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src/EvtGenModels/EvtEtaLLPiPi.cpp

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* Copyright 1998-2020 CERN for the benefit of the EvtGen authors *
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* This file is part of EvtGen. *
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* GNU General Public License for more details. *
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#include "EvtGenModels/EvtEtaLLPiPi.hh"
#include "EvtGenBase/EvtId.hh"
#include "EvtGenBase/EvtKine.hh"
#include "EvtGenBase/EvtPDL.hh"
#include "EvtGenBase/EvtParticle.hh"
#include "EvtGenBase/EvtSpinType.hh"
#include "EvtGenBase/EvtVector4R.hh"
#include <cmath>
// eta' -> mu+ mu- pi+ pi- or e+ e- pi+ pi-
// From Zhang Zhen-Yu et al, Chinese Phys. C 36, p926, 2012
void EvtEtaLLPiPi::init()
{
// Check for 0 or 1 (maxProb) arguments
checkNArg( 0, 1 );
// Check particle types
checkSpinParent( EvtSpinType::SCALAR );
checkSpinDaughter( 0, EvtSpinType::DIRAC );
checkSpinDaughter( 1, EvtSpinType::DIRAC );
checkSpinDaughter( 2, EvtSpinType::SCALAR );
checkSpinDaughter( 3, EvtSpinType::SCALAR );
// Mass and width of rho0 from particle properties file
m_rhoMass = EvtPDL::getMeanMass( EvtPDL::getId( "rho0" ) );
m_rhoMassSq = m_rhoMass * m_rhoMass;
m_rhoGamma = EvtPDL::getWidth( EvtPDL::getId( "rho0" ) );
// Mixing parameter squared, using Eq 6
const double denom = 8.0 * pow( EvtConst::pi * m_fPi, 2 );
const double factor = m_eSq / ( denom * denom * 3.0 );
const double fTerm8 = sin( m_thetaMix ) / m_f8;
const double fTerm0 = 2.0 * sqrt( 2.0 ) * cos( m_thetaMix ) / m_f0;
tlathamUnsubmitted
Done
Inline Actions

Would be better to give these values in the header.
Then this constructor can be removed from here and just be '=default' in the header.

tlatham: Would be better to give these values in the header. Then this constructor can be removed from…
m_mixSq = factor * pow( fTerm8 + fTerm0, 2 );
}
void EvtEtaLLPiPi::initProbMax()
{
if ( getNArg() == 1 ) {
setProbMax( getArg( 0 ) );
} else {
int lepId = getDaug( 0 ).getId();
if ( lepId == EvtPDL::getId( "e-" ).getId() ||
lepId == EvtPDL::getId( "e+" ).getId() ) {
setProbMax( 27500.0 );
} else if ( lepId == EvtPDL::getId( "mu-" ).getId() ||
lepId == EvtPDL::getId( "mu+" ).getId() ) {
setProbMax( 20.0 );
}
}
}
std::string EvtEtaLLPiPi::getName()
{
return "ETA_LLPIPI";
}
EvtDecayBase* EvtEtaLLPiPi::clone()
{
return new EvtEtaLLPiPi();
}
void EvtEtaLLPiPi::decay( EvtParticle* p )
{
p->initializePhaseSpace( getNDaug(), getDaugs() );
const double mLep = p->getDaug( 0 )->mass();
const double mPi = p->getDaug( 2 )->mass();
updateMassPars( mLep, mPi );
const double prob = ampSquared( p );
setProb( prob );
}
void EvtEtaLLPiPi::updateMassPars( double mLep, double mPi )
{
// Update mass parameters used in various functions
m_lepMass = mLep;
m_lepMassSq = mLep * mLep;
m_4LepMassSq = 4.0 * m_lepMassSq;
m_piMass = mPi;
m_piMassSq = mPi * mPi;
m_4PiMassSq = 4.0 * m_piMassSq;
}
double EvtEtaLLPiPi::rhoWidth( double s, double m ) const
{
// Define width of rho using modified vector meson dynamics, Eq 8
double gamma( 0.0 );
if ( s >= m_4PiMassSq ) {
const double mSq = m * m;
const double num = 1.0 - ( 4.0 * mSq / s );
const double denom = 1.0 - ( 4.0 * mSq / m_rhoMassSq );
const double ratio = denom > 0.0 ? num / denom : 0.0;
gamma = m_rhoGamma * ( s / m_rhoMassSq ) * pow( ratio, 1.5 );
}
return gamma;
}
double EvtEtaLLPiPi::F0( double sLL, double sPiPi ) const
{
// Equation 7
double ampSq( 0.0 );
const double rhoWidthL = rhoWidth( sLL, m_lepMass );
const double rhoWidthPi = rhoWidth( sPiPi, m_piMass );
const double mSqDiffL = m_rhoMassSq - sLL;
const double mRhoWidthL = m_rhoMass * rhoWidthL;
const double mSqDiffPi = m_rhoMassSq - sPiPi;
const double mRhoWidthPi = m_rhoMass * rhoWidthPi;
const double denomLL = mSqDiffL * mSqDiffL + mRhoWidthL * mRhoWidthL;
const double denomPiPi = mSqDiffPi * mSqDiffPi + mRhoWidthPi * mRhoWidthPi;
if ( denomLL > 0.0 && denomPiPi > 0.0 ) {
const double mRho4 = m_rhoMassSq * m_rhoMassSq;
const double denomProd = denomLL * denomPiPi;
double realAmp = m_par1 + m_parLL * ( m_rhoMassSq * mSqDiffL / denomLL );
realAmp += m_parPiPi * mRho4 *
( ( mSqDiffPi * mSqDiffL ) - mRhoWidthL * mRhoWidthPi ) /
denomProd;
double imagAmp = m_parLL * ( m_rhoMassSq * mRhoWidthL / denomLL );
imagAmp += m_parPiPi * mRho4 *
( mRhoWidthPi * mSqDiffL + mRhoWidthL * mSqDiffPi ) /
denomProd;
ampSq = realAmp * realAmp + imagAmp * imagAmp;
}
return ampSq;
}
double EvtEtaLLPiPi::lambda( double a, double b, double c ) const
{
const double sumSq = a * a + b * b + c * c;
const double prod = a * b + b * c + c * a;
const double L = sumSq - 2.0 * prod;
return L;
}
double EvtEtaLLPiPi::ampSquared( EvtParticle* p ) const
{
// Equation 3
const double zeroProb( 0.0 );
// Mass of eta' meson
const double mEta = p->mass();
const EvtVector4R pL1 = p->getDaug( 0 )->getP4();
const EvtVector4R pL2 = p->getDaug( 1 )->getP4();
const EvtVector4R pPi1 = p->getDaug( 2 )->getP4();
const EvtVector4R pPi2 = p->getDaug( 3 )->getP4();
const EvtVector4R pLL = pL1 + pL2;
const double sLL = pLL.mass2();
const EvtVector4R pPiPi = pPi1 + pPi2;
const double sPiPi = pPiPi.mass2();
if ( sLL < 1e-4 || sPiPi < m_4PiMassSq || sLL < m_4LepMassSq ) {
// To avoid negative square roots etc
return zeroProb;
}
// Angles theta_p, theta_k and phi defined in Fig 1
const EvtVector4R pSum = pLL + pPiPi;
// Helicity angle of first lepton
const double cosThp = -EvtDecayAngle( pSum, pLL, pL1 );
const double sinThp = sqrt( 1.0 - cosThp * cosThp );
// Helicity angle of first pion
const double cosThk = -EvtDecayAngle( pSum, pPiPi, pPi2 );
const double sinThk = sqrt( 1.0 - cosThk * cosThk );
// Angle between the dilepton and dipion decay planes
const double phi = EvtDecayAngleChi( pSum, pL1, pL2, pPi1, pPi2 );
const double sinPhi = sin( phi );
const double betaLL = sqrt( 1.0 - ( m_4LepMassSq / sLL ) );
const double betaPiPi = sqrt( 1.0 - ( m_4PiMassSq / sPiPi ) );
const double betaProd = ( 1.0 - pow( betaLL * sinThp * sinPhi, 2 ) ) *
sPiPi * pow( betaPiPi * sinThk, 2 );
const double L = lambda( mEta * mEta, sLL, sPiPi );
const double ampSq = m_eSq * F0( sLL, sPiPi ) * m_mixSq * L * betaProd /
( 8.0 * sLL );
return ampSq;
}