From 80ed3cfbda25fe3d0e4757166208c7f2a0fcca86 Mon Sep 17 00:00:00 2001
From: Marco Giacalone <mgiacalo@gmail.com>
Date: Mon, 14 Sep 2026 14:05:43 +0200
Subject: [PATCH] Added user hook for triggered events

---
 .../generator_pythia8_hfhadron_to_nuclei.C       | 16 ++++++++++++++++
 .../pythia8/hooks/pythia8_userhooks_qqbar.C      |  8 ++++++++
 2 files changed, 24 insertions(+)

diff --git a/MC/config/PWGHF/external/generator/generator_pythia8_hfhadron_to_nuclei.C b/MC/config/PWGHF/external/generator/generator_pythia8_hfhadron_to_nuclei.C
index a2aadef89..339b79eb8 100644
--- a/MC/config/PWGHF/external/generator/generator_pythia8_hfhadron_to_nuclei.C
+++ b/MC/config/PWGHF/external/generator/generator_pythia8_hfhadron_to_nuclei.C
@@ -9,6 +9,7 @@
 
 R__ADD_INCLUDE_PATH($O2DPG_MC_CONFIG_ROOT)
 #include "MC/config/common/external/generator/CoalescencePythia8.h"
+#include "MC/config/PWGHF/pythia8/hooks/pythia8_userhooks_qqbar.C"
 
 using namespace Pythia8;
 
@@ -60,6 +61,16 @@ class GeneratorPythia8HFHadToNuclei : public o2::eventgen::GeneratorPythia8
   {
     addSubGenerator(0, "Minimum bias");
     addSubGenerator(1, "HF + Coalescence");
+
+    // Bias parton-level generation towards bbbar production at midrapidity, but only
+    // while generating the triggered events. Without this, finding the requested b-hadron 
+    // can stall the jobs for a very long time
+    mBbbarBiasHook = new UserHooks_qqbar();
+    mBbbarBiasHook->setPDG(5);
+    mBbbarBiasHook->setRapidity(mHadRapidityMin, mHadRapidityMax);
+    mBbbarBiasHook->setActive(false);
+    setUserHooks(mBbbarBiasHook);
+
     return o2::eventgen::GeneratorPythia8::Init();
   }
 
@@ -93,11 +104,13 @@ class GeneratorPythia8HFHadToNuclei : public o2::eventgen::GeneratorPythia8
 
       // Generate event of interest
       bool genOk = false;
+      mBbbarBiasHook->setActive(true);
       while (!genOk) {
         if (GeneratorPythia8::generateEvent()) {
           genOk = selectEvent(mPythia.event);
         }
       }
+      mBbbarBiasHook->setActive(false);
       notifySubGenerator(1);
     } else {
       // Generate minimum-bias event
@@ -178,6 +191,9 @@ class GeneratorPythia8HFHadToNuclei : public o2::eventgen::GeneratorPythia8
   float mHadRapidityMax;
   unsigned int mUsedSeed;
 
+  // Bias towards bbbar at midrapidity, active only while generating HF-triggered events
+  UserHooks_qqbar* mBbbarBiasHook{nullptr};
+
   // Control gap-triggering
   unsigned long long mGeneratedEvents;
   int mInverseTriggerRatio;
diff --git a/MC/config/PWGHF/pythia8/hooks/pythia8_userhooks_qqbar.C b/MC/config/PWGHF/pythia8/hooks/pythia8_userhooks_qqbar.C
index 9cbf6fe27..8dcf2b4cd 100644
--- a/MC/config/PWGHF/pythia8/hooks/pythia8_userhooks_qqbar.C
+++ b/MC/config/PWGHF/pythia8/hooks/pythia8_userhooks_qqbar.C
@@ -16,6 +16,10 @@ class UserHooks_qqbar : public Pythia8::UserHooks
   bool canVetoPartonLevel() override { return true; };
   bool doVetoPartonLevel(const Pythia8::Event& event) override
   {
+    // Veto runtime condition
+    if (!mActive) {
+      return false;
+    }
     // search for c-cbar mother with at least one c at midrapidity
     for (int ipa = 0; ipa < event.size(); ++ipa) {
       auto daughterList = event[ipa].daughterList();
@@ -40,11 +44,15 @@ class UserHooks_qqbar : public Pythia8::UserHooks
     mRapidityMin = valMin;
     mRapidityMax = valMax;
   };
+  // Allows the veto to be switched on/off at run time, e.g. so that a generator
+  // alternating between biased and unbiased events can apply the bias selectively.
+  void setActive(bool val) { mActive = val; };
 
  private:
   int mPDG = 4;
   double mRapidityMin = -1.5;
   double mRapidityMax = 1.5;
+  bool mActive = true;
 };
 
 Pythia8::UserHooks*