Rivet analyses

title: BESIII_2019_I1747092

Analysis of $\psi(2S)$ decays to $\Xi^{-}\bar\Xi^{+}$

Experiment: BESIII (BEPC)

Inspire ID: 1747092

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 100 (2019) 5, 051101

Beams: e- e+

Beam energies: (1.8, 1.8)GeV

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

Analysis of the angular distribution of the baryons produced in $e^+e^-\to \psi(2S) \to \Xi^{-}\bar\Xi^{+}$. Gives information about the decay and is useful for testing correlations in hadron decays.

Source code:BESIII_2019_I1747092.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 BES psi2s -> Xi- Xibar+ class BESIII_2019_I1747092 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2019_I1747092);


/// @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");

  book(_h_xim, 1, 1, 1);
  book(_h_xip, 1, 1, 2);
}

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;
  }

  const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
  for (const Particle& p : ufs.particles(Cuts::abspid == 3314)) {
    if (p.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    bool 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;
          }
        }
        // fond baryon and antibaryon
        if (matched) {
          // calc cosine
          double ctheta1 = p.momentum().p3().unit().dot(axis);
          double ctheta2 = p2.momentum().p3().unit().dot(axis);
          if (p.pid() < 0) swap(ctheta1, ctheta2);
          _h_xim->fill(ctheta1);
          _h_xip->fill(ctheta2);
          break;
        }
      }
    }
    if (matched) break;
  }
}

pair<double, pair<double, double>> calcAlpha(Histo1DPtr hist) {
  if (hist->numEntries() == 0.) return make_pair(0., make_pair(0., 0.));
  double d = 3. / (pow(hist->xMax(), 3) - pow(hist->xMin(), 3));
  double c = 3. * (hist->xMax() - hist->xMin()) / (pow(hist->xMax(), 3) - pow(hist->xMin(), 3));
  double sum1(0.), sum2(0.), sum3(0.), sum4(0.), sum5(0.);
  for (const auto& bin : hist->bins()) {
    double Oi = bin.sumW();
    if (Oi == 0.) continue;
    double a = d * (bin.xMax() - bin.xMin());
    double b = d / 3. * (pow(bin.xMax(), 3) - pow(bin.xMin(), 3));
    double Ei = bin.errW();
    sum1 += a * Oi / sqr(Ei);
    sum2 += b * Oi / sqr(Ei);
    sum3 += sqr(a) / sqr(Ei);
    sum4 += sqr(b) / sqr(Ei);
    sum5 += a * b / sqr(Ei);
  }
  // calculate alpha
  double alpha = (-c * sum1 + sqr(c) * sum2 + sum3 - c * sum5) / (sum1 - c * sum2 + c * sum4 - sum5);
  // and error
  double cc = -pow((sum3 + sqr(c) * sum4 - 2 * c * sum5), 3);
  double bb = -2 * sqr(sum3 + sqr(c) * sum4 - 2 * c * sum5) * (sum1 - c * sum2 + c * sum4 - sum5);
  double aa = sqr(sum1 - c * sum2 + c * sum4 - sum5)
      * (-sum3 - sqr(c) * sum4 + sqr(sum1 - c * sum2 + c * sum4 - sum5) + 2 * c * sum5);
  double dis = sqr(bb) - 4. * aa * cc;
  if (dis > 0.) {
    dis = sqrt(dis);
    return make_pair(alpha, make_pair(0.5 * (-bb + dis) / aa, -0.5 * (-bb - dis) / aa));
  }
  else {
    return make_pair(alpha, make_pair(0., 0.));
  }
}

/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_xim, 1., false);
  normalize(_h_xip, 1., false);
  Estimate0DPtr _h_alpha_xi;
  book(_h_alpha_xi, 2, 1, 1);
  pair<double, pair<double, double>> alpha = calcAlpha(_h_xim);
  _h_alpha_xi->set(alpha.first, make_pair(alpha.second.first, alpha.second.second));
}

/// @}


/// @name Histograms
/// @{+
Histo1DPtr _h_xim, _h_xip;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2019_I1747092);

} ```