Rivet analyses

title: LHCB_2013_I1243156

Helicity angle distributions in excited $D$ meson decays

Experiment: LHCB (LHC)

Inspire ID: 1243156

Status: VALIDATED

Authors: - Peter Richardson

References: none listed

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 $D_1(2420)^0$, $D^_2(2460)^0$, $D(2650)^0$, $D(2760)$, $D(2580)$, $D(2740)$, $D(3000)$ were measured,currently the $D(3000)^0$ is not implemented. 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:LHCB_2013_I1243156.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Helicity angle distributions in excited $D$ meson decays class LHCB_2013_I1243156 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(LHCB_2013_I1243156);


/// @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, 8, 1, 1);
  book(_h_D2_ctheta, 8, 1, 2);
  book(_h_DStar2S_ctheta, 9, 1, 1);
  book(_h_D3_ctheta, 9, 1, 2);
  book(_h_D2S_ctheta, 10, 1, 1);
  book(_h_D21D_ctheta, 10, 1, 2);
}


/// 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) {
  const Particles usps = apply<UnstableParticles>(event, "UFS")
                             .particles(Cuts::abspid == 425 || Cuts::abspid == 10423
                                        || Cuts::abspid == 100423 || Cuts::abspid == 427
                                        || Cuts::abspid == 100421 || Cuts::abspid == 10425
                                        || Cuts::abspid == 20425);
  for (const Particle& p : usps) {
    // 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() == 100423)
      _h_DStar2S_ctheta->fill(cTheta);
    else if (p.abspid() == 427)
      _h_D3_ctheta->fill(cTheta);
    else if (p.abspid() == 100421)
      _h_D2S_ctheta->fill(cTheta);
    else if (p.abspid() == 10425 || p.abspid() == 20425)
      _h_D21D_ctheta->fill(cTheta);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_D1_ctheta);
  normalize(_h_D2_ctheta);
  normalize(_h_DStar2S_ctheta);
  normalize(_h_D3_ctheta);
  normalize(_h_D2S_ctheta);
  normalize(_h_D21D_ctheta);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_D1_ctheta, _h_D2_ctheta;
Histo1DPtr _h_DStar2S_ctheta, _h_D3_ctheta;
Histo1DPtr _h_D2S_ctheta, _h_D21D_ctheta;
/// @}

};

RIVET_DECLARE_PLUGIN(LHCB_2013_I1243156);

} ```