Rivet analyses

title: BABAR_2012_I1125973

Semi-leptonic $B$ to $\pi$, $\omega$ and $\eta$ decays

Experiment: BABAR (PEP-II)

Inspire ID: 1125973

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 86 (2012) 092004

Beams: * *

Beam energies: ANY

Run details: - Any process producing B+ or B0, original e+e->Upsilon(4S)

Implementation of Lorentz invariant $q^2$ distributions ("form factor") for semileptonic $B^0$ and $B^+$ decays. Includes $\pi$, $\omega$ and $\eta$ decays.

Source code:BABAR_2012_I1125973.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief B -> pi, eta and omega decays class BABAR_2012_I1125973 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2012_I1125973);


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

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

  // Initialise and register projections
  declare(UnstableParticles(Cuts::pid == PID::BPLUS || Cuts::pid == PID::B0), "UFS");

  // histograms
  for (unsigned int ix = 0; ix < 2; ++ix) {
    book(_h_B0_pi[ix], 1, 1, ix + 1);
    book(_h_Bp_pi[ix], 2, 1, ix + 1);
    book(_nB[ix], "TMP/nB_" + toString(ix + 1));
  }
  book(_h_omega, 3, 1, 1);
  book(_h_eta, 4, 1, 1);
}

// Calculate the Q2 using mother and daughter charged lepton
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) {
  // Get B+ Mesons
  for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles()) {
    if (p.pid() == PID::B0) {
      _nB[0]->fill();
      if (isSemileptonicDecay(p, {PID::PIMINUS, PID::POSITRON, PID::NU_E})
          || isSemileptonicDecay(p, {PID::PIMINUS, PID::ANTIMUON, PID::NU_MU})) {
        double Q2 = q2(p, PID::PIMINUS);
        _h_B0_pi[0]->fill(Q2);
        _h_B0_pi[1]->fill(Q2);
      }
    }
    else {
      _nB[1]->fill();
      if (isSemileptonicDecay(p, {PID::PI0, PID::POSITRON, PID::NU_E})
          || isSemileptonicDecay(p, {PID::PI0, PID::ANTIMUON, PID::NU_MU})) {
        double Q2 = q2(p, PID::PI0);
        _h_Bp_pi[0]->fill(Q2);
        _h_Bp_pi[1]->fill(Q2);
      }
      else if (isSemileptonicDecay(p, {PID::OMEGA, PID::POSITRON, PID::NU_E})
               || isSemileptonicDecay(p, {PID::OMEGA, PID::ANTIMUON, PID::NU_MU})) {
        _h_omega->fill(q2(p, PID::OMEGA));
      }
      else if (isSemileptonicDecay(p, {PID::ETA, PID::POSITRON, PID::NU_E})
               || isSemileptonicDecay(p, {PID::ETA, PID::ANTIMUON, PID::NU_MU})) {
        _h_eta->fill(q2(p, PID::ETA));
      }
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  // BR in units of 10^{-7}
  for (unsigned int ix = 0; ix < 2; ++ix) {
    scale(_h_B0_pi[ix], 1e7 / *_nB[0]);
    scale(_h_Bp_pi[ix], 1e7 / *_nB[1]);
  }
  scale(_h_omega, 1e7 / *_nB[1]);
  scale(_h_eta, 1e7 / *_nB[1]);
}

/// @}


/// @name Histograms
/// @{
CounterPtr _nB[2];
Histo1DPtr _h_B0_pi[2], _h_Bp_pi[2], _h_omega, _h_eta;
/// @}

};

RIVET_DECLARE_PLUGIN(BABAR_2012_I1125973);

} ```