Rivet analyses

title: BABAR_2009_I827985

Helicity angle distributions in excited $D_s$ 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 $D_s$ mesons

The decays $D_s^{} \to D^{+}K^0_S\to D^0\pi^+K^0_S$ 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 $D_{s1}^(2700)^+$ and with mass 2.8 GeV were measured. It is unclear if this is the $D_{s1}^(2860)^+$, $D_{s3}^*(2860)^+$ 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++ // -- 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);

} ```