[PWGEM] add additional max. mass pair cut to reduce number of events (#17571)

Author: Steffimro

PWGEM/Dilepton/TableProducer/filterEoI.cxx

@@ -19,6 +19,7 @@
 
 #include "Common/Core/TableHelper.h"
 
+#include <Framework/ASoAHelpers.h>
 #include <Framework/AnalysisDataModel.h>
 #include <Framework/AnalysisHelpers.h>
 #include <Framework/AnalysisTask.h>
@@ -28,6 +29,7 @@
 #include <Framework/InitContext.h>
 #include <Framework/runDataProcessing.h>
 
+#include <Math/Vector4D.h>
 #include <TH1.h>
 
 #include <cstdint>
@@ -53,6 +55,7 @@ struct filterEoI {
   Configurable<int> minNphotons{"minNphotons", 1, "min number of photon candidates per collision"};
   Configurable<std::string> taskNameForNelectron{"taskNameForNelectron", "skimmer-primary-electron", "task name where minNelectron is defined."};
   Configurable<std::string> varNameForNelectron{"varNameForNelectron", "minNelectron", "variable name for minNelectron"};
+  Configurable<float> maxMinvPair{"maxMinvPair", -1.f, "keep events only if at least one photon pair has q_inv below this (GeV/c); negative = disabled"};
 
   HistogramRegistry fRegistry{"output"};
   void init(o2::framework::InitContext& initContext)
@@ -65,8 +68,9 @@ struct filterEoI {
     LOGF(info, "minNelectron = %d", minNelectron.value);
     LOGF(info, "minNmuon = %d", minNmuon.value);
     LOGF(info, "minNphotons = %d", minNphotons.value);
+    LOGF(info, "maxMinvPair = %f", static_cast<float>(maxMinvPair.value));
 
-    auto hEventCounter = fRegistry.add<TH1>("hEventCounter", "hEventCounter", kTH1D, {{8, 0.5f, 8.5f}});
+    auto hEventCounter = fRegistry.add<TH1>("hEventCounter", "hEventCounter", kTH1D, {{10, 0.5f, 10.5f}});
     hEventCounter->GetXaxis()->SetBinLabel(1, "all");
     hEventCounter->GetXaxis()->SetBinLabel(2, "event with electron");
     hEventCounter->GetXaxis()->SetBinLabel(3, "event with forward muon");
@@ -75,6 +79,8 @@ struct filterEoI {
     hEventCounter->GetXaxis()->SetBinLabel(6, "event with electron and forward muon");
     hEventCounter->GetXaxis()->SetBinLabel(7, "event with electron or forward muon or v0");
     hEventCounter->GetXaxis()->SetBinLabel(8, "event with v0 or electrons from dalitz");
+    hEventCounter->GetXaxis()->SetBinLabel(9, "event with minNphotons v0s selection");
+    hEventCounter->GetXaxis()->SetBinLabel(10, "event with minNphotons v0s and low-M pair selection");
   }
 
   SliceCache cache;
@@ -109,10 +115,27 @@ struct filterEoI {
       }
       if constexpr (static_cast<bool>(system & kPCM)) {
         auto v0s_coll = v0s.sliceBy(perCollision_v0, collision.globalIndex());
-        if (v0s_coll.size() >= minNphotons) {
-          does_pcm_exist = true;
+        if (v0s_coll.size() >= 1) {
           fRegistry.fill(HIST("hEventCounter"), 4);
         }
+        if (v0s_coll.size() >= minNphotons) {
+          fRegistry.fill(HIST("hEventCounter"), 9);
+          bool hasLowMPair = (maxMinvPair < 0.f);
+          if (!hasLowMPair) {
+            for (const auto& [g1, g2] : combinations(CombinationsStrictlyUpperIndexPolicy(v0s_coll, v0s_coll))) {
+              ROOT::Math::PtEtaPhiMVector v1(g1.pt(), g1.eta(), g1.phi(), 0.f);
+              ROOT::Math::PtEtaPhiMVector v2(g2.pt(), g2.eta(), g2.phi(), 0.f);
+              if ((v1 + v2).M() < maxMinvPair) {
+                hasLowMPair = true;
+                break;
+              }
+            }
+          }
+          if (hasLowMPair) {
+            does_pcm_exist = true;
+            fRegistry.fill(HIST("hEventCounter"), 10);
+          }
+        }
       }
       if constexpr (static_cast<bool>(system & kElectronFromDalitz)) {
         auto electronsda_coll = electronsda.sliceBy(perCollision_elda, collision.globalIndex());