LauLASSBWRes.cc

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// Copyright University of Warwick 2008 - 2013.
// 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 "LauLASSBWRes.hh"

ClassImp(LauLASSBWRes)


LauLASSBWRes::LauLASSBWRes(TString resName, Double_t resMass, Double_t resWidth, Int_t resSpin,
                Int_t resCharge, Int_t resPairAmpInt, const LauDaughters* daughters) :
        LauAbsResonance(resName, resMass, resWidth, resSpin, resCharge, resPairAmpInt, daughters),
        q0_(0.0),
        mDaugSum_(0.0),
        mDaugSumSq_(0.0),
        mDaugDiff_(0.0),
        mDaugDiffSq_(0.0),
        resMassSq_(0.0),
        r_(0.0),
        a_(0.0),
        B_(0.0),
        R_(0.0),
        phiB_(0.0),
        phiR_(0.0),
        cutOff_(0.0)
{
        // Default values for LASS parameters
        r_ = 3.32;
        a_ = 2.07;
        R_ = 1.0;
        B_ = 1.0;
        phiR_ = 0.0;
        phiB_ = 0.0;
        cutOff_ = 1.8;
}

LauLASSBWRes::~LauLASSBWRes()
{
}

void LauLASSBWRes::initialise()
{
        // Create the mass sums and differences
        Double_t massDaug1 = this->getMassDaug1();
        Double_t massDaug2 = this->getMassDaug2();

        mDaugSum_  = massDaug1 + massDaug2;
        mDaugSumSq_ = mDaugSum_*mDaugSum_;

        mDaugDiff_ = massDaug1 - massDaug2;
        mDaugDiffSq_ = mDaugDiff_*mDaugDiff_;

        // Decay momentum of either daughter in the resonance rest frame
        // when resonance mass = rest-mass value, m_0 (PDG value)

        Double_t resMass = this->getMass();
        resMassSq_ = resMass*resMass;

        q0_ = TMath::Sqrt((resMassSq_ - mDaugSumSq_)*(resMassSq_ - mDaugDiffSq_))/(2.0*resMass);

        Int_t resSpin = this->getSpin();
        if (resSpin != 0) {
                std::cerr << "WARNING in LauLASSBWRes::amplitude : Resonance spin is " << resSpin << "." << std::endl;
                std::cerr << "                                   : LASS amplitude is only for scalers, resetting spin to 0." << std::endl;
                this->changeResonance( -1.0, -1.0, 0 );
        }
}

LauComplex LauLASSBWRes::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 LauLASSBWRes::amplitude : mass < 1e-10." << std::endl;
                return LauComplex(0.0, 0.0);
        }

        // Calculate the width of the resonance (as a function of mass)
        // q is the momentum of either daughter in the resonance rest-frame
        Double_t q = this->getQ();
        Double_t resMass = this->getMass();
        Double_t resWidth = this->getWidth();

        Double_t qRatio = q/q0_;
        Double_t totWidth = resWidth*qRatio*(resMass/mass);

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

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

        // Scale by the denominator factor
        resAmplitude.rescale(spinTerm*(resMassSq_*resWidth/q0_)/(massSqTerm*massSqTerm + resMassSq_*totWidth*totWidth));

        // Calculate the phase shift term
        Double_t deltaB = TMath::ATan((2.0*a_*q)/(2.0 + a_*r_*q*q));
        Double_t cos2PhaseShift = TMath::Cos(2.0*(deltaB + phiB_));
        Double_t sin2PhaseShift = TMath::Sin(2.0*(deltaB + phiB_));
        LauComplex phaseShift(cos2PhaseShift, sin2PhaseShift);

        // Add in the R e^{i phiR} term
        Double_t reR = R_ * TMath::Cos(phiR_);
        Double_t imR = R_ * TMath::Sin(phiR_);
        LauComplex R(reR, imR);

        // Multiply by the phase shift and R e^{i phiR}
        resAmplitude = resAmplitude * phaseShift * R;

        // There is no spin term for the LASS shape 
        // Just set it 1.0 in case anyone decides to use it at a later date.
        spinTerm = 1.0;

        return resAmplitude;

}

void LauLASSBWRes::setResonanceParameter(const TString& name, const Double_t value) 
{
        // Set various parameters for the LASS lineshape dynamics
        if (name == "a") {
                this->setScatteringLength(value);
                std::cout << "INFO in LauLASSBWRes::setResonanceParameter : Setting LASS Scattering Length = " << this->getScatteringLength() << std::endl;
        }
        else if (name == "r") {
                this->setEffectiveRange(value);
                std::cout << "INFO in LauLASSBWRes::setResonanceParameter : Setting LASS Effective Range = " << this->getEffectiveRange() << std::endl;
        }
        else if (name == "R") {
                this->setResonanceMag(value);
                std::cout << "INFO in LauLASSBWRes::setResonanceParameter : Setting LASS Resonance Magnitude = " << this->getResonanceMag() << std::endl;
        }
        else if (name == "B") {
                this->setBackgroundMag(value);
                std::cout << "INFO in LauLASSBWRes::setResonanceParameter : Setting LASS Background Magnitude = " << this->getBackgroundMag() << std::endl;
        }
        else if (name == "phiR") {
                this->setResonancePhase(value);
                std::cout << "INFO in LauLASSBWRes::setResonanceParameter : Setting LASS Resonance Phase = " << this->getResonancePhase() << std::endl;
        }
        else if (name == "phiB") {
                this->setBackgroundPhase(value);
                std::cout << "INFO in LauLASSBWRes::setResonanceParameter : Setting LASS Background Phase = " << this->getBackgroundPhase() << std::endl;
        }
        else if (name == "cutOff") {
                this->setCutOff(value);
                std::cout << "INFO in LauLASSBWRes::setResonanceParameter : Setting LASS Cut Off = " << this->getCutOff() << std::endl;
        }
        else {
                std::cerr << "WARNING in LauLASSBWRes::setResonanceParameter: Parameter name not reconised.  No parameter changes made." << std::endl;
        }
}