Rivet analyses

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** → D*+π− → D0π+π− 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 D1(2420)0, D2*(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 D1(2420)0, D2*(2460)0 the error bars are the size of the points in the plots.

Source code:BABAR_2010_I867611.cc

// -*- 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);

}