Rivet analyses

title: BESIII_2019_I1712742

Semileptonic $D_s^+\to \eta e^+\nu_e$ and $\eta^\prime e^+\nu_e$ decays

Experiment: BESIII (BEPC)

Inspire ID: 1712742

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 122 (2019) 12, 121801

Beams: * *

Beam energies: ANY

Run details: - Any process producing D_s mesons

Differential decay rates for semileptonic $D_s^+\to \eta e^+\nu_e$ and $\eta^\prime e^+\nu_e$ decays

Source code:BESIII_2019_I1712742.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief D_s -> eta, eta' semi-leptonic class BESIII_2019_I1712742 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2019_I1712742);


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

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

  // Initialise and register projections
  declare(UnstableParticles(), "UFS");
  // histograms
  for (unsigned int ix = 0; ix < 2; ++ix)
    for (unsigned int iy = 0; iy < 2; ++iy) book(_h[ix][iy], ix + 1, 1, iy + 1);
  // counter for normalization
  book(_nDs, "TMP/nDs");
}

// Calculate the Q2 using mother and daugher meson
double q2(const Particle& B, int mesonID) {
  FourMomentum q = B.mom() - select(B.children(), Cuts::pid == mesonID)[0];
  return q * q;
}

// Check for explicit decay into pdgids
bool isSemileptonicDecay(const Particle& mother, vector<int> ids) {
  // Trivial check to ignore any other decays but the one in question modulo photons
  const Particles children = mother.children(Cuts::pid != PID::PHOTON);
  if (children.size() != ids.size()) return false;
  // Check for the explicit decay
  return all(ids, [&](int i) { return count(children, hasPID(i)) == 1; });
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  // Loop over Ds mesons
  for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == 431)) {
    _nDs->fill();
    if (isSemileptonicDecay(p, {PID::ETA, PID::POSITRON, PID::NU_E})) {
      _h[0][0]->fill(q2(p, PID::ETA));
      _h[0][1]->fill(q2(p, PID::ETA));
    }
    else if (isSemileptonicDecay(p, {PID::ETAPRIME, PID::POSITRON, PID::NU_E})) {
      _h[1][0]->fill(q2(p, PID::ETAPRIME));
      _h[1][1]->fill(q2(p, PID::ETAPRIME));
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  // D_s lifetime pdg2018 (ns)
  double tau = 504e-6;
  for (unsigned int ix = 0; ix < 2; ++ix)
    for (unsigned int iy = 0; iy < 2; ++iy) scale(_h[ix][iy], 1. / tau / *_nDs);
}

/// @}


/// @name Histograms
/// @{
CounterPtr _nDs;
Histo1DPtr _h[2][2];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2019_I1712742);

} ```