main62

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// main62.cc is a part of the PYTHIA event generator.
// Copyright (C) 2020 Torbjorn Sjostrand.
// PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
// Please respect the MCnet Guidelines, see GUIDELINES for details.

// Keywords:
//            Userhooks
//            LHE file
//            Resonance decay
//            External resonance

// Example how you can use UserHooks to set angular decay distributions
// for undecayed resonances from Les Houches input using the polarization
// information of the boson defined in its rest frame.

#include "Pythia8/Pythia.h"
using namespace Pythia8;

//==========================================================================

// Write own derived UserHooks class.
// Assumptions in this particular case:
// The W+- bosons were undecayed in the Les Houches Events input file,
// and subsequently decayed isotropically by the Pythia machinery.
// Now the angular distribution will be corrected for each W,
// based on the polarization value stored in the LHEF.
// For W- this is (1 -+ cos(theta))^2 for +-1, sin^2(theta) for 0,
// and isotropic for 9. For W+ it is flipped (i.e. theta->pi-theta).
// The Pythia decay products (i.e. the branching ratios) are retained.

class MyUserHooks : public UserHooks {

public:

  // Constructor can set helicity definition and book a histogram.
  MyUserHooks(const Info* infoPtrIn, bool inputOption = true)
    : infoPtr(infoPtrIn), helicityDefinedByMother(inputOption) {
    // Book a histogram to test the angular distribution in the UserHook.
    cosRaw = new Hist("cos(the*) raw", 100, -1., 1.);
  }

  // Destructor prints histogram and deletes it.
  ~MyUserHooks() { cout << *cosRaw; delete cosRaw; }

  // Allow a veto for the process level, to gain access to decays.
  bool canVetoProcessLevel() {return true;}

  // Access the event after resonance decays.
  bool doVetoProcessLevel(Event& process) {

    // Identify decayed W+- bosons for study.
    // Assume isotropic decay if polarization is unphysically big (|pol|>2)
    for (int i = 0; i < process.size(); ++i) {
      if (process[i].idAbs() == 24 && process[i].status() == -22
          && abs(process[i].pol()) < 2.) {

        // Pick decay angles according to desired distribution
        // based on polarization and particle/antiparticle.
        double cosThe = selectAngle( process[i].pol(), process[i].id() );
        // Accumulate the raw angular distribution.
        cosRaw->fill( cosThe );
        double sinThe = sqrt(1.0 - pow2(cosThe));
        double phi    = 2.0 * M_PI * rndmPtr->flat();

        // Identify W+- daughters, properly ordered.
        int idV = process[i].id();
        int i1  = process[i].daughter1();
        int i2  = process[i].daughter2();
        // The current distributions are for the particle with the
        // same charge sign as the mother, i.e. W- -> e-.
        if (process[i1].id() * idV > 0) swap( i1, i2);

        // Set up decay in rest frame of W+-.
        double mV = process[i].m();
        double m1 = process[i1].m();
        double m2 = process[i2].m();
        // Energy and absolute momentum of first decay product in W rest frame.
        double e1 = 0.5* (pow2(mV) + pow2(m1) - pow2(m2))/mV;
        double pA = sqrt(pow2(e1) - pow2(m1));
        // Four-vectors for the two decay products.
        Vec4 p1( pA * sinThe * cos(phi), pA *sinThe * sin(phi),
          pA * cosThe, e1);
        Vec4 p2   = Vec4(0,0,0,mV) - p1;

        // Reference four-vector for helicity definition.
        Vec4 pM;
        // Helicity is defined in the mother frame.
        if( helicityDefinedByMother ) {
          pM = process[process[i].mother1()].p();
        // Helicity is defined in the process CMS frame.
        // This is the convention for MadGraph.
        } else {
          pM = Vec4( 0., 0., 0., infoPtr->mHat());
        }

        // Angular reference axis defined as opposite the mother
        // direction in W rest frame.
        pM.bstback( process[i].p() );
        pM.flip3();
        RotBstMatrix Mrotbst;
        Mrotbst.rot( pM);

        // Rotate and boost W decay products.
        Mrotbst.bst( process[i].p() );
        p1.rotbst(Mrotbst);
        p2.rotbst(Mrotbst);
        process[i1].p( p1 );
        process[i2].p( p2 );
      }
      // End of loop over W's. Do not veto any events.
    }
    return false;
  }

  // Select polar angle for the W decay.
  double selectAngle( double inputSpin, double inputId ) {

    // Set up initial angles.
    double rdNow = rndmPtr->flat();
    double cosThe;
    // Small number to distinguish -1, 1, and 0 with round-off.
    double eps = 1e-10;

    // W+ distribution is "opposite" of W-.
    if (inputId > 0) inputSpin *= -1;

    // Different decay angular distributions.
    // 3/8 * (1 - cos(theta))^2  ++
    if (inputSpin > eps) {
      cosThe = max( 1.0 - 2.0 * pow(rdNow, 1./3.), -1.0);
    // 3/8 * (1 + cos(theta))^2  --
    } else if (inputSpin < -eps) {
      cosThe = min( 2.0 * pow(rdNow, 1./3.) - 1.0,  1.0);
    // 3/4 * sin(theta)^2        00
    // Solution of cubic equation that yields the correct result.
    } else {
      double theA = (acos(1.0 - 2.0 * rdNow) + 4.0 * M_PI) / 3.0;
      cosThe = 2.0 * cos(theA);
    }

    // Return the selected cos(theta) value.
    return cosThe;
  }

private:

  const Info* infoPtr;
   // bool to define the frame for helicity.
  bool helicityDefinedByMother;
  Hist* cosRaw;

};

//==========================================================================

int main() {

  // Generator. Shorthand for the event.
  Pythia pythia;
  Event& event = pythia.event;

  // Set up to do a user veto and send it in. Initialize.
  //  Use this line for CMS definition of helicity.
  //  MyUserHooks* myUserHooks = new MyUserHooks(&pythia.info,false);
  // Default constructor uses mother frame for helicity.
  shared_ptr myUserHooks = make_shared(&pythia.info);
  pythia.setUserHooksPtr( (UserHooksPtr)myUserHooks);
  pythia.readFile("main62.cmnd");
  pythia.init();

  // Histograms.
  Hist polarization("W polarization", 99, -9.9, 9.9);
  Hist cosPlus( "cos(theta) W- -> f",    100, -1.0, 1.0);
  Hist cosMinus("cos(theta) W+ -> fbar", 100, -1.0, 1.0);
  Hist energy("daughter energy in W rest frame", 100, 0.0, 100.0);

  // Extract settings to be used in the main program.
  int nEvent = pythia.mode("Main:numberOfEvents");

  // Begin event loop.
  for (int iEvent = 0; iEvent < nEvent; ++iEvent) {

    // Generate events.
    pythia.next();

    // Loop through event, looking for a W and its daughters.
    for (int i = 0; i < event.size(); ++i) {
      // Select W boson when it decays to two partons,
      // not when it is a recoil in FSR.
      if (event[i].idAbs() == 24
          && event[i].daughter1() != event[i].daughter2() ) {
        int i1 = event[i].daughter1();
        // Angular distribution is defined with respect to the decay product
        // with the same sign charge as the W boson.
        if (event[i1].id() * event[i].id() > 0 ) i1 = event[i].daughter2();

        // Reconstruct W+- decay angle by boosting daughter and mother to W
        // rest frame. W direction in mother rest frame opposite to mother now.
        RotBstMatrix Mrotbst;
        Mrotbst.bstback( event[i].p() );
        Vec4 p1 = event[i1].p();
        p1.rotbst( Mrotbst );
        Vec4 pM = event[event[i].mother1()].p();
        pM.rotbst( Mrotbst );
        pM.flip3();
        double costhe = costheta( p1, pM );

        // Histogram information.
        polarization.fill( event[i].pol() );
        if ( event[i].id() > 0 ) cosPlus.fill( costhe );
        else                    cosMinus.fill( costhe );
        energy.fill( p1.e() );
      }
    }

  // End of event loop.
  }

  // Statistics. Histograms.
  pythia.stat();
  cout << polarization << cosPlus << cosMinus << energy;

  // Done.
  return 0;
}