From 779f3a1c89db62746f462dbbcbce90f288704693 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Viktor=20P=C4=9B=C4=8D?=
 <36884673+vpec0@users.noreply.github.com>
Date: Fri, 7 Feb 2025 16:50:03 +0100
Subject: [PATCH] Fixing time synchronisation for PhotonBacktracker for
 ProtoDUNE HD MC.

I have observed the backtracker did not match OpHits to correct true particles and in general it failed to find corresponding true depositions. Looking at the OpFlash times w.r.t. the true times, the flash times came 250 us earlier. This fix is configuring the backtracker to account for that.
---
 dunecore/Utilities/services_protodunehd.fcl | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/dunecore/Utilities/services_protodunehd.fcl b/dunecore/Utilities/services_protodunehd.fcl
index fb41c388..128d318b 100644
--- a/dunecore/Utilities/services_protodunehd.fcl
+++ b/dunecore/Utilities/services_protodunehd.fcl
@@ -45,6 +45,12 @@ protodunehd_simulation_services:
   DetectorClocksService: @local::protodunehd_detectorclocks
 }
 protodunehd_simulation_services.OpDetResponseInterface.QuantumEfficiency: 1
+# Make sure that the backtracker looks at the correct time interval. 
+# The recob::OpFlash::Time() is at -250 us w.r.t. the true times and so should be the hit peak times. 
+# recob::OpFlash::AbsTime() is corrected for this offset. 
+# The Delay configurable is expected to be in ns.
+protodunehd_simulation_services.PhotonBackTrackerService.PhotonBackTracker.Delay: -250e3
+
 
 
 # Reco services for ProtoDUNE simulation.