Rivet analyses

title: MARKII_1990_I304882

$\gamma\gamma\to\pi^+\pi^-$ for centre-of-mass energies between 0.35 and 1.6 GeV

Experiment: MARKII (PEP)

Inspire ID: 304882

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.D 42 (1990) 1350-1367

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons

Measurement of the differential cross section for $\gamma\gamma\to\pi^+\pi^-$ for $0.35 \text{GeV} < W < 1.6 \text{GeV}$. Both the cross section as a function of the centre-of-mass energy of the photonic collision, and the differential cross section with respect to the pion scattering angle are measured.

Source code:MARKII_1990_I304882.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief gamma gamma -> pi+ pi- class MARKII_1990_I304882 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(MARKII_1990_I304882);


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

/// Book histograms and initialise projections before the run
void init() {
  // Final state
  declare(FinalState(), "FS");
  // check CMS energy in range
  if (sqrtS() < 0.35 * GeV || sqrtS() > 1.6 * GeV)
    throw Error("Invalid CMS energy for MARKII_1990_I304882");
  for (unsigned int ix = 0; ix < 7; ++ix) {
    std::ostringstream title;
    title << "/TMP/nPi_" << ix;
    book(_cPi[ix], title.str());
  }
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  Particles part = apply<FinalState>(event, "FS").particles();
  if (part.size() != 2) vetoEvent;
  double cTheta(0.);
  bool foundP(false), foundM(false);
  for (const Particle& p : part) {
    if (p.pid() == PID::PIPLUS) {
      foundP = true;
      cTheta = abs(p.momentum().z() / p.momentum().p3().mod());
    }
    else if (p.pid() == PID::PIMINUS)
      foundM = true;
  }
  if (!foundP || !foundM) vetoEvent;
  int ibin = cTheta / 0.1;
  if (ibin > 5) vetoEvent;
  _cPi[0]->fill();
  _cPi[ibin + 1]->fill();
}


/// Normalise histograms etc., after the run
void finalize() {
  double fact = crossSection() / nanobarn / sumOfWeights();
  for (unsigned int ih = 0; ih < 7; ++ih) {
    unsigned int ix = 2, iy = ih;
    if (ih == 0) {
      ix = 1;
      iy = 1;
    }
    double sigma = _cPi[ih]->val() * fact;
    double error = _cPi[ih]->err() * fact;
    Estimate1DPtr mult;
    book(mult, ix, 1, iy);
    for (auto& b : mult->bins()) {
      if (inRange(sqrtS(), b.xMin(), b.xMax())) {
        b.set(sigma, error);
      }
    }
  }
}

///@}


/// @name Histograms
///@{
CounterPtr _cPi[7];
///@}

};

RIVET_DECLARE_PLUGIN(MARKII_1990_I304882);

} ```