Rivet analyses

title: BESIII_2022_I2099126

Measurement of $\Lambda\to n\gamma$ decay asymmetry using $J/\psi$ decays to $\Lambda^0\bar\Lambda^0$

Experiment: BESIII (BEPC)

Inspire ID: 2099126

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - arXiv: 2206.10791

Beams: e- e+

Beam energies: (1.6, 1.6)GeV

Run details: none listed

Analysis of the angular distribution of the baryons, and decay products, produced in $e^+e^-\to J/\psi \to \Lambda^0\bar\Lambda^0$ with the decay $\Lambda\to n\gamma$. Gives information about the decay and is useful for testing correlations in hadron decays. N.B. the moment data is not corrected and should only be used qualatively.

Source code:BESIII_2022_I2099126.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 JPsi > Lambda, Lambdabar with Lambda -> n gamma class BESIII_2022_I2099126 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2022_I2099126);


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

/// Book histograms and initialise projections before the run
void init() {

  // Initialise and register projections
  declare(Beam(), "Beams");
  declare(UnstableParticles(), "UFS");
  declare(FinalState(), "FS");
  for (unsigned int ix = 0; ix < 2; ++ix) {
    book(_n[ix], "TMP/n_" + toString(ix + 1));
    book(_t[ix], "TMP/t_" + toString(ix + 1));
    for (unsigned int iy = 0; iy < 2; ++iy) {
      book(_h_mu[ix][iy], 1, 1, 2 * ix + iy + 1);
    }
  }
  book(_n[2], "TMP/n_3");
  book(_t[2], "TMP/t_3");
}

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 lambda0 baryons
  const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
  Particle Lambda, LamBar;
  bool matched(false);
  for (const Particle& p : ufs.particles(Cuts::abspid == 3122)) {
    if (p.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    matched = false;
    // check for antiparticle
    for (const Particle& p2 : ufs.particles(Cuts::pid == -p.pid())) {
      if (p2.children().empty()) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(p2, nRes2, ncount2);
      if (ncount2 == 0) {
        matched = true;
        for (const auto& val : nRes2) {
          if (val.second != 0) {
            matched = false;
            break;
          }
        }
        // found baryon and antibaryon
        if (matched) {
          if (p.pid() > 0) {
            Lambda = p;
            LamBar = p2;
          }
          else {
            Lambda = p2;
            LamBar = p;
          }
          break;
        }
      }
    }
    if (matched) break;
  }
  if (!matched) vetoEvent;
  // check the Lambda decay mode
  bool radiative[2] = {false, false};
  // identifyt Lambda decay
  Particle baryon1;
  if ((Lambda.children()[0].pid() == PID::PROTON && Lambda.children()[1].pid() == PID::PIMINUS)) {
    radiative[0] = false;
    baryon1 = Lambda.children()[0];
  }
  else if ((Lambda.children()[1].pid() == PID::PROTON && Lambda.children()[0].pid() == PID::PIMINUS)) {
    radiative[0] = false;
    baryon1 = Lambda.children()[1];
  }
  else if ((Lambda.children()[0].pid() == PID::NEUTRON && Lambda.children()[1].pid() == PID::PHOTON)) {
    radiative[0] = true;
    baryon1 = Lambda.children()[0];
  }
  else if ((Lambda.children()[1].pid() == PID::NEUTRON && Lambda.children()[0].pid() == PID::PHOTON)) {
    radiative[0] = true;
    baryon1 = Lambda.children()[1];
  }
  else
    vetoEvent;
  Particle baryon2;
  if ((LamBar.children()[0].pid() == PID::ANTIPROTON && LamBar.children()[1].pid() == PID::PIPLUS)) {
    radiative[1] = false;
    baryon2 = LamBar.children()[0];
  }
  else if ((LamBar.children()[1].pid() == PID::ANTIPROTON && LamBar.children()[0].pid() == PID::PIPLUS)) {
    radiative[1] = false;
    baryon2 = LamBar.children()[1];
  }
  else if ((LamBar.children()[0].pid() == PID::ANTINEUTRON
            && LamBar.children()[1].pid() == PID::PHOTON)) {
    radiative[1] = true;
    baryon2 = LamBar.children()[0];
  }
  else if ((LamBar.children()[1].pid() == PID::ANTINEUTRON
            && LamBar.children()[0].pid() == PID::PHOTON)) {
    radiative[1] = true;
    baryon2 = LamBar.children()[1];
  }
  else
    vetoEvent;
  if (radiative[0] == radiative[1]) vetoEvent;
  // boost to the Lambda rest frame
  LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambda.momentum().betaVec());
  Vector3 e1z = Lambda.momentum().p3().unit();
  Vector3 e1y = e1z.cross(axis).unit();
  Vector3 e1x = e1y.cross(e1z).unit();
  Vector3 axis1 = boost1.transform(baryon1.momentum()).p3().unit();
  double n1x(e1x.dot(axis1)), n1y(e1y.dot(axis1)), n1z(e1z.dot(axis1));
  // boost to the Lambda bar
  LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(LamBar.momentum().betaVec());
  Vector3 axis2 = boost2.transform(baryon2.momentum()).p3().unit();
  double n2x(e1x.dot(axis2)), n2y(e1y.dot(axis2)), n2z(e1z.dot(axis2));
  double cosL = axis.dot(Lambda.momentum().p3().unit());
  double sinL = sqrt(1. - sqr(cosL));
  double T1 = sqr(sinL) * n1x * n2x + sqr(cosL) * n1z * n2z;
  // lambda -> n gamma
  if (radiative[0]) {
    _h_mu[0][0]->fill(cosL, n2y);
    _h_mu[0][1]->fill(cosL, n1y);
    _n[0]->fill();
    _n[2]->fill();
    _t[0]->fill(T1);
    _t[2]->fill(T1);
  }
  // lambdabar -> nbar gamma
  else {
    _h_mu[1][0]->fill(cosL, n1y);
    _h_mu[1][1]->fill(cosL, n2y);
    _n[1]->fill();
    _n[2]->fill();
    _t[1]->fill(T1);
    _t[2]->fill(T1);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  // values of constants
  double aPsi = 0.461;
  double aPlus = -0.758;
  double factor = 45. * (3. + aPsi) / (11. + 5. * aPsi) / aPlus;
  // plots
  for (unsigned int ix = 0; ix < 2; ++ix) {
    for (unsigned int iy = 0; iy < 2; ++iy) {
      scale(_h_mu[ix][iy], 10. * 0.2 / *_n[ix]);
    }
  }
  // alpha from the moments
  for (unsigned int ix = 0; ix < 3; ++ix) {
    double value = _t[ix]->val() / _n[ix]->val();
    double error = _t[ix]->err() / _n[ix]->val();
    value *= factor;
    error *= abs(factor);
    if (ix == 1) value *= -1.;
    Estimate0DPtr alpha;
    book(alpha, 2, 1, 1 + ix);
    alpha->set(value, error);
  }
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_mu[2][2];
CounterPtr _n[3];
Histo1DPtr _h_ctheta[3];
CounterPtr _t[3];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2022_I2099126);

} ```