EvtVubHybrid.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: EvtVubHybrid.cc
//
// Description: Routine to decay a particle according to phase space.
//
// Modification history:
//
//   Jochen Dingfelder February 1, 2005  Created Module as update of the
//                                       original module EvtVub by Sven Menke
//---------------------------------------------------------------------------
//
#include "EvtVubHybrid.hh"
#include "CLHEP/Random/RandGeneral.h"
#include "../EvtGenBase/EvtGenKine.hh"
#include "../EvtGenBase/EvtPDL.hh"
#include "../EvtGenBase/EvtParticle.hh"
#include "../EvtGenBase/EvtPatches.hh"
#include "../EvtGenBase/EvtRandom.hh"
#include "../EvtGenBase/EvtReport.hh"
#include "../EvtGenBase/EvtVector4R.hh"
#include "EvtPFermi.hh"
#include "EvtVubdGamma.hh"
#include <stdlib.h>
 
#include <fstream>
#include <iostream>
#include <string>
 
typedef double HepDouble;
// #include "CLHEP/config/CLHEP.h" //maqm add
// #include "CLHEP/config/TemplateFunctions.h" //maqm add
 
using std::cout;
using std::endl;
using std::ifstream;
 
// _noHybrid will be set TRUE if the DECAY.DEC file has no binning or weights
// _storeQplus should alwasy be TRUE: writes out Fermi motion parameter
 
EvtVubHybrid::EvtVubHybrid()
    : _noHybrid( false )
    , _storeQplus( true )
    , _mb( 4.62 )
    , _a( 2.27 )
    , _alphas( 0.22 )
    , _dGMax( 3. )
    , _nbins_mX( 0 )
    , _nbins_q2( 0 )
    , _nbins_El( 0 )
    , _nbins( 0 )
    , _masscut( 0.28 )
    , _bins_mX( 0 )
    , _bins_q2( 0 )
    , _bins_El( 0 )
    , _weights( 0 )
    , _dGamma( 0 ) {}
 
EvtVubHybrid::~EvtVubHybrid() {
  delete _dGamma;
  delete[] _bins_mX;
  delete[] _bins_q2;
  delete[] _bins_El;
  delete[] _weights;
}
 
void EvtVubHybrid::getName( std::string& model_name ) { model_name = "VUBHYBRID"; }
 
EvtDecayBase* EvtVubHybrid::clone() { return new EvtVubHybrid; }
 
void EvtVubHybrid::init() {
 
  // check that there are at least 3 arguments
  if ( getNArg() < EvtVubHybrid::nParameters )
  {
    report( ERROR, "EvtVubHybrid" )
        << "EvtVub generator expected "
        << "at least " << EvtVubHybrid::nParameters << " arguments but found: " << getNArg()
        << "\nWill terminate execution!" << endl;
    ::abort();
  }
  else if ( getNArg() == EvtVubHybrid::nParameters )
  {
    report( WARNING, "EvtVubHybrid" ) << "EvtVub: generate B -> Xu l nu events "
                                      << "without using the hybrid reweighting." << endl;
    _noHybrid = true;
  }
  else if ( getNArg() < EvtVubHybrid::nParameters + EvtVubHybrid::nVariables )
  {
    report( ERROR, "EvtVubHybrid" ) << "EvtVub could not read number of bins for "
                                    << "all variables used in the reweighting\n"
                                    << "Will terminate execution!" << endl;
    ::abort();
  }
 
  // check that there are 3 daughters
  checkNDaug( 3 );
 
  // read minimum required parameters from decay.dec
  _mb     = getArg( 0 );
  _a      = getArg( 1 );
  _alphas = getArg( 2 );
 
  // 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
 
  static double mB0 = EvtPDL::getMaxMass( EvtPDL::getId( "B0" ) );
  static double mBP = EvtPDL::getMaxMass( EvtPDL::getId( "B+" ) );
  static 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 ( int 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 ( int i = 0; i < _pf.size(); i++ ) _pf[i] /= _pf[_pf.size() - 1];
 
  _dGamma = new EvtVubdGamma( _alphas );
 
  if ( _noHybrid ) return; // Without hybrid weighting, nothing else to do
 
  _nbins_mX = abs( (int)getArg( 3 ) );
  _nbins_q2 = abs( (int)getArg( 4 ) );
  _nbins_El = abs( (int)getArg( 5 ) );
 
  int nextArg = EvtVubHybrid::nParameters + EvtVubHybrid::nVariables;
 
  _nbins = _nbins_mX * _nbins_q2 * _nbins_El; // Binning of weight table
 
  int expectArgs = nextArg + _nbins_mX + _nbins_q2 + _nbins_El + _nbins;
 
  if ( getNArg() < expectArgs )
  {
    report( ERROR, "EvtVubHybrid" )
        << " finds " << getNArg() << " arguments, expected " << expectArgs
        << ".  Something is wrong with the tables of weights or thresholds."
        << "\nWill terminate execution!" << endl;
    ::abort();
  }
 
  // read bin boundaries from decay.dec
  int i;
 
  _bins_mX = new double[_nbins_mX];
  for ( i = 0; i < _nbins_mX; i++, nextArg++ ) { _bins_mX[i] = getArg( nextArg ); }
  _masscut = _bins_mX[0];
 
  _bins_q2 = new double[_nbins_q2];
  for ( i = 0; i < _nbins_q2; i++, nextArg++ ) { _bins_q2[i] = getArg( nextArg ); }
 
  _bins_El = new double[_nbins_El];
  for ( i = 0; i < _nbins_El; i++, nextArg++ ) { _bins_El[i] = getArg( nextArg ); }
 
  // read in weights (and rescale to range 0..1)
  readWeights( nextArg );
}
 
void EvtVubHybrid::initProbMax() { noProbMax(); }
 
void EvtVubHybrid::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;
  double q2, mX;
 
  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-_masscut^2)
    kplus = 2 * xhigh;
 
    while (
        kplus >= xhigh || kplus <= xlow ||
        ( _alphas == 0 && kplus >= mB / 2 - _mb + sqrt( mB * mB / 4 - _masscut * _masscut ) ) )
    {
      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 > _masscut * _masscut && 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, "EvtVubHybrid" )
                  << "EvtVubHybrid decay probability > 1 found: " << y << endl;
            if ( y >= xran ) tryit = 0;
          }
        }
      }
    }
 
    // compute all kinematic variables needed for reweighting (J. Dingfelder)
    mX = sqrt( sh );
    q2 = mB * mB + sh - 2 * mB * Eh;
 
    // Reweighting in bins of mX, q2, El (J. Dingfelder)
    if ( _nbins > 0 )
    {
      double xran1 = EvtRandom::Flat();
      double w     = 1.0;
      if ( !_noHybrid ) w = getWeight( mX, q2, El );
      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 EvtVubHybrid::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;
}
 
double EvtVubHybrid::getWeight( double mX, double q2, double El ) {
 
  int ibin_mX = -1;
  int ibin_q2 = -1;
  int ibin_El = -1;
 
  for ( int i = 0; i < _nbins_mX; i++ )
  {
    if ( mX >= _bins_mX[i] ) ibin_mX = i;
  }
  for ( int i = 0; i < _nbins_q2; i++ )
  {
    if ( q2 >= _bins_q2[i] ) ibin_q2 = i;
  }
  for ( int i = 0; i < _nbins_El; i++ )
  {
    if ( El >= _bins_El[i] ) ibin_El = i;
  }
  int ibin = ibin_mX + ibin_q2 * _nbins_mX + ibin_El * _nbins_mX * _nbins_q2;
 
  if ( ( ibin_mX < 0 ) || ( ibin_q2 < 0 ) || ( ibin_El < 0 ) )
  {
    report( ERROR, "EvtVubHybrid" ) << "Cannot determine hybrid weight "
                                    << "for this event "
                                    << "-> assign weight = 0" << endl;
    return 0.0;
  }
 
  return _weights[ibin];
}
 
void EvtVubHybrid::readWeights( int startArg ) {
  _weights = new double[_nbins];
 
  double maxw = 0.0;
  for ( int i = 0; i < _nbins; i++, startArg++ )
  {
    _weights[i] = getArg( startArg );
    if ( _weights[i] > maxw ) maxw = _weights[i];
  }
 
  if ( maxw == 0 )
  {
    report( ERROR, "EvtVubHybrid" ) << "EvtVub generator expected at least one "
                                    << " weight > 0, but found none! "
                                    << "Will terminate execution!" << endl;
    ::abort();
  }
 
  // rescale weights (to be in range 0..1)
  for ( int i = 0; i < _nbins; i++ ) { _weights[i] /= maxw; }
}