EvtVub.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.
//
// Copyright Information:
//      Copyright (C) 1998      Caltech, UCSB
//
// Module: EvtVub.cc
//
// Description: Routine to decay a particle according th phase space
//
// Modification history:
//
//    Sven Menke       January 17, 2001       Module created
//
//------------------------------------------------------------------------
//
#include "EvtVub.hh"
#include "../EvtGenBase/EvtGenKine.hh"
#include "../EvtGenBase/EvtPDL.hh"
#include "../EvtGenBase/EvtParticle.hh"
#include "../EvtGenBase/EvtPatches.hh"
#include "../EvtGenBase/EvtReport.hh"
#include "../EvtGenBase/EvtVector4R.hh"
#include <stdlib.h>
#include <string>
// #include "EvtHepRandomEngine.hh"
#include "CLHEP/Random/RandGeneral.h"
#include "../EvtGenBase/EvtRandom.hh"
#include "EvtPFermi.hh"
#include "EvtVubdGamma.hh"
using std::endl;
using namespace CLHEP;
typedef double HepDouble;
 
// #include "CLHEP/config/CLHEP.h" //maqm add
// #include "CLHEP/config/TemplateFunctions.h" //maqm add
 
EvtVub::~EvtVub() {
  delete _dGamma;
  delete[] _masses;
  delete[] _weights;
}
 
void EvtVub::getName( std::string& model_name ) { model_name = "VUB"; }
 
EvtDecayBase* EvtVub::clone() { return new EvtVub; }
 
void EvtVub::init() {
 
  // check that there are at least 6 arguments
 
  if ( getNArg() < 6 )
  {
 
    report( ERROR, "EvtGen" )
        << "EvtVub generator expected "
        << " at least 6 arguments (mb,a,alpha_s,Nbins,m1,w1,...) but found: " << getNArg()
        << endl;
    report( ERROR, "EvtGen" ) << "Will terminate execution!" << endl;
    ::abort();
  }
 
  _mb         = getArg( 0 );
  _a          = getArg( 1 );
  _alphas     = getArg( 2 );
  _nbins      = abs( (int)getArg( 3 ) );
  _storeQplus = ( getArg( 3 ) < 0 ? 1 : 0 );
  _masses     = new double[_nbins];
  _weights    = new double[_nbins];
 
  if ( getNArg() - 4 != 2 * _nbins )
  {
    report( ERROR, "EvtGen" ) << "EvtVub generator expected " << _nbins
                              << " masses and weights but found: " << ( getNArg() - 4 ) / 2
                              << endl;
    report( ERROR, "EvtGen" ) << "Will terminate execution!" << endl;
    ::abort();
  }
  int    i, j = 4;
  double maxw = 0.;
  for ( i = 0; i < _nbins; i++ )
  {
    _masses[i] = getArg( j++ );
    if ( i > 0 && _masses[i] <= _masses[i - 1] )
    {
      report( ERROR, "EvtGen" ) << "EvtVub generator expected "
                                << " mass bins in ascending order!"
                                << "Will terminate execution!" << endl;
      ::abort();
    }
    _weights[i] = getArg( j++ );
    if ( _weights[i] < 0 )
    {
      report( ERROR, "EvtGen" ) << "EvtVub generator expected "
                                << " weights >= 0, but found: " << _weights[i] << endl;
      report( ERROR, "EvtGen" ) << "Will terminate execution!" << endl;
      ::abort();
    }
    if ( _weights[i] > maxw ) maxw = _weights[i];
  }
  if ( maxw == 0 )
  {
    report( ERROR, "EvtGen" ) << "EvtVub generator expected at least one "
                              << " weight > 0, but found none! "
                              << "Will terminate execution!" << endl;
    ::abort();
  }
  for ( i = 0; i < _nbins; i++ ) _weights[i] /= maxw;
 
  // the maximum dGamma*p2 value depends on alpha_s only:
 
  const double dGMax0 = 3.;
  _dGMax              = 0.21344 + 8.905 * _alphas;
  if ( _dGMax < dGMax0 ) _dGMax = dGMax0;
 
  // for the Fermi Motion we need a B-Meson mass - but it's not critical
  // to get an exact value; in order to stay in the phase space for
  // B+- and B0 use the smaller mass
 
  EvtId BP = EvtPDL::getId( "B+" );
  EvtId B0 = EvtPDL::getId( "B0" );
 
  double mB0 = EvtPDL::getMaxMass( B0 );
  double mBP = EvtPDL::getMaxMass( BP );
 
  double mB = ( mB0 < mBP ? mB0 : mBP );
 
  const double xlow  = -_mb;
  const double xhigh = mB - _mb;
  const int    aSize = 10000;
 
  EvtPFermi pFermi( _a, mB, _mb );
  // pf is the cumulative distribution
  // normalized to 1.
  _pf.resize( aSize );
  for ( i = 0; i < aSize; i++ )
  {
    double kplus = xlow + (double)( i + 0.5 ) / ( (double)aSize ) * ( xhigh - xlow );
    if ( i == 0 ) _pf[i] = pFermi.getFPFermi( kplus );
    else _pf[i] = _pf[i - 1] + pFermi.getFPFermi( kplus );
  }
  for ( i = 0; i < _pf.size(); i++ ) _pf[i] /= _pf[_pf.size() - 1];
 
  //  static EvtHepRandomEngine myEngine;
 
  //  _pFermi = new RandGeneral(myEngine,pf,aSize,0);
  _dGamma = new EvtVubdGamma( _alphas );
 
  // check that there are 3 daughters
  checkNDaug( 3 );
}
 
void EvtVub::initProbMax() { noProbMax(); }
 
void EvtVub::decay( EvtParticle* p ) {
 
  int j;
  // B+ -> u-bar specflav l+ nu
 
  EvtParticle *xuhad, *lepton, *neutrino;
  EvtVector4R  p4;
  // R. Faccini 21/02/03
  // move the reweighting up , before also shooting the fermi distribution
  double       x, z, p2;
  double       sh = 0.0;
  double       mB, ml, xlow, xhigh, qplus;
  double       El = 0.0;
  double       Eh = 0.0;
  double       kplus;
  const double lp2epsilon = -10;
  bool         rew( true );
  while ( rew )
  {
 
    p->initializePhaseSpace( getNDaug(), getDaugs() );
 
    xuhad    = p->getDaug( 0 );
    lepton   = p->getDaug( 1 );
    neutrino = p->getDaug( 2 );
 
    mB = p->mass();
    ml = lepton->mass();
 
    xlow  = -_mb;
    xhigh = mB - _mb;
 
    // Fermi motion does not need to be computed inside the
    // tryit loop as m_b in Gamma0 does not need to be replaced by (m_b+kplus).
    // The difference however should be of the Order (lambda/m_b)^2 which is
    // beyond the considered orders in the paper anyway ...
 
    // for alpha_S = 0 and a mass cut on X_u not all values of kplus are
    // possible. The maximum value is mB/2-_mb + sqrt(mB^2/4-_masses[0]^2)
    kplus = 2 * xhigh;
 
    while ( kplus >= xhigh || kplus <= xlow ||
            ( _alphas == 0 &&
              kplus >= mB / 2 - _mb + sqrt( mB * mB / 4 - _masses[0] * _masses[0] ) ) )
    {
      kplus = findPFermi(); //_pFermi->shoot();
      kplus = xlow + kplus * ( xhigh - xlow );
    }
    qplus = mB - _mb - kplus;
    if ( ( mB - qplus ) / 2. <= ml ) continue;
 
    int tryit = 1;
    while ( tryit )
    {
 
      x  = EvtRandom::Flat();
      z  = EvtRandom::Flat( 0, 2 );
      p2 = EvtRandom::Flat();
      p2 = pow( 10, lp2epsilon * p2 );
 
      El = x * ( mB - qplus ) / 2;
      if ( El > ml && El < mB / 2 )
      {
 
        Eh = z * ( mB - qplus ) / 2 + qplus;
        if ( Eh > 0 && Eh < mB )
        {
 
          sh = p2 * pow( mB - qplus, 2 ) + 2 * qplus * ( Eh - qplus ) + qplus * qplus;
          if ( sh > _masses[0] * _masses[0] && mB * mB + sh - 2 * mB * Eh > ml * ml )
          {
 
            double xran = EvtRandom::Flat();
 
            double y = _dGamma->getdGdxdzdp( x, z, p2 ) / _dGMax * p2;
 
            if ( y > 1 )
              report( WARNING, "EvtGen" )
                  << "EvtVub decay probability > 1 found: " << y << endl;
            if ( y >= xran ) tryit = 0;
          }
        }
      }
    }
    // reweight the Mx distribution
    if ( _nbins > 0 )
    {
      double xran1 = EvtRandom::Flat();
      double m     = sqrt( sh );
      j            = 0;
      while ( j < _nbins && m > _masses[j] ) j++;
      double w = _weights[j - 1];
      if ( w >= xran1 ) rew = false;
    }
    else { rew = false; }
  }
 
  // o.k. we have the three kineamtic variables
  // now calculate a flat cos Theta_H [-1,1] distribution of the
  // hadron flight direction w.r.t the B flight direction
  // because the B is a scalar and should decay isotropic.
  // Then chose a flat Phi_H [0,2Pi] w.r.t the B flight direction
  // and and a flat Phi_L [0,2Pi] in the W restframe w.r.t the
  // W flight direction.
 
  double ctH = EvtRandom::Flat( -1, 1 );
  double phH = EvtRandom::Flat( 0, 2 * M_PI );
  double phL = EvtRandom::Flat( 0, 2 * M_PI );
 
  // now compute the four vectors in the B Meson restframe
 
  double ptmp, sttmp;
  // calculate the hadron 4 vector in the B Meson restframe
 
  sttmp         = sqrt( 1 - ctH * ctH );
  ptmp          = sqrt( Eh * Eh - sh );
  double pHB[4] = { Eh, ptmp * sttmp * cos( phH ), ptmp * sttmp * sin( phH ), ptmp * ctH };
  p4.set( pHB[0], pHB[1], pHB[2], pHB[3] );
  xuhad->init( getDaug( 0 ), p4 );
 
  if ( _storeQplus )
  {
    // cludge to store the hidden parameter q+ with the decay;
    // the lifetime of the Xu is abused for this purpose.
    // tau = 1 ps corresponds to ctau = 0.3 mm -> in order to
    // stay well below BaBars sensitivity we take q+/(10000 GeV) which
    // goes up to 0.0005 in the most extreme cases as ctau in mm.
    // To extract q+ back from the StdHepTrk its necessary to get
    // delta_ctau = Xu->anyDaughter->getVertexTime()-Xu->getVertexTime()
    // where these pseudo calls refere to the StdHep time stored at
    // the production vertex in the lab for each particle. The boost
    // has to be reversed and the result is:
    //
    // q+ = delta_ctau * 10000 GeV/mm * Mass_Xu/Energy_Xu
    //
    xuhad->setLifetime( qplus / 10000. );
  }
 
  // calculate the W 4 vector in the B Meson restrframe
 
  double apWB   = ptmp;
  double pWB[4] = { mB - Eh, -pHB[1], -pHB[2], -pHB[3] };
 
  // first go in the W restframe and calculate the lepton and
  // the neutrino in the W frame
 
  double mW2   = mB * mB + sh - 2 * mB * Eh;
  double beta  = ptmp / pWB[0];
  double gamma = pWB[0] / sqrt( mW2 );
 
  double pLW[4];
 
  ptmp   = ( mW2 - ml * ml ) / 2 / sqrt( mW2 );
  pLW[0] = sqrt( ml * ml + ptmp * ptmp );
 
  double ctL = ( El - gamma * pLW[0] ) / beta / gamma / ptmp;
  if ( ctL < -1 ) ctL = -1;
  if ( ctL > 1 ) ctL = 1;
  sttmp = sqrt( 1 - ctL * ctL );
 
  // eX' = eZ x eW
  double xW[3] = { -pWB[2], pWB[1], 0 };
  // eZ' = eW
  double zW[3] = { pWB[1] / apWB, pWB[2] / apWB, pWB[3] / apWB };
 
  double lx = sqrt( xW[0] * xW[0] + xW[1] * xW[1] );
  for ( j = 0; j < 2; j++ ) xW[j] /= lx;
 
  // eY' = eZ' x eX'
  double yW[3] = { -pWB[1] * pWB[3], -pWB[2] * pWB[3], pWB[1] * pWB[1] + pWB[2] * pWB[2] };
  double ly    = sqrt( yW[0] * yW[0] + yW[1] * yW[1] + yW[2] * yW[2] );
  for ( j = 0; j < 3; j++ ) yW[j] /= ly;
 
  // p_lep = |p_lep| * (  sin(Theta) * cos(Phi) * eX'
  //                    + sin(Theta) * sin(Phi) * eY'
  //                    + cos(Theta) *            eZ')
  for ( j = 0; j < 3; j++ )
    pLW[j + 1] = sttmp * cos( phL ) * ptmp * xW[j] + sttmp * sin( phL ) * ptmp * yW[j] +
                 ctL * ptmp * zW[j];
 
  double apLW = ptmp;
  // calculate the neutrino 4 vector in the W restframe
  // double pNW[4] = {sqrt(mW2)-pLW[0],-pLW[1],-pLW[2],-pLW[3]};
 
  // boost them back in the B Meson restframe
 
  double appLB = beta * gamma * pLW[0] + gamma * ctL * apLW;
 
  ptmp        = sqrt( El * El - ml * ml );
  double ctLL = appLB / ptmp;
 
  if ( ctLL > 1 ) ctLL = 1;
  if ( ctLL < -1 ) ctLL = -1;
 
  double pLB[4] = { El, 0, 0, 0 };
  double pNB[4] = { pWB[0] - El, 0, 0, 0 };
 
  for ( j = 1; j < 4; j++ )
  {
    pLB[j] = pLW[j] + ( ctLL * ptmp - ctL * apLW ) / apWB * pWB[j];
    pNB[j] = pWB[j] - pLB[j];
  }
 
  p4.set( pLB[0], pLB[1], pLB[2], pLB[3] );
  lepton->init( getDaug( 1 ), p4 );
 
  p4.set( pNB[0], pNB[1], pNB[2], pNB[3] );
  neutrino->init( getDaug( 2 ), p4 );
 
  return;
}
 
double EvtVub::findPFermi() {
 
  double ranNum     = EvtRandom::Flat();
  double oOverBins  = 1.0 / ( float( _pf.size() ) );
  int    nBinsBelow = 0;          // largest k such that I[k] is known to be <= rand
  int    nBinsAbove = _pf.size(); // largest k such that I[k] is known to be >  rand
  int    middle;
 
  while ( nBinsAbove > nBinsBelow + 1 )
  {
    middle = ( nBinsAbove + nBinsBelow + 1 ) >> 1;
    if ( ranNum >= _pf[middle] ) { nBinsBelow = middle; }
    else { nBinsAbove = middle; }
  }
 
  double bSize = _pf[nBinsAbove] - _pf[nBinsBelow];
  // binMeasure is always aProbFunc[nBinsBelow],
 
  if ( bSize == 0 )
  {
    // rand lies right in a bin of measure 0.  Simply return the center
    // of the range of that bin.  (Any value between k/N and (k+1)/N is
    // equally good, in this rare case.)
    return ( nBinsBelow + .5 ) * oOverBins;
  }
 
  HepDouble bFract = ( ranNum - _pf[nBinsBelow] ) / bSize;
 
  return ( nBinsBelow + bFract ) * oOverBins;
}