EvtBtoXsll.cc

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//--------------------------------------------------------------------------
//
// Environment:
//      This software is part of the EvtGen package developed jointly
//      for the BaBar and CLEO collaborations.  If you use all or part
//      of it, please give an appropriate acknowledgement.
//
// Module: EvtBtoXsll.cc
//
// Description: Routine to generate non-resonant B -> Xs l+ l- decays.
// It generates a dilepton mass spectrum according to Kruger and Sehgal
// and then generates the two lepton momenta accoring to Ali et al.
// The resultant X_s particles may be decayed by JETSET.
//
// Modification history:
//
//    Stephane Willocq     Jan  17, 2001    Module created
//    Stephane Willocq     Jul  15, 2003    Input model parameters
//
//------------------------------------------------------------------------
//
#include "EvtGenBase/EvtPatches.hh"
 
#include <stdlib.h>
#include "EvtGenBase/EvtRandom.hh"
#include "EvtGenBase/EvtParticle.hh"
#include "EvtGenBase/EvtGenKine.hh"
#include "EvtGenBase/EvtPDL.hh"
#include "EvtGenBase/EvtReport.hh"
#include "EvtGenModels/EvtbTosllAmp.hh"
#include "EvtGenModels/EvtBtoXsll.hh"
#include "EvtGenModels/EvtBtoXsllUtil.hh"
#include "EvtGenBase/EvtConst.hh"
#include "EvtGenBase/EvtId.hh"
using std::endl;
 
EvtBtoXsll::~EvtBtoXsll() {}
 
void EvtBtoXsll::getName(std::string& model_name){
 
  model_name="BTOXSLL";     
 
}
 
EvtDecayBase* EvtBtoXsll::clone(){
 
  return new EvtBtoXsll;
 
}
 
 
void EvtBtoXsll::init(){
 
  // check that there are no arguments
 
  checkNArg(0,4,5);
 
  checkNDaug(3);
 
  // Check that the two leptons are the same type
 
  EvtId lepton1type = getDaug(1);
  EvtId lepton2type = getDaug(2);
 
  int etyp   = 0;
  int mutyp  = 0;
  int tautyp = 0;
  if ( lepton1type == EvtPDL::getId("e+") ||
       lepton1type == EvtPDL::getId("e-") ) { etyp++;}
  if ( lepton2type == EvtPDL::getId("e+") ||
       lepton2type == EvtPDL::getId("e-") ) { etyp++;}
  if ( lepton1type == EvtPDL::getId("mu+") ||
       lepton1type == EvtPDL::getId("mu-") ) { mutyp++;}
  if ( lepton2type == EvtPDL::getId("mu+") ||
       lepton2type == EvtPDL::getId("mu-") ) { mutyp++;}
  if ( lepton1type == EvtPDL::getId("tau+") ||
       lepton1type == EvtPDL::getId("tau-") ) { tautyp++;}
  if ( lepton2type == EvtPDL::getId("tau+") ||
       lepton2type == EvtPDL::getId("tau-") ) { tautyp++;}
 
  if ( etyp != 2 && mutyp != 2 && tautyp != 2 ) {
 
     report(ERROR,"EvtGen") << "Expect two leptons of the same type in EvtBtoXsll.cc\n";
     ::abort();
  }
 
  // Check that the second and third entries are leptons with positive
  // and negative charge, respectively
 
  int lpos = 0;
  int lneg = 0;
  if ( lepton1type == EvtPDL::getId("e+")  ||
       lepton1type == EvtPDL::getId("mu+") ||
       lepton1type == EvtPDL::getId("tau+") ) { lpos++;}
  if ( lepton2type == EvtPDL::getId("e-")  ||
       lepton2type == EvtPDL::getId("mu-") ||
       lepton2type == EvtPDL::getId("tau-") ) { lneg++;}
 
  if ( lpos != 1 || lneg != 1 ) {
 
     report(ERROR,"EvtGen") << "Expect 2nd and 3rd particles to be positive and negative leptons in EvtBtoXsll.cc\n";
     ::abort();
  }
 
 
  _mb=4.8;
  _ms=0.2;
  _mq=0.;
  _pf=0.41;
  _mxmin=1.1;
  if ( getNArg()==4) 
  {
    // b-quark mass
    _mb = getArg(0);
    // s-quark mass
    _ms = getArg(1);
    // spectator quark mass
    _mq = getArg(2);
    // Fermi motion parameter
    _pf = getArg(3);
  }
  if ( getNArg()==5) 
  {
    _mxmin = getArg(4);
  }
 
  _calcprob = new EvtBtoXsllUtil;
 
  double ml = EvtPDL::getMeanMass(getDaug(1));
 
  // determine the maximum probability density from dGdsProb
 
  int i, j;
  int nsteps  = 100;
  double s    = 0.0;
  double smin = 4.0 * ml * ml;
  double smax = (_mb - _ms)*(_mb - _ms);
  double probMax  = -10000.0;
  double sProbMax = -10.0;
  double uProbMax = -10.0;
 
  for (i=0;i<nsteps;i++)
  {
    s = smin + (i+0.0005)*(smax - smin)/(double)nsteps;
    double prob = _calcprob->dGdsProb(_mb, _ms, ml, s);
    if (prob > probMax)
    {
      sProbMax = s;
      probMax  = prob;
    }
  }
 
  _dGdsProbMax = probMax;
 
  if ( verbose() ) {
    report(INFO,"EvtGen") << "dGdsProbMax = " << probMax << " for s = "  << sProbMax << endl;
  }
 
  // determine the maximum probability density from dGdsdupProb
 
  probMax  = -10000.0;
  sProbMax = -10.0;
 
  for (i=0;i<nsteps;i++)
  {
    s = smin + (i+0.0005)*(smax - smin)/(double)nsteps;
    double umax = sqrt((s - (_mb + _ms)*(_mb + _ms)) * (s - (_mb - _ms)*(_mb - _ms)));
    for (j=0;j<nsteps;j++)
    {
      double u = -umax + (j+0.0005)*(2.0*umax)/(double)nsteps;
      double prob = _calcprob->dGdsdupProb(_mb, _ms, ml, s, u);
      if (prob > probMax)
      {
        sProbMax = s;
        uProbMax = u;
        probMax  = prob;
      }
    }
  }
 
  _dGdsdupProbMax = probMax;
 
  if ( verbose() ) {
   report(INFO,"EvtGen") << "dGdsdupProbMax = " << probMax << " for s = "  << sProbMax
              << " and u = " << uProbMax << endl;
  }
 
}
 
void EvtBtoXsll::initProbMax(){
 
  noProbMax();
 
}
 
void EvtBtoXsll::decay( EvtParticle *p ){
 
  p->makeDaughters(getNDaug(),getDaugs());
 
  EvtParticle* xhadron = p->getDaug(0);
  EvtParticle* leptonp = p->getDaug(1);
  EvtParticle* leptonn = p->getDaug(2);
 
  double mass[3];
 
  findMasses( p, getNDaug(), getDaugs(), mass );
 
  double mB = p->mass();
  double ml = mass[1];
  double pb;
 
  int im = 0;
  static int nmsg = 0;
  double xhadronMass = -999.0;
 
  EvtVector4R p4xhadron;
  EvtVector4R p4leptonp;
  EvtVector4R p4leptonn;
 
  // require the hadronic system has mass greater than that of a Kaon pion pair
 
  //  while (xhadronMass < 0.6333)
  // the above minimum value of K+pi mass appears to be too close
  // to threshold as far as JETSET is concerned
  // (JETSET gets caught in an infinite loop)
  // so we choose a lightly larger value for the threshold
  while (xhadronMass < _mxmin)
  {
    im++;
 
    // Apply Fermi motion and determine effective b-quark mass
 
    // Old BaBar MC parameters
    //    double pf = 0.25;
    //    double ms = 0.2;
    //    double mq = 0.3;
 
    double mb = 0.0;
 
    double xbox, ybox;
 
    while (mb <= 0.0)
    {
      pb = _calcprob->FermiMomentum(_pf);
 
      // effective b-quark mass
      mb = mB*mB + _mq*_mq - 2.0*mB*sqrt(pb*pb + _mq*_mq);
    }
    mb = sqrt(mb);
  
    //    cout << "b-quark momentum = " << pb << " mass = " <<  mb << endl;
 
    // generate a dilepton invariant mass
 
    double s    = 0.0;
    double smin = 4.0 * ml * ml;
    double smax = (mb - _ms)*(mb - _ms);
 
    while (s == 0.0)
    {
      xbox = EvtRandom::Flat(smin, smax);
      ybox = EvtRandom::Flat(_dGdsProbMax);
      if (ybox < _calcprob->dGdsProb(mb, _ms, ml, xbox)) { s = xbox;}
    }
 
    //    cout << "dGdsProb(s) = " << _calcprob->dGdsProb(mb, _ms, ml, s)
    //         << " for s = " << s << endl;
 
    // two-body decay of b quark at rest into s quark and dilepton pair:
    // b -> s (ll)
 
    EvtVector4R p4sdilep[2];
 
    double msdilep[2];
    msdilep[0] = _ms;
    msdilep[1] = sqrt(s);
 
    EvtGenKine::PhaseSpace(2, msdilep, p4sdilep, mb);
 
    // generate dilepton decay with the expected asymmetry: (ll) -> l+ l-
 
    EvtVector4R p4ll[2];
 
    double mll[2];
    mll[0] = ml;
    mll[1] = ml;
 
    double tmp = 0.0;
 
    while (tmp == 0.0)
    {
      // (ll) -> l+ l- decay in dilepton rest frame
 
      EvtGenKine::PhaseSpace(2, mll, p4ll, msdilep[1]);
 
      // boost to b-quark rest frame
 
      p4ll[0] = boostTo(p4ll[0], p4sdilep[1]);
      p4ll[1] = boostTo(p4ll[1], p4sdilep[1]);
 
      // compute kinematical variable u
 
      EvtVector4R p4slp = p4sdilep[0] + p4ll[0];
      EvtVector4R p4sln = p4sdilep[0] + p4ll[1];
 
      double u = p4slp.mass2() - p4sln.mass2();
 
      ybox = EvtRandom::Flat(_dGdsdupProbMax);
 
      double prob = _calcprob->dGdsdupProb(mb, _ms, ml, s, u);
      if (prob > _dGdsdupProbMax && nmsg < 20)
      {
        report(INFO,"EvtGen") << "d2gdsdup GT d2gdsdup_max:" << prob
             << " " << _dGdsdupProbMax
             << " for s = " << s << " u = " << u << " mb = " << mb << endl;
        nmsg++;
      }
      if (ybox < prob)
      {
        tmp = 1.0;
    //        cout << "dGdsdupProb(s) = " << prob
    //             << " for u = " << u << endl;
      }
    }
 
    // assign 4-momenta to valence quarks inside B meson in B rest frame
 
    double phi   = EvtRandom::Flat( EvtConst::twoPi );
    double costh = EvtRandom::Flat( -1.0, 1.0 );
    double sinth = sqrt(1.0 - costh*costh);
 
    // b-quark four-momentum in B meson rest frame
 
    EvtVector4R p4b(sqrt(mb*mb + pb*pb),
                    pb*sinth*sin(phi),
                    pb*sinth*cos(phi),
                    pb*costh);
 
    // B meson in its rest frame
    //
    //    EvtVector4R p4B(mB, 0.0, 0.0, 0.0);
    //
    // boost B meson to b-quark rest frame
    //
    //    p4B = boostTo(p4B, p4b);
    //
    //    cout << " B meson mass in b-quark rest frame = " << p4B.mass() << endl;
 
    // boost s, l+ and l- to B meson rest frame
 
    //    EvtVector4R p4s = boostTo(p4sdilep[0], p4B);
    //    p4leptonp       = boostTo(p4ll[0],     p4B);
    //    p4leptonn       = boostTo(p4ll[1],     p4B);
 
    EvtVector4R p4s = boostTo(p4sdilep[0], p4b);
    p4leptonp       = boostTo(p4ll[0],     p4b);
    p4leptonn       = boostTo(p4ll[1],     p4b);
 
    // spectator quark in B meson rest frame
 
    EvtVector4R p4q( sqrt(pb*pb + _mq*_mq), -p4b.get(1), -p4b.get(2), -p4b.get(3) );
 
    // hadron system in B meson rest frame
 
    p4xhadron = p4s + p4q;
    xhadronMass = p4xhadron.mass();
 
    //    cout << "Xs mass = " << xhadronMass << " trial " << im << endl;
  }
 
  // initialize the decay products
 
  xhadron->init(getDaug(0), p4xhadron);
 
  // For B-bar mesons (i.e. containing a b quark) we have the normal
  // order of leptons
  if ( p->getId() == EvtPDL::getId("anti-B0") ||
       p->getId() == EvtPDL::getId("B-") )
  {
    leptonp->init(getDaug(1), p4leptonp);
    leptonn->init(getDaug(2), p4leptonn);
  }
  // For B mesons (i.e. containing a b-bar quark) we need to flip the
  // role of the positive and negative leptons in order to produce the
  // correct forward-backward asymmetry between the two leptons
  else
  {
    leptonp->init(getDaug(1), p4leptonn);
    leptonn->init(getDaug(2), p4leptonp);
  }
 
  return ;
}