Rivet analyses

title: BABAR_2008_I789011

Angular distributions in $e^+e^-\to\rho^+\rho^-$

Experiment: BABAR (PEP-II)

Inspire ID: 789011

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 78 (2008) 071103

Beams: e+ e-

Beam energies: (5.3, 5.3)GeV

Run details: - e+ e- > hadrons

Measurement of helicty angle distributions in $e^+e^-\to\rho^+\rho^-$. The corrected data were read from fgiure 4 in the paper

Source code:BABAR_2008_I789011.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- > rho+ rho- class BABAR_2008_I789011 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2008_I789011);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(Beam(), "Beams");
  declare(UnstableParticles(Cuts::abspid == 213), "UFS");
  declare(FinalState(), "FS");
  // histos
  for (unsigned int ix = 0; ix < 5; ++ix) book(_h[ix], 1, 1, 1 + ix);
}

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 rho mesons
  const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
  Particle rhoP, rhoM;
  bool matched(false);
  for (const Particle& p : ufs.particles()) {
    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 rho+/-
        if (matched) {
          if (p.pid() > 0) {
            rhoP = p;
            rhoM = p2;
          }
          else {
            rhoP = p2;
            rhoM = p;
          }
          break;
        }
      }
    }
    if (matched) break;
  }
  if (!matched) vetoEvent;
  Particle piP, piM;
  matched = false;
  if (rhoP.children().size() != 2) vetoEvent;
  if (rhoM.children().size() != 2) vetoEvent;
  if (rhoP.children()[0].pid() == 211 && rhoP.children()[1].pid() == 111)
    piP = rhoP.children()[0];
  else if (rhoP.children()[1].pid() == 211 && rhoP.children()[0].pid() == 111)
    piP = rhoP.children()[1];
  else
    vetoEvent;
  if (rhoM.children()[0].pid() == -211 && rhoM.children()[1].pid() == 111)
    piM = rhoM.children()[0];
  else if (rhoM.children()[1].pid() == -211 && rhoM.children()[0].pid() == 111)
    piM = rhoM.children()[1];
  else
    vetoEvent;
  // boost to the rho+ rest frame
  LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(rhoP.momentum().betaVec());
  Vector3 e1z = rhoP.momentum().p3().unit();
  Vector3 e1y = e1z.cross(axis).unit();
  Vector3 e1x = e1y.cross(e1z).unit();
  Vector3 axis1 = boost1.transform(piP.momentum()).p3().unit();
  double n1x(e1x.dot(axis1)), n1y(e1y.dot(axis1)), n1z(e1z.dot(axis1));
  _h[0]->fill(n1z);
  _h[2]->fill(atan2(n1y, n1x));
  // boost to the rho- frame
  LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(rhoM.momentum().betaVec());
  Vector3 axis2 = boost2.transform(piM.momentum()).p3().unit();
  double n2x(e1x.dot(axis2)), n2y(e1y.dot(axis2)), n2z(e1z.dot(axis2));
  _h[1]->fill(n2z);
  _h[3]->fill(atan2(n2y, n2x));
  _h[4]->fill(axis.dot(rhoP.momentum().p3().unit()));
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 5; ++ix) normalize(_h[ix], 1., false);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[5];
/// @}

};

RIVET_DECLARE_PLUGIN(BABAR_2008_I789011);

} ```