Rivet analyses

Measurement of Semi-Leptonic Tau Decays into π±π0

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 Υ(4S) resonance, with CoM boosts of 8.0 GeV~(e−) and 3.5GeV(e+)

High-statistics measurement of the branching fraction for τ → π−π0ντ and the invariant mass spectrum of the produced π−π0 system using 72.2fb−1of data recorded with the Belle detector at the KEKB asymmetric-energy e+e− collider.

Source code:BELLE_2008_I786560.cc

// -*- 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:

    BELLE_2008_I786560()
      : Analysis("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, 1. / *_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);

}