Rivet analyses

title: BABAR_2012_I1122034

$X(3915)$ production in $\gamma\gamma\to J/\psi \omega$

Experiment: BABAR (PEP-II)

Inspire ID: 1122034

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 86 (2012) 072002

Beams: e+ e-

Beam energies: (5.3, 5.3)GeV

Run details: - e+ e- > e+ e- gamma gamma with gamma gamma -> X(3915)

Measurement of the mass and angle distributions in $\gamma\gamma\to J/\psi \omega$.

Source code:BABAR_2012_I1122034.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 gamma gamma -> X(3915) -> J/psi omega class BABAR_2012_I1122034 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2012_I1122034);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(Beam(), "Beams");
  declare(FinalState(), "FS");
  declare(UnstableParticles(Cuts::pid == 223 or Cuts::pid == 443), "UFS");
  // histograms
  book(_h_mass, 1, 1, 1);
  for (unsigned int ix = 0; ix < 4; ++ix) {
    book(_h_angle1[ix], 2, 1, 1 + ix);
    if (ix < 3) book(_h_angle2[ix], 3, 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()];
      --ncount;
    }
    else {
      findChildren(child, nRes, ncount);
    }
  }
}

bool findScattered(Particle beam, double& q2) {
  bool found = false;
  Particle scat = beam;
  while (!scat.children().empty()) {
    found = false;
    for (const Particle& p : scat.children()) {
      if (p.pid() == scat.pid()) {
        scat = p;
        found = true;
        break;
      }
    }
    if (!found) break;
  }
  if (!found) return false;
  q2 = -(beam.mom() - scat.mom()).mass2();
  return true;
}

void findChildren(const Particle& p,
                  Particles& pim,
                  Particles& pip,
                  Particles& pi0,
                  unsigned int& ncount) {
  for (const Particle& child : p.children()) {
    if (child.pid() == PID::PIPLUS) {
      pip.push_back(child);
      ncount += 1;
    }
    else if (child.pid() == PID::PIMINUS) {
      pim.push_back(child);
      ncount += 1;
    }
    else if (child.pid() == PID::PI0) {
      pi0.push_back(child);
      ncount += 1;
    }
    else if (child.children().empty()) {
      ncount += 1;
    }
    else {
      findChildren(child, pim, pip, pi0, ncount);
    }
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  // find scattered leptons and calc Q2
  const Beam& beams = apply<Beam>(event, "Beams");
  double q12 = -1, q22 = -1;
  if (!findScattered(beams.beams().first, q12)) vetoEvent;
  if (!findScattered(beams.beams().second, q22)) vetoEvent;
  // check the final state
  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;
  }
  // find the J/psi
  const FinalState& ufs = apply<FinalState>(event, "UFS");
  Particle omega, psi;
  bool found = false;
  for (const Particle& p1 : ufs.particles(Cuts::pid == 443)) {
    if (p1.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p1, nRes, ntotal);
    for (const Particle& p2 : ufs.particles(Cuts::pid == 223)) {
      if (p2.children().empty()) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(p2, nRes2, ncount2);
      found = true;
      for (const auto& val : nRes2) {
        if (abs(val.first) == 11) {
          if (val.second != 1) {
            found = false;
            break;
          }
        }
        else if (val.second != 0) {
          found = false;
          break;
        }
      }
      if (found) {
        psi = p1;
        omega = p2;
        break;
      }
    }
  }
  if (!found) vetoEvent;
  FourMomentum psum = omega.mom() + psi.mom();
  if (psum.pT() > 0.2) vetoEvent;
  // mass distribution
  _h_mass->fill(psum.mass());
  // from now on we need specific decay modes of J/psi and omega
  // first J/psi -> l+l-
  if (psi.children().size() != 2) vetoEvent;
  if (psi.children()[0].pid() != -psi.children()[1].pid()) vetoEvent;
  if (psi.children()[0].abspid() != 11 && psi.children()[0].abspid() != 13) vetoEvent;
  Particle ep = psi.children()[0];
  Particle em = psi.children()[1];
  if (ep.pid() > 0) swap(ep, em);
  // omega decay
  unsigned int ncount = 0;
  Particles pip, pim, pi0;
  findChildren(omega, pim, pip, pi0, ncount);
  if (ncount != 3 || !(pim.size() == 1 && pip.size() == 1 && pi0.size() == 1)) vetoEvent;
  // boost to gamma gamma frame
  LorentzTransform boostCMS = LorentzTransform::mkFrameTransformFromBeta(psum.betaVec());
  FourMomentum pPsi = boostCMS.transform(psi.mom());
  FourMomentum pOmega = boostCMS.transform(omega.mom());
  FourMomentum pLp = boostCMS.transform(ep.mom());
  FourMomentum pPip = boostCMS.transform(pip[0].mom());
  FourMomentum pPim = boostCMS.transform(pim[0].mom());
  Vector3 axis(0., 0., 1.);
  // lepton angle
  double cosL = pLp.p3().unit().dot(axis);
  _h_angle1[0]->fill(cosL);
  LorentzTransform boostOmega = LorentzTransform::mkFrameTransformFromBeta(pOmega.betaVec());
  pPip = boostOmega.transform(pPip);
  pPim = boostOmega.transform(pPim);
  // omega decay plane normal angle
  Vector3 axisOmega = pPip.p3().cross(pPim.p3()).unit();
  double cosN = axisOmega.dot(axis);
  _h_angle1[1]->fill(cosN);
  // angle lepton and omega
  _h_angle1[2]->fill(pLp.p3().unit().dot(axisOmega));
  // helicity angle
  _h_angle1[3]->fill(pPsi.p3().unit().dot(psum.p3().unit()));
  // now the new frame
  Vector3 axisZ = pOmega.p3().unit();
  Vector3 axisY = axisZ.cross(axisOmega);
  Vector3 axisX = axisY.cross(axisZ);
  // second set of angles
  LorentzTransform boostPsi = LorentzTransform::mkFrameTransformFromBeta(pPsi.betaVec());
  Vector3 axisL = boostPsi.transform(pLp).p3().unit();
  _h_angle2[0]->fill(axisZ.dot(axisOmega));
  _h_angle2[1]->fill(axisL.dot(pPsi.p3().unit()));
  axisZ *= -1.;
  axisX *= -1.;
  Vector3 axisnp = axisL.cross(axisZ).unit();
  double phiL = atan2(axisnp.dot(axisY), axisnp.dot(axisX));
  double phiN = atan2(axisOmega.dot(axisY), axisOmega.dot(axisX));
  _h_angle2[2]->fill(mapAngleMPiToPi(phiL - phiN) / M_PI * 180.);
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_mass, 1.0, false);
  normalize(_h_angle1, 1.0, false);
  normalize(_h_angle2, 1.0, false);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_mass, _h_angle1[4], _h_angle2[3];
/// @}

};

RIVET_DECLARE_PLUGIN(BABAR_2012_I1122034);

} ```