TPC: add ITS-TPC combined-momentum sampling trigger (#15599)

Add a third sampling trigger bit for tracks with matched ITS-TPC
combined momentum. The new trigger uses the Tsallis sampling function on
the combined-track pT instead of the TPC-only pT and stores the
corresponding weight as weight_ITSTPC.

This extends the existing trigger mask scheme:
  0x1: minimum-bias sampling
  0x2: Tsallis sampling using TPC-only track pT
  0x4: Tsallis sampling using ITS-TPC combined-track pT

The output condition is updated so tracks selected by the new ITS-TPC
sampling path are written even when they are not selected by the existing
TPC-only or minimum-bias triggers.

Motivation:
sampling only on TPC-only momentum can bias correlated observables such
as qpt_TPC - qpt_Comb. The new trigger provides a dedicated sampled
stream for combined-track performance studies while preserving the
existing TPC-only and minimum-bias sampling streams.

Co-authored-by: miranov25 <marian.ivanov@cern.cg>

Author: 2 people

Detectors/TPC/workflow/src/TPCTimeSeriesSpec.cxx

@@ -1324,12 +1324,17 @@ class TPCTimeSeries : public Task
     if (mUnbinnedWriter && mStreamer[iThread]) {
       const float factorPt = mSamplingFactor;
       bool writeData = true;
+      bool writeDataITSTPC = false;
       float weight = 0;
+      float weightITSTPC = 0;
       if (mSampleTsallis) {
         std::uniform_real_distribution<> distr(0., 1.);
         writeData = o2::math_utils::Tsallis::downsampleTsallisCharged(tracksTPC[iTrk].getPt(), factorPt, mSqrt, weight, distr(mGenerator[iThread]));
+        if (hasITSTPC) {
+          writeDataITSTPC = o2::math_utils::Tsallis::downsampleTsallisCharged(tracksITSTPC[idxITSTPC.front()].getPt(), factorPt, mSqrt, weightITSTPC, distr(mGenerator[iThread]));
+        }
       }
-      if (writeData || minBiasOk) {
+      if (writeData || writeDataITSTPC || minBiasOk) {
         auto clusterMask = makeClusterBitMask(trackFull);
         const auto& trkOrig = tracksTPC[iTrk];
         const bool isNearestVtx = (idxITSTPC.back() == -1); // is nearest vertex in case no vertex was found
@@ -1384,7 +1389,11 @@ class TPCTimeSeries : public Task
             }
           }
         }
-        const int triggerMask = 0x1 * minBiasOk + 0x2 * writeData;
+        // triggerMask bits:
+        // 0x1: flat minimum-bias stream
+        // 0x2: Tsallis stream sampled with TPC-only pT
+        // 0x4: Tsallis stream sampled with ITS-TPC combined pT
+        const int triggerMask = 0x1 * minBiasOk + 0x2 * writeData + 0x4 * writeDataITSTPC;
 
         float deltaP2ConstrVtx = -999;
         float deltaP3ConstrVtx = -999;
@@ -1435,6 +1444,7 @@ class TPCTimeSeries : public Task
                             << "factorMinBias=" << factorMinBias
                             << "factorPt=" << factorPt
                             << "weight=" << weight
+                            << "weight_ITSTPC=" << weightITSTPC
                             << "dcar_tpc_vertex=" << dcaTPCAtVertex
                             << "dcar_tpc=" << dca[0]
                             << "dcaz_tpc=" << dca[1]