Rivet analyses

title: BABAR_2010_I867611

Helicity angle distributions in excited $D$ meson decays

Experiment: BABAR (PEP-II)

Inspire ID: 867611

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D82 (2010) 111101

Beams: * *

Beam energies: ANY

Run details: - Any process producing excited $D$ mesons

The decays $D^{} \to D^{+}\pi^-\to D^0\pi^+\pi^-$ are used to measure the helicity angle, i.e. the angle between the two pions in the rest frame of the $D^$. The decays of $D_1(2420)^0$, $D^_2(2460)^0$, $D(2550)^0$, $D^(2600)$, $D(2750)$ were measured. The data were read from the plots in the papers and therefore for the $D_1(2420)^0$, $D^*_2(2460)^0$ the error bars are the size of the points in the plots.

Source code:BABAR_2010_I867611.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief D_1 and D_2 decay distributions class BABAR_2010_I867611 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2010_I867611);


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

/// Book histograms and initialise projections before the run
void init() {

  // Initialise and register projections
  declare(UnstableParticles(), "UFS");

  // Book histograms
  book(_h_D1_ctheta, 1, 1, 1);
  book(_h_D2_ctheta, 1, 1, 2);
  book(_h_D02S_ctheta, 1, 1, 3);
  book(_h_DStar02S_ctheta, 1, 1, 4);
  book(_h_D3_ctheta, 1, 1, 5);
}

/// Recursively walk the decay tree to find decay products of @a p
void findDecayProducts(Particle mother,
                       Particles& dstar,
                       Particles& d0,
                       Particles& pi,
                       unsigned int& ncount) {
  for (const Particle& p : mother.children()) {
    if (p.abspid() == 413)
      dstar.push_back(p);
    else if (p.abspid() == 421)
      d0.push_back(p);
    else if (p.abspid() == 211)
      pi.push_back(p);
    ncount += 1;
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  for (const Particle& p :
       apply<UnstableParticles>(event, "UFS")
           .particles(Cuts::abspid == 425 || Cuts::abspid == 10423 || Cuts::abspid == 100421
                      || Cuts::abspid == 100423 || Cuts::abspid == 427)) {
    // decay products
    Particles dstar, d0, pi;
    unsigned int ncount = 0;
    findDecayProducts(p, dstar, d0, pi, ncount);
    if (ncount != 2 || dstar.size() != 1 || pi.size() != 1 || d0.size() != 0) continue;
    if (dstar[0].pid() / p.pid() < 0) continue;
    Particle p2 = dstar[0];
    LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(p2.momentum().betaVec());
    Vector3 d1 = boost.transform(pi[0].momentum()).p3().unit();
    ncount = 0;
    dstar.clear();
    d0.clear();
    pi.clear();
    findDecayProducts(p2, dstar, d0, pi, ncount);
    if (ncount != 2 || dstar.size() != 0 || pi.size() != 1 || d0.size() != 1) continue;
    if (pi[0].pid() / p2.pid() < 0) continue;
    Vector3 d2 = boost.transform(pi[0].momentum()).p3().unit();
    double cTheta = d1.dot(d2);
    // decay angles
    if (p.abspid() == 425)
      _h_D2_ctheta->fill(cTheta);
    else if (p.abspid() == 10423)
      _h_D1_ctheta->fill(cTheta);
    else if (p.abspid() == 100421)
      _h_D02S_ctheta->fill(cTheta);
    else if (p.abspid() == 100423)
      _h_DStar02S_ctheta->fill(cTheta);
    else if (p.abspid() == 427)
      _h_D3_ctheta->fill(cTheta);
  }
}

/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_D1_ctheta);
  normalize(_h_D2_ctheta);
  normalize(_h_D02S_ctheta);
  normalize(_h_DStar02S_ctheta);
  normalize(_h_D3_ctheta);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_D1_ctheta, _h_D2_ctheta, _h_D02S_ctheta;
Histo1DPtr _h_DStar02S_ctheta, _h_D3_ctheta;
/// @}

};

RIVET_DECLARE_PLUGIN(BABAR_2010_I867611);

} ```