Rivet analyses

title: BESIII_2015_I1382146

$e^+e^-\to (D^\bar{D}^)^0\pi^0$ for $\sqrt{s}=4.23$ and 4.26 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1382146

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 115 (2015) 18, 182002

Beams: e+ e-

Beam energies: (2.1, 2.1); (2.1, 2.1)GeV

Run details: - e+ e- to hadrons, pi0 set stable

Measurement of mass distributions for $e^+e^-\to (D^\bar{D}^)^0\pi^0$ for $\sqrt{s}=4.23$ and 4.26 GeV by BES. The cross section for $e^+e^-\to Z_c(4025)^0\pi^0\to (D^\bar{D}^)^0\pi^0$ is also measured. As there is no PDG code for the $Z_c(4025)^0$ we take it to be the first unused exotic $c\bar{c}$ value 9030443, although this can be changed using the PID option.

Source code:BESIII_2015_I1382146.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+e- -> → D D pi0 class BESIII_2015_I1382146 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2015_I1382146);


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

/// Book histograms and initialise projections before the run
void init() {
  // set the PDG code
  _pid = getOption<double>("PID", 9030443);
  // projections
  declare(FinalState(), "FS");
  // histograms
  book(_sigma, 1, 1, 1);
  book(_h["avg"], 2, 1, 1);
  size_t ih = 1;
  for (double eVal : allowedEnergies()) {
    const string en = toString(round(eVal / MeV));
    if (isCompatibleWithSqrtS(eVal)) _sqs = en;
    book(_h[en], 2, 1, 1 + ih);
    ++ih;
  }
  raiseBeamErrorIf(_sqs.empty());
  _edge = (_sqs == "4230"s) ? "4.23" : "4.26";
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  Particles fs = apply<FinalState>(event, "FS").particles();
  Particles DD, other;
  for (const Particle& p : fs) {
    Particle parent = p;
    while (!parent.parents().empty()) {
      parent = parent.parents()[0];
      if (parent.abspid() == 413 || parent.abspid() == 423) break;
    }
    if (parent.abspid() == 413 || parent.abspid() == 423) {
      bool found = false;
      for (const auto& D : DD) {
        // D already in list
        if (fuzzyEquals(D.mom(), parent.mom())) {
          found = true;
          break;
        }
      }
      if (!found) DD.push_back(parent);
    }
    else {
      other.push_back(p);
    }
  }
  // D Dbar + neutral pion
  if (DD.size() != 2 || other.size() != 1) vetoEvent;
  if (DD[0].pid() != -DD[1].pid()) vetoEvent;
  if (other[0].pid() != 111) vetoEvent;
  const double mass = (DD[0].mom() + DD[1].mom()).mass();
  _h[_sqs]->fill(mass / GeV);
  _h["avg"]->fill(mass / GeV);
  // parent Z0
  if (DD[0].parents()[0].pid() == _pid && DD[1].parents()[0].pid() == _pid
      && fuzzyEquals(DD[0].parents()[0].mom(), DD[1].parents()[0].mom()))
    _sigma->fill(_edge);
}


/// Normalise histograms etc., after the run
void finalize() {
  // distributions
  normalize(_h, 1.0, false);
  // cross section
  scale(_sigma, crossSection() / sumOfWeights() / picobarn);
}
/// @}


/// @name Histograms
/// @{
BinnedHistoPtr<string> _sigma;
map<string, Histo1DPtr> _h;
string _sqs = "", _edge = "";
int _pid;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2015_I1382146);

} ```