Rivet analyses

Helicity angle distributions in excited Ds meson decays

Experiment: BABAR (PEP-II)

Inspire ID: 827985

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.D 80 (2009) 092003

Beams: * *

Beam energies: ANY

Run details: - Any process producing excited Ds mesons

The decays Ds** → D*+KS0 → D0π+KS0 are used to measure the helicity angle, i.e. the angle between the pion and kaon in the rest frame of the D*. The decays of Ds1*(2700)+ and with mass 2.8 GeV were measured. It is unclear if this is the Ds1*(2860)+, Ds3*(2860)+ or a mixture of the two. This histogram is therefore filled we with each state and the admixture. The data were read from the plots in the papers.

Source code:BABAR_2009_I827985.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief D**_s decays
  class BABAR_2009_I827985 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2009_I827985);


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

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

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

      // Book histograms
      book(_h_DStar_ctheta, 1, 1, 1);
      book(_h_D3_ctheta[0], 1, 1, 2);
      book(_h_D3_ctheta[1], 1, 1, 3);
      book(_h_D3_ctheta[2], 1, 1, 4);
    }

    /// Recursively walk the decay tree to find decay products of @a p
    void findDecayProducts(Particle mother,
                           Particles& dstar,
                           Particles& d0,
                           Particles& K0,
                           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() == 130 || p.abspid() == 130 || p.abspid() == 311)
          K0.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 == 100433 || Cuts::abspid == 437 || Cuts::abspid == 30433)) {
        // decay products
        Particles dstar, d0, K0, pi;
        unsigned int ncount = 0;
        findDecayProducts(p, dstar, d0, K0, pi, ncount);
        if (ncount != 2 || dstar.size() != 1 || K0.size() != 1) continue;
        if (dstar[0].pid() / p.pid() < 0) continue;
        Particle p2 = dstar[0];
        LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(p2.momentum().betaVec());
        Vector3 d1 = boost.transform(K0[0].momentum()).p3().unit();
        ncount = 0;
        dstar.clear();
        d0.clear();
        pi.clear();
        findDecayProducts(p2, dstar, d0, K0, pi, ncount);
        if (ncount != 2 || 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() == 100433)
          _h_DStar_ctheta->fill(cTheta);
        else if (p.abspid() == 30433) {
          _h_D3_ctheta[0]->fill(cTheta);
          _h_D3_ctheta[2]->fill(cTheta);
        }
        else if (p.abspid() == 437) {
          _h_D3_ctheta[1]->fill(cTheta);
          _h_D3_ctheta[2]->fill(cTheta);
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      normalize(_h_DStar_ctheta);
      normalize(_h_D3_ctheta[0]);
      normalize(_h_D3_ctheta[1]);
      normalize(_h_D3_ctheta[2]);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_DStar_ctheta, _h_D3_ctheta[3];
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BABAR_2009_I827985);

}