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Fix independent LF resonance gun RNG seeding #2456

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Merged
njacazio merged 1 commit into from
Sep 9, 2026
Merged

Fix independent LF resonance gun RNG seeding #2456

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13 changes: 13 additions & 0 deletions MC/config/PWGLF/pythia8/generator_pythia8_LF_rapidity_width.C
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Original file line number Diff line number Diff line change
Expand Up @@ -153,6 +153,12 @@ class GeneratorPythia8LFRapidity : public o2::eventgen::GeneratorPythia8
/** switch off process level **/
mPythiaGun.readString("ProcessLevel:all off");

// The gun owns a separate RNG: seeding the inherited mPythia does not
// seed its mass sampler or decays. Derive a valid, nonzero Pythia seed
// from ROOT's RNG; explicit decayer configuration can still override it.
mPythiaGun.readString("Random:setSeed = on");
mPythiaGun.readString("Random:seed = " + std::to_string(1 + gRandom->Integer(900000000)));

auto& param = o2::eventgen::DecayerPythia8Param::Instance();
LOG(info) << "Init \'GeneratorPythia8LFRapidity\' with following parameters";
LOG(info) << param;
Expand All @@ -177,6 +183,13 @@ class GeneratorPythia8LFRapidity : public o2::eventgen::GeneratorPythia8
mPythiaGun.readString(std::string("Init:showChangedParticleData off"));
}

LOG(info) << "LF gun RNG: Random:setSeed=" << mPythiaGun.settings.flag("Random:setSeed")
<< ", Random:seed=" << mPythiaGun.settings.mode("Random:seed");
if (!mPythiaGun.settings.flag("Random:setSeed") || mPythiaGun.settings.mode("Random:seed") <= 0) {
LOG(warn) << "Decayer configuration overrides the ROOT-derived gun seed; "
<< "the gun may use default or time-based seeding";
}

/** initialise **/
if (!mPythiaGun.init()) {
LOG(fatal) << "Failed to init \'DecayerPythia8\': init returned with error";
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