LauRelBreitWignerRes.cc

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// Copyright University of Warwick 2004 - 2014.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

// Authors:
// Thomas Latham
// John Back
// Paul Harrison

#include <iostream>

#include "LauConstants.hh"
#include "LauRelBreitWignerRes.hh"

ClassImp(LauRelBreitWignerRes)


LauRelBreitWignerRes::LauRelBreitWignerRes(LauResonanceInfo* resInfo, const Int_t resPairAmpInt, const LauDaughters* daughters) :
        LauAbsResonance(resInfo, resPairAmpInt, daughters),
        q0_(0.0),
        p0_(0.0),
        pstar0_(0.0),
        resMass_(0.0),
        resMassSq_(0.0),
        resWidth_(0.0),
        resRadius_(0.0),
        parRadius_(0.0),
        mDaugSum_(0.0),
        mDaugSumSq_(0.0),
        mDaugDiff_(0.0),
        mDaugDiffSq_(0.0),
        mParentSq_(0.0),
        mBachSq_(0.0),
        FR0_(1.0),
        FP0_(1.0)
{
}

LauRelBreitWignerRes::~LauRelBreitWignerRes()
{
}

void LauRelBreitWignerRes::initialise()
{
        // Set-up various constants. This must be called again if the mass/width/spin
        // of a resonance changes...  

        resMass_   = this->getMass();
        resWidth_  = this->getWidth();
        resRadius_ = this->getResRadius();
        parRadius_ = this->getParRadius();

        Double_t massDaug1 = this->getMassDaug1();
        Double_t massDaug2 = this->getMassDaug2();
        Double_t massBachelor = this->getMassBachelor();
        Double_t massParent = this->getMassParent();

        // Create the mass squares, sums, differences etc.
        resMassSq_ = resMass_*resMass_;
        mDaugSum_  = massDaug1 + massDaug2;
        mDaugSumSq_ = mDaugSum_*mDaugSum_;
        mDaugDiff_ = massDaug1 - massDaug2;
        mDaugDiffSq_ = mDaugDiff_*mDaugDiff_;
        mParentSq_ = massParent*massParent;
        mBachSq_ = massBachelor*massBachelor;

        // Create an effective resonance pole mass to protect against resonances
        // that are below threshold
        Double_t effResMass = resMass_;
        Double_t effResMassSq = resMassSq_;
        if (resMassSq_ - mDaugSumSq_ < 0.0 ){
                Double_t minMass = mDaugSum_;
                Double_t maxMass = massParent - massBachelor;
                Double_t tanhTerm = std::tanh( (resMass_ - ((minMass + maxMass)/2))/(maxMass-minMass));
                effResMass = minMass + (maxMass-minMass)*(1+tanhTerm)/2;
                effResMassSq = effResMass*effResMass;
        }

        // Decay momentum of either daughter in the resonance rest frame
        // when resonance mass = rest-mass value, m_0 (PDG value)
        Double_t term1 = effResMassSq - mDaugSumSq_;
        Double_t term2 = effResMassSq - mDaugDiffSq_;
        Double_t term12 = term1*term2;
        if (term12 > 0.0) {
                q0_ = TMath::Sqrt(term12)/(2.0*effResMass);
        } else {
                q0_ = 0.0;
        }

        // Momentum of the bachelor particle in the resonance rest frame
        // when resonance mass = rest-mass value, m_0 (PDG value)
        Double_t eBach = (mParentSq_ - effResMassSq - mBachSq_)/(2.0*effResMass);
        Double_t termBach = eBach*eBach - mBachSq_;
        if ( eBach<0.0 || termBach<0.0 ) {
                p0_ = 0.0;
        } else {
                p0_ = TMath::Sqrt( termBach );
        }

        // Momentum of the bachelor particle in the parent rest frame
        // when resonance mass = rest-mass value, m_0 (PDG value)
        Double_t eStarBach = (mParentSq_ + mBachSq_ - effResMassSq)/(2.0*massParent);
        Double_t termStarBach = eStarBach*eStarBach - mBachSq_;
        if ( eStarBach<0.0 || termStarBach<0.0 ) {
                pstar0_ = 0.0;
        } else {
                pstar0_ = TMath::Sqrt( termStarBach );
        }

        // Calculate the Blatt-Weisskopf form factor for the case when m = m_0
        const LauBlattWeisskopfFactor* resBWFactor = this->getResBWFactor();
        const LauBlattWeisskopfFactor* parBWFactor = this->getParBWFactor();
        FR0_ = (resBWFactor!=0) ? resBWFactor->calcFormFactor(q0_) : 1.0;
        FP0_ = (parBWFactor!=0) ? parBWFactor->calcFormFactor(p0_) : 1.0;
}

LauComplex LauRelBreitWignerRes::resAmp(Double_t mass, Double_t spinTerm)
{
        // This function returns the complex dynamical amplitude for a Breit-Wigner resonance,
        // given the invariant mass and cos(helicity) values.

        LauComplex resAmplitude(0.0, 0.0);

        if (mass < 1e-10) {
                std::cerr << "WARNING in LauRelBreitWignerRes::amplitude : mass < 1e-10." << std::endl;
                return LauComplex(0.0, 0.0);
        } else if (q0_ < 1e-30) {
                return LauComplex(0.0, 0.0);
        }

        // Calculate the width of the resonance (as a function of mass)
        // First, calculate the various form factors.
        // NB
        // q is the momentum of either daughter in the resonance rest-frame,
        // p is the momentum of the bachelor in the resonance rest-frame,
        // pstar is the momentum of the bachelor in the parent rest-frame.
        // These quantities have been calculate in LauAbsResonance::amplitude(...)

        const Int_t resSpin = this->getSpin();
        const Double_t resMass = this->getMass();
        const Double_t resWidth = this->getWidth();
        const Double_t resRadius = this->getResRadius();
        const Double_t parRadius = this->getParRadius();
        const Double_t q = this->getQ();
        const Double_t p = this->getP();
        //const Double_t pstar = this->getPstar();

        // If the mass is floating and its value has changed we need to
        // recalculate everything that assumes that value
        // Similarly for the BW radii
        if ( ( (!this->fixMass()) && resMass != resMass_ ) ||
             ( (!this->fixResRadius()) && resRadius != resRadius_ ) ||
             ( (!this->fixParRadius()) && parRadius != parRadius_ ) ) {
                this->initialise();
        }

        const LauBlattWeisskopfFactor* resBWFactor = this->getResBWFactor();
        const LauBlattWeisskopfFactor* parBWFactor = this->getParBWFactor();
        const Double_t fFactorR = (resBWFactor!=0) ? resBWFactor->calcFormFactor(q) : 1.0;
        const Double_t fFactorB = (parBWFactor!=0) ? parBWFactor->calcFormFactor(p) : 1.0;
        const Double_t fFactorRRatio = fFactorR/FR0_;
        const Double_t fFactorBRatio = fFactorB/FP0_;

        const Double_t qRatio = q/q0_;
        Double_t qTerm(0.0);
        if (resSpin == 0) {
                qTerm = qRatio;
        } else if (resSpin == 1) {
                qTerm = qRatio*qRatio*qRatio;
        } else if (resSpin == 2) {
                qTerm = qRatio*qRatio*qRatio*qRatio*qRatio;
        } else {
                qTerm = TMath::Power(qRatio, 2*resSpin + 1);  
        }

        const Double_t totWidth = resWidth*qTerm*(resMass/mass)*fFactorRRatio*fFactorRRatio;

        const Double_t massSq = mass*mass;
        const Double_t massSqTerm = resMassSq_ - massSq;

        // Compute the complex amplitude
        resAmplitude = LauComplex(massSqTerm, resMass*totWidth);

        // Scale by the denominator factor, as well as the spin term and Blatt-Weisskopf factors
        resAmplitude.rescale((fFactorRRatio*fFactorBRatio*spinTerm)/(massSqTerm*massSqTerm + resMassSq_*totWidth*totWidth));

        return resAmplitude;
}

const std::vector<LauParameter*>& LauRelBreitWignerRes::getFloatingParameters()
{
        this->clearFloatingParameters();

        if ( ! this->fixMass() ) {
                this->addFloatingParameter( this->getMassPar() );
        }
        if ( ! this->fixWidth() ) {
                this->addFloatingParameter( this->getWidthPar() );
        }
        if ( ! this->fixResRadius() ) {
                this->addFloatingParameter( this->getResBWFactor()->getRadiusParameter() );
        }
        if ( ! this->fixParRadius() ) {
                this->addFloatingParameter( this->getParBWFactor()->getRadiusParameter() );
        }

        return this->getParameters();
}