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Fix px/py bug in generator_pythia8_gun.C #1683

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merged 1 commit into from
Jul 5, 2024
+8 −10
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Fix px/py bug in generator_pythia8_gun.C #1683

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18 changes: 8 additions & 10 deletions MC/config/PWGHF/pythia8_gun/generator_pythia8_gun.C
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Original file line number Diff line number Diff line change
Expand Up @@ -135,6 +135,8 @@ class GeneratorPythia8Gun : public o2::eventgen::GeneratorPythia8
/// generate uniform eta and uniform momentum
void genUniformMomentumEta(double minP, double maxP, double minEta, double maxEta)
{
// Warning: this generator samples randomly in p and not in pT. Care is advised

// random generator
std::unique_ptr<TRandom3> ranGenerator{new TRandom3()};
ranGenerator->SetSeed(0);
Expand All @@ -146,15 +148,11 @@ class GeneratorPythia8Gun : public o2::eventgen::GeneratorPythia8
// z-component momentum from eta
const double cosTheta = (exp(2 * gen_eta) - 1) / (exp(2 * gen_eta) + 1); // starting from eta = -ln(tan(theta/2)) = 1/2*ln( (1+cos(theta))/(1-cos(theta)) ) ---> NB: valid for cos(theta)!=1
const double gen_pz = gen_p * cosTheta;
// y-component: random uniform
const double maxVal = sqrt(gen_p * gen_p - gen_pz * gen_pz);
double sign_py = ranGenerator->Uniform(0, 1);
sign_py = (sign_py > 0.5) ? 1. : -1.;
const double gen_py = ranGenerator->Uniform(0., maxVal) * sign_py;
// x-component momentum
double sign_px = ranGenerator->Uniform(0, 1);
sign_px = (sign_px > 0.5) ? 1. : -1.;
const double gen_px = sqrt(gen_p * gen_p - gen_pz * gen_pz - gen_py * gen_py) * sign_px;
// phi: random uniform, X, Y conform
const double pT = sqrt(gen_p * gen_p - gen_pz * gen_pz);
double phi = ranGenerator->Uniform(0., 2.0f*TMath::Pi());
const double gen_px = pT*TMath::Cos(phi);
const double gen_py = pT*TMath::Sin(phi);

set4momentum(gen_px, gen_py, gen_pz);
}
Expand Down Expand Up @@ -288,4 +286,4 @@ FairGenerator* generateOmegaAndPions_RandomCharge(const int nPions)
myGen->setAddFurtherPrimaries(-211, nPions / 2); // pi-

return myGen;
}
}