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Rivet analyses reference
BESIII_2019_I1716256
Form factors for the decays $\chi_{c(1,2)}\to J/\psi \mu^+\mu^-$
Experiment: BESIII (BEPC)
Inspire ID: 1716256
Status: VALIDATED NOHEPDATA
Authors:
References:
- Phys.Rev.D 99 (2019) 5, 051101
Beams: * *
Beam energies: ANY
Measurement of the form factors and differential branching ratios for the decays $\chi_{c(1,2)}\to J/\psi \mu^+\mu^-$ by the BESIII collaboration.
Source code:
BESIII_2019_I1716256.cc
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93 | // -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"
#include "Rivet/Projections/DecayedParticles.hh"
namespace Rivet {
/// @brief chi_c -> J/psi mu+mu-
class BESIII_2019_I1716256 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2019_I1716256);
/// @name Analysis methods
/// @{
/// Book histograms and initialise projections before the run
void init() {
UnstableParticles ufs = UnstableParticles(Cuts::pid==20443||Cuts::pid==445);
declare(ufs, "UFS");
DecayedParticles chi(ufs);
chi.addStable(PID::JPSI);
declare(chi, "chi");
// Initialise and register projections
for(unsigned int ix=0;ix<2;++ix) {
for(unsigned int iy=0;iy<2;++iy) {
book(_h[ix][iy],1+ix,1,1+iy);
book(_n[ix][iy],"TMP/n_"+toString(ix+1)+"_"+toString(iy+1));
}
}
}
/// Perform the per-event analysis
void analyze(const Event& event) {
static double alpha= 7.2973525664e-3;
static const map<PdgId,unsigned int> & mode1 = { { 443,1}, {22,1} };
static const map<PdgId,unsigned int> & mode2 = { { 443,1}, {13,1}, {-13,1} };
DecayedParticles chi = apply<DecayedParticles>(event, "chi");
// loop over particles
for(unsigned int ix=0;ix<chi.decaying().size();++ix) {
unsigned int iloc=chi.decaying()[ix].pid()==20443 ? 0 : 1;
_n[iloc][0]->fill();
if(chi.modeMatches(ix,2,mode1)) {
_n[iloc][1]->fill();
}
else if(chi.modeMatches(ix,3,mode2)) {
const Particle & mum = chi.decayProducts()[ix].at( 13)[0];
const Particle & mup = chi.decayProducts()[ix].at(-13)[0];
const Particle & Jpsi = chi.decayProducts()[ix].at(443)[0];
double q = (mum.momentum()+mup.momentum()).mass();
// br
_h[iloc][0]->fill(q);
double M2 = chi.decaying()[ix].mass2();
double m = Jpsi.mass();
// factor for form factor (eqn 2 arXiv:1709.02444)
double fact = alpha/3./M_PI/sqr(q)*
sqrt((1.-sqr(m+q)/M2)*(1.-sqr(m-q)/M2))/(1.-sqr(m)/M2)*
(1.+2.*sqr(mum.mass()/q))*sqrt(1.-4.*sqr(mum.mass()/q));
// additional factor of 2q from dq^2 = 2q dq
_h[iloc][1]->fill(q,1./fact/2/q);
}
}
}
/// Normalise histograms etc., after the run
void finalize() {
for(unsigned int ix=0;ix<2;++ix) {
for(unsigned int iy=0;iy<2;++iy) {
scale(_h[ix][iy], 1./ *_n[ix][iy]);
}
scale(_h[ix][0], 1e5);
}
}
/// @}
/// @name Histograms
///@{
Histo1DPtr _h[2][2];
CounterPtr _n[2][2];
///@}
};
RIVET_DECLARE_PLUGIN(BESIII_2019_I1716256);
}
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