update default beam origin to align with standard beam gun

Tracking/src/Tracking/Reco/TruthSeedProcessor.cxx

@@ -61,11 +61,11 @@ void TruthSeedProcessor::configure(framework::config::Parameters& parameters) {
   // In tracking frame: where do these numbers come from?
   // These numbers come from approximating the path of the beam up
   // until it is about to enter the first detector volume (TriggerPad1).
-  // In detector coordinates, (x,y,z) = (-44,0,-880) is _roughly_
+  // In detector coordinates, (x,y,z) = (-21.7, -883) is
   // where the beam arrives (if no smearing is applied) and we simply
   // reorder these values so that they are in tracking coordinates.
   beamOrigin_ = parameters.getParameter<std::vector<double>>(
-      "beamOrigin", {-880.1, -44., 0.});
+      "beamOrigin", {-883.0, -21.745876, 0.0});
 
   // Skip the tagger or recoil trackers if wanted
   skip_tagger_ = parameters.getParameter<bool>("skip_tagger", false);