Rivet analyses

title: ARGUS_1994_I375417

Pion helicity angle in $\tau^-\to\pi^-\pi^0\nu_\tau$

Experiment: ARGUS (DORIS)

Inspire ID: 375417

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Lett.B 337 (1994) 383-392

Beams: * *

Beam energies: ANY

Run details: - Any process producing tau leptons, originally e+e-

Measurement of the pion helicity angle in the $\pi^-\pi^0$ frame for $\tau^-\to\pi^-\pi^0\nu_\tau$. The corrected data were read from figure 3 in the paper.

Source code:ARGUS_1994_I375417.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

include "Rivet/Tools/HistoGroup.hh"

namespace Rivet {

/// @brief tau -> pi- pi0 nu_tau class ARGUS_1994_I375417 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ARGUS_1994_I375417);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::abspid == 15);
  declare(ufs, "UFS");
  DecayedParticles TAU(ufs);
  TAU.addStable(310);
  TAU.addStable(111);
  TAU.addStable(221);
  declare(TAU, "TAU");
  // histos
  book(_b, {0.5, 0.69, 0.776, 0.85, 1.1});
  for (auto& b : _b->bins()) {
    book(b, 1, 1, b.index());
  }
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  DecayedParticles TAU = apply<DecayedParticles>(event, "TAU");
  // loop over particles
  for (unsigned int ix = 0; ix < TAU.decaying().size(); ++ix) {
    int sign = TAU.decaying()[ix].pid() > 0 ? 1 : -1;
    if (!(TAU.modeMatches(ix, 3, mode) || TAU.modeMatches(ix, 3, modeCC))) continue;
    const Particle& pi0 = TAU.decayProducts()[ix].at(111)[0];
    const Particle& pim = TAU.decayProducts()[ix].at(-211 * sign)[0];
    FourMomentum phad = pim.mom() + pi0.mom();
    LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(
        TAU.decaying()[ix].mom().betaVec());
    phad = boost.transform(phad);
    LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(phad.betaVec());
    FourMomentum ppi = boost2.transform(boost.transform(pim.mom()));
    const double cAlpha = ppi.p3().unit().dot(phad.p3().unit());
    _b->fill(phad.mass(), cAlpha);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_b, 1.0, false);
}

/// @}


/// @name Histograms
/// @{
Histo1DGroupPtr _b;
const map<PdgId, unsigned int> mode = {{111, 1}, {-211, 1}, {16, 1}};
const map<PdgId, unsigned int> modeCC = {{111, 1}, {211, 1}, {-16, 1}};
/// @}

};

RIVET_DECLARE_PLUGIN(ARGUS_1994_I375417);

} ```