/ILD/compact/ILD_common_v02/SEcal06_hybrid_Barrel.xml
index 457246d..d66595f 100644
--- a/ILD/compact/ILD_common_v02/SEcal06_hybrid_Barrel.xml
+++ b/ILD/compact/ILD_common_v02/SEcal06_hybrid_Barrel.xml
@@ -5,8 +5,35 @@
     </region>
   </regions>
 
+  <regions>
+    <region name="EcalBarrelFastSimTriggerRegion">
+    </region>
+  </regions>
+
+  <readouts>
+        <readout name="ECalBarrelFaceReadout">
+            <id>system:5,side:2,layer:9,module:8,sensor:8</id>
+        </readout>
+    </readouts>
+
   <detectors>
 
+    <detector name="EcalBarrelFastSimTrigger" type="PolyhedralBarrelSurfaces" region="EcalBarrelFastSimTriggerRegion" readout="ECalBarrelFaceReadout">
+      <comment> EM Calorimeter Fast Sim Trigger Region </comment>
+       
+      <envelope vis="ILD_ECALVis">
+        <shape type="PolyhedraRegular" numsides="Ecal_symmetry"  rmin="Ecal_inner_radius - 3*env_safety" 
+        rmax="Ecal_inner_radius - 2*env_safety" dz="2.*Ecal_half_length + env_safety"  material = "Air" />
+        <rotation x="0*deg" y="0*deg" z="-180*deg/Ecal_symmetry"/>
+      </envelope>
+      
+      <dimensions numsides="Ecal_symmetry" rmin="Ecal_inner_radius - 3*env_safety" 
+      rmax="Ecal_inner_radius - 2*env_safety" zhalf="Ecal_half_length"
+      phi0="-180*deg/Ecal_symmetry" material = "Air" />
+
+  </detector>
+
+
     <detector name="EcalBarrel" type="SEcal06_Barrel" id="ILDDetID_ECAL" readout="EcalBarrelCollection" vis="ILD_BlueVis"
      region="EcalBarrelRegion">

2. Ensure that backscatter from the calorimeter does not cause model triggers (see changes to src/OctogonalBarrelTrigger.cc)

Validation checks performed so far (with 2025-01-28 key4hep release):

• particle gun into calo in FastSim region
  position: (0, 0, 0), direction: (0 1 0), particle: gamma, energy: 50 GeV
  Results: Triggers Fast Sim (expected behavior)

• particle gun inside calo (firing backwards) in FastSim region
  position: (0, 1860, 0), direction: 0 -1 0, particle: gamma, energy: 50 GeV
  Results: Does not trigger Fast Sim (expected behavior)

• particle gun into calo region excluded from FastSim (corner of barrel)
  position: (0, 0, 0), direction: (-0.4152 1 0), particle: gamma, energy: 50 GeV
  Results: Does not trigger Fast Sim (expected behavior)

[tmadlener]

Is there an easy way to check whether this works. Mainly to avoid future regressions?