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);

}