Rivet analyses

title: BESIII_2015_I1393996

$e^+e^-\to (DD^*)^0\pi^0$ for $\sqrt{s}=4.226$ and 4.257 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1393996

Status: VALIDATED NOHEPDATA SINGLEWEIGHT

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 115 (2015) 22, 222002

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 $e^+e^-\to (DD^)^0\pi^0$ for $\sqrt{s}=4.226$ and 4.257 GeV by BES. The cross section for $e^+e^-\to Z_c(3885)^0\pi^0\to (DD^)^0\pi^0$ is also measured. As there is no PDG code for the $Z_c(3885)^0$ 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_I1393996.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+ e- > Z0 pi0 class BESIII_2015_I1393996 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2015_I1393996);


/// @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
  if (isCompatibleWithSqrtS(4.226))
    _sqs = "4.226";
  else if (isCompatibleWithSqrtS(4.257))
    _sqs = "4.257";
  raiseBeamErrorIf(_sqs.empty());
  for (size_t ix = 0; ix < 2; ++ix) {
    book(_h["4.257"s + toString(ix)], 2, ix + 1, 1);
    book(_h["4.226"s + toString(ix)], 2, ix + 1, 2);
  }
  book(_h["2"], 3, 1, 1);
  book(_sigma, 1, 1, 1);
}


/// 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() == 411 || parent.abspid() == 413 || parent.abspid() == 421
          || parent.abspid() == 423)
        break;
    }
    if ((parent.abspid() == 411 || parent.abspid() == 421) && !parent.parents().empty()) {
      Particle Dstar = parent.parents()[0];
      if (Dstar.abspid() == 413 || Dstar.abspid() == 423) parent = Dstar;
    }
    if (parent.abspid() == 411 || parent.abspid() == 413 || parent.abspid() == 421
        || 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() > 0) vetoEvent;
  if (other[0].pid() != 111) vetoEvent;
  // D pi mass greater than 2.1
  for (unsigned int ix = 0; ix < 2; ++ix) {
    if (DD[ix].abspid() % 10 == 1 && (DD[ix].momentum() + other[0].momentum()).mass() < 2.1) vetoEvent;
  }
  double mass = (DD[0].momentum() + DD[1].momentum()).mass();
  unsigned int iloc = 0;
  // D+ D*+
  if ((DD[0].abspid() == 413 && DD[1].abspid() == 411)
      || (DD[1].abspid() == 413 && DD[0].abspid() == 411)) {
    iloc = 0;
  }
  // D0 D*0
  else if ((DD[0].abspid() == 423 && DD[1].abspid() == 421)
           || (DD[1].abspid() == 423 && DD[0].abspid() == 421)) {
    iloc = 1;
  }
  // otherwise veto event
  else
    vetoEvent;
  _h[_sqs + toString(iloc)]->fill(mass);
  _h["2"]->fill(mass);
  // 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(_sqs);
}


/// 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 = "";
int _pid;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2015_I1393996);

} ```