Rivet analyses

title: BESIII_2018_I1662660

$D^+\to \eta e^+\nu_e$ $q^2$ spectrum

Experiment: BESIII (BEPC)

Inspire ID: 1662660

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 121 (2018) 171803

Beams: * *

Beam energies: ANY

Run details: - Any process producing D0 and D+

Measurement of the $q^2$ spectrum in the decay $D^+\to \eta e^+\nu_e$ BES-III.

Source code:BESIII_2018_I1662660.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief q^2 in D+ -> eta e+ nu_e class BESIII_2018_I1662660 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1662660);


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

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

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

  // Book histograms
  book(_h_q2, 1, 1, 1);
}

// 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 D+ mesons
  for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == PID::DPLUS)) {
    if (p.pid() == PID::DPLUS && isSemileptonicDecay(p, {PID::ETA, PID::EPLUS, PID::NU_E})) {
      _h_q2->fill(q2(p, PID::ETA));
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_q2);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_q2;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2018_I1662660);

} ```