Rivet analyses

title: BESIII_2013_I1256939

$e^+e^-\to (D\bar{D}^*)^\pm\pi^\mp$ for $\sqrt{s}=4.26$ GeV

Experiment: BESIII (BEPC)

Inspire ID: 1256939

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 112 (2014) 2, 022001

Beams: e+ e-

Beam energies: (2.1, 2.1)GeV

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

Measurement of mass distributions for $e^+e^-\to (D\bar{D}^)^\pm\pi^\mp$ for $\sqrt{s}=4.26$ GeV by BES. The cross section for $e^+e^-\to Z_c(3885)^\pm\pi^\mp\to (D\bar{D}^)^\pm\pi^\mp$ is also measured. As there is no PDG code for the $Z_c(3885)^+ we take it to be 9044213, although this can be changed using the PID option.

Source code:BESIII_2013_I1256939.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 e+e-> Z+- pi-+ class BESIII_2013_I1256939 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2013_I1256939);


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

/// Book histograms and initialise projections before the run
void init() {
  // set the PDG code
  _pid = getOption<double>("PID", 9044213);
  // projections
  declare(Beam(), "Beams");
  declare(FinalState(), "FS");
  // histograms
  book(_sigma, 1, 1, 1);
  for (unsigned int ix = 0; ix < 2; ++ix) {
    book(_h[ix], 2, 1, 1 + ix);
  }
  book(_h[2], 3, 1, 1);
}


/// 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();
  }
  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.momentum(), parent.momentum())) {
          found = true;
          break;
        }
      }
      if (!found) DD.push_back(parent);
    }
    else {
      other.push_back(p);
    }
  }
  // D Dbar + charged pion
  if (DD.size() != 2 || other.size() != 1) vetoEvent;
  if (DD[0].pid() * DD[1].pid() > 0) vetoEvent;
  if (other[0].abspid() != 211) vetoEvent;
  if (DD[0].abspid() % 10 != 3) swap(DD[0], DD[1]);
  if (DD[0].abspid() % 10 != 3 || DD[1].abspid() % 10 != 1) vetoEvent;
  const double mass = (DD[0].momentum() + DD[1].momentum()).mass();
  const double cTheta = abs(axis.dot(other[0].momentum().p3().unit()));
  unsigned int iloc = 0;
  // D0 D*- pi+ +cc
  if ((DD[0].pid() == -413 && DD[1].pid() == 421 && other[0].pid() == 211)
      || (DD[0].pid() == 413 && DD[1].pid() == -421 && other[0].pid() == -211)) {
    iloc = 0;
  }
  // D- D*0 pi+ +cc
  else if ((DD[0].pid() == 423 && DD[1].pid() == -411 && other[0].pid() == 211)
           || (DD[0].pid() == -423 && DD[1].pid() == 411 && other[0].pid() == -211)) {
    iloc = 1;
  }
  // otherwise veto event
  else {
    vetoEvent;
  }
  _h[iloc]->fill(mass);
  // parent Z+/-
  if (DD[0].parents()[0].abspid() == _pid && DD[1].parents()[0].abspid() == _pid
      && fuzzyEquals(DD[0].parents()[0].momentum(), DD[1].parents()[0].momentum())) {
    _sigma->fill("4.26"s);
    _h[2]->fill(cTheta);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_sigma, crossSection() / sumOfWeights() / picobarn);
  for (unsigned int ix = 0; ix < 3; ++ix) {
    normalize(_h[ix], 1.0, false);
  }
}

/// @}


/// @name Histograms
/// @{
BinnedHistoPtr<string> _sigma;
Histo1DPtr _h[3];
int _pid;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2013_I1256939);

} ```