Rivet analyses

title: BABAR_2018_I1679886

Mass spectrum for $K^-K_S^0$ decays of the $\tau^-$ lepton

Experiment: BABAR (PEP-II)

Inspire ID: 1679886

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D98 (2018) no.3, 032010

Beams: * *

Beam energies: ANY

Run details: - e+ e- > tau+ tau-, or any process producing tau leptons

Mass spectra for $K^-K_S^0$ in tau decays measured by BaBar. Useful for testing models of the hadronic current in $\tau$ decays.

Source code:BABAR_2018_I1679886.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief tau -> K+ KS nu_tau class BABAR_2018_I1679886 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2018_I1679886);


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

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

  declare(UnstableParticles(), "UFS");
  book(_h_KK, 1, 1, 1);
  book(_v_KK, 1, 1, 2);
  book(_c, "TMP/c");
}

void findDecayProducts(const Particle& mother,
                       unsigned int& nstable,
                       unsigned int& nK0,
                       unsigned int& nKp,
                       unsigned int& nKm,
                       FourMomentum& ptot) {
  for (const Particle& p : mother.children()) {
    int id = p.pid();
    if (id == PID::KPLUS) {
      ++nKp;
      ++nstable;
      ptot += p.momentum();
    }
    else if (id == PID::KMINUS) {
      ++nKm;
      ++nstable;
      ptot += p.momentum();
    }
    else if (id == PID::K0S) {
      ++nK0;
      ++nstable;
      ptot += p.momentum();
    }
    else if (id == PID::PI0 || id == PID::PIPLUS || id == PID::PIMINUS) {
      ++nstable;
    }
    else if (!p.children().empty()) {
      findDecayProducts(p, nstable, nK0, nKp, nKm, ptot);
    }
    else
      ++nstable;
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {

  // Loop over taus
  for (const Particle& tau : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid == PID::TAU)) {
    unsigned int nstable(0), nK0(0), nKp(0), nKm(0);
    FourMomentum p_tot(0, 0, 0, 0);
    findDecayProducts(tau, nstable, nK0, nKp, nKm, p_tot);
    if (tau.pid() < 0) {
      swap(nKp, nKm);
    }
    if (nstable != 3) continue;
    if (nKm == 1 && nK0 == 1) {
      double q = p_tot.mass();
      _h_KK->fill(q);
      double mtau = tau.mass();
      double Cq = q * sqr(sqr(mtau) - sqr(q)) * (sqr(mtau) + 2. * sqr(q));
      double fact = pow(mtau, 8) / Cq;
      _v_KK->fill(q, fact);
      _c->fill();
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_KK);
  // values from PDG 2023 for constants, see eqn 1 of paper
  double brKK = 0.5 * 1.486e-3; // PDG is K0 divide by 2 to get KS0
  double brE = 0.1782;
  double Vud = .97367;
  scale(_v_KK, 1000. * brKK / brE / sqr(Vud) / 12. / M_PI / *_c);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_KK, _v_KK;
CounterPtr _c;
/// @}

};

RIVET_DECLARE_PLUGIN(BABAR_2018_I1679886);

} ```