Rivet analyses

title: BESIII_2017_I1624548

Analysis of $\psi(2S)\to\gamma\chi_{c2}$ decays using $\chi_{c2}\to\gamma\gamma$

Experiment: BESIII (BEPC)

Inspire ID: 1624548

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 96 (2017) 9, 092007

Beams: e- e+

Beam energies: (1.8, 1.8)GeV

Run details: - e+e- > psi(2S)

Analysis of the angular distribution of the photons and leptons produced in $e^+e^-\to \psi(2S) \to \gamma\chi_{c2}$ followed by $\chi_{c2}\to\gamma \gamma$. Gives information about the decay and is useful for testing correlations in charmonium decays. N.B. the data was read from the figures in the paper and is not corrected and should only be used qualatively.

Source code:BESIII_2017_I1624548.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/Beam.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief psi(2S) -> gamma chi_c1,2 class BESIII_2017_I1624548 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2017_I1624548);


/// @name Analysis methods
/// @{

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(Beam(), "Beams");
  declare(UnstableParticles(Cuts::pid == 445), "UFS");
  declare(FinalState(), "FS");
  for (unsigned int ix = 0; ix < 3; ++ix) book(_h[ix], 1, 1, 1 + ix);
}

void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) {
  for (const Particle& child : p.children()) {
    if (child.children().empty()) {
      nRes[child.pid()] -= 1;
      --ncount;
    }
    else
      findChildren(child, nRes, ncount);
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  // get the axis, direction of incoming electron
  const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
  Vector3 axis;
  if (beams.first.pid() > 0)
    axis = beams.first.momentum().p3().unit();
  else
    axis = beams.second.momentum().p3().unit();
  // types of final state particles
  const FinalState& fs = apply<FinalState>(event, "FS");
  map<long, int> nCount;
  int ntotal(0);
  for (const Particle& p : fs.particles()) {
    nCount[p.pid()] += 1;
    ++ntotal;
  }
  // loop over chi_c states
  Particle chi;
  bool matched = false;
  const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
  for (const Particle& p : ufs.particles()) {
    if (p.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    if (ncount == 1) {
      matched = true;
      for (const auto& val : nRes) {
        if (val.first == PID::PHOTON) {
          if (val.second != 1) {
            matched = false;
            break;
          }
        }
        else if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        chi = p;
        break;
      }
    }
  }
  if (!matched) vetoEvent;
  // have chi_c find psi2S
  if (chi.parents().empty() || chi.children().size() != 2) vetoEvent;
  Particle psi2S = chi.parents()[0];
  if (psi2S.pid() != 100443 || psi2S.children().size() != 2) vetoEvent;
  // then the first photon
  Particle gamma1;
  if (psi2S.children()[0].pid() == PID::PHOTON)
    gamma1 = psi2S.children()[0];
  else if (psi2S.children()[1].pid() == PID::PHOTON)
    gamma1 = psi2S.children()[1];
  else
    vetoEvent;
  // and second photon
  Particle gamma2;
  if (chi.children()[0].pid() == PID::PHOTON && chi.children()[1].pid() == PID::PHOTON) {
    gamma2 = chi.children()[0];
  }
  else
    vetoEvent;
  // first angle of gamma1 w.r.t beam
  _h[0]->fill(axis.dot(gamma1.momentum().p3().unit()));
  // axis in the chi frame
  LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(chi.momentum().betaVec());
  Vector3 e1z = gamma1.momentum().p3().unit();
  Vector3 e1y = e1z.cross(axis).unit();
  Vector3 e1x = e1y.cross(e1z).unit();
  // cos theta_2 and phi 2 distributions
  FourMomentum pGamma2 = boost1.transform(gamma2.momentum());
  Vector3 axis1 = pGamma2.p3().unit();
  _h[1]->fill(e1z.dot(axis1));
  double phi2 = atan2(e1y.dot(axis1), e1x.dot(axis1));
  if (phi2 < 0) phi2 += 2. * M_PI;
  _h[2]->fill(phi2);
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 3; ++ix) {
    normalize(_h[ix], 1., false);
  }
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[3];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2017_I1624548);

} ```