Rivet analyses

title: BELLE_2008_I786560

Measurement of Semi-Leptonic Tau Decays into $\pi^\pm\pi^0$

Experiment: Belle (KEKB)

Inspire ID: 786560

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D78 (2008) 072006 - arXiv: 0805.3773 - DOI: 10.1103/PhysRevD.78.072006

Beams: * *

Beam energies: ANY

Run details: - Tau production, can be any process but original data was in $e^+ e^-$ at the $\Upsilon(4S)$ resonance, with CoM boosts of 8.0 GeV~($e^-$) and 3.5~GeV~($e^+$)

High-statistics measurement of the branching fraction for $\tau\to\pi^-\pi^0\nu_\tau$ and the invariant mass spectrum of the produced $\pi^-\pi^0$ system using 72.2fb$^{-1}$of data recorded with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider.

Source code:BELLE_2008_I786560.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief BELLE tau lepton to pi pi /// @author Peter Richardson class BELLE_2008_I786560 : public Analysis { public:

RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2008_I786560);


void init() {
  declare(UnstableParticles(), "UFS");
  book(_hist_pipi, 1, 1, 1);
  book(_weight_total, "TMP/weight_total");
  book(_weight_pipi, "TMP/weight_pipi");
}


void analyze(const Event& e) {
  // Find the taus
  Particles taus;
  const UnstableParticles& ufs = apply<UnstableParticles>(e, "UFS");
  for (const Particle& p : ufs.particles()) {
    if (p.abspid() != PID::TAU) continue;
    _weight_total->fill();
    Particles pip, pim, pi0;
    unsigned int nstable = 0;
    // find the decay products we want
    findDecayProducts(p, nstable, pip, pim, pi0);
    if (p.pid() < 0) {
      swap(pip, pim);
    }
    if (nstable != 3) continue;
    // pipi
    if (pim.size() == 1 && pi0.size() == 1) {
      _weight_pipi->fill();
      _hist_pipi->fill((pi0[0].momentum() + pim[0].momentum()).mass2());
    }
  }
}


void finalize() {
  if (_weight_pipi->val() > 0.) scale(_hist_pipi, 1000. / *_weight_pipi);
}

private:

/// @{

// Histograms
Histo1DPtr _hist_pipi;

// Weights counters
CounterPtr _weight_total, _weight_pipi;

/// @}

void findDecayProducts(const Particle& mother,
                       unsigned int& nstable,
                       Particles& pip,
                       Particles& pim,
                       Particles& pi0) {
  for (const Particle& p : mother.children()) {
    long id = p.pid();
    if (id == PID::PI0) {
      pi0.push_back(p);
      ++nstable;
    }
    else if (id == PID::K0S)
      ++nstable;
    else if (id == PID::PIPLUS) {
      pip.push_back(p);
      ++nstable;
    }
    else if (id == PID::PIMINUS) {
      pim.push_back(p);
      ++nstable;
    }
    else if (id == PID::KPLUS) {
      ++nstable;
    }
    else if (id == PID::KMINUS) {
      ++nstable;
    }
    else if (!p.children().empty()) {
      findDecayProducts(p, nstable, pip, pim, pi0);
    }
    else
      ++nstable;
  }
}

};

RIVET_DECLARE_PLUGIN(BELLE_2008_I786560);

} ```