Rivet analyses

title: ALEPH_2003_I626022

$\gamma\gamma\to\pi^+\pi^-$ ($2<\sqrt{s}<6$ GeV) and $K^+K^-$ ($2.25<\sqrt{s}<4$ GeV)

Experiment: ALEPH (LEP)

Inspire ID: 626022

Status: VALIDATED

Authors: none listed

References: - Phys.Lett.B 569 (2003) 140-150

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons

Measurement of the differential cross section for $\gamma\gamma\to\pi^+\pi^-$ ($2<\sqrt{s}<6$ GeV) and $K^+K^-$ ($2.25<\sqrt{s}<4$ GeV). The cross section as a function of the centre-of-mass energy of the photonic collision is measured, and the differential cross section with respect to the meson scattering angle are measured.

Source code:ALEPH_2003_I626022.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FastJets.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/LeptonFinder.hh"

include "Rivet/Projections/MissingMomentum.hh"

include "Rivet/Projections/PromptFinalState.hh"

namespace Rivet {

/// @brief gamma gamma -> pi+pi-/K+ K- class ALEPH_2003_I626022 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ALEPH_2003_I626022);


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

/// Book histograms and initialise projections before the run
void init() {
  // check CMS energy in range
  if (sqrtS() < 2. * GeV || sqrtS() > 6. * GeV) throw Error("Invalid CMS energy for ");
  // Final state
  declare(FinalState(), "FS");
  // histos
  book(_h_Pi, 1, 1, 1);
  if (sqrtS() < 4.) book(_h_K, 2, 1, 1);
  book(_cPi, "/TMP/nPi_");
  book(_cK, "/TMP/nK_");
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  Particles part = apply<FinalState>(event, "FS").particles();
  if (part.size() != 2) vetoEvent;
  if (part[0].pid() != -part[1].pid()) vetoEvent;
  double cTheta(0.);
  bool foundPi(false), foundK(false);
  for (const Particle& p : part) {
    if (p.pid() == PID::PIPLUS) {
      foundPi = true;
      cTheta = abs(p.momentum().z() / p.momentum().p3().mod());
    }
    else if (p.pid() == PID::KPLUS) {
      foundK = true;
      cTheta = abs(p.momentum().z() / p.momentum().p3().mod());
    }
  }
  if (!foundPi && !foundK) vetoEvent;
  if (foundPi && _h_Pi) _h_Pi->fill(cTheta);
  if (foundK && _h_K) _h_K->fill(cTheta);
  if (foundPi)
    _cPi->fill();
  else if (foundK)
    _cK->fill();
}


/// Normalise histograms etc., after the run
void finalize() {
  double fact = crossSection() / nanobarn / sumOfWeights();
  if (_h_Pi) scale(_h_Pi, fact);
  if (_h_K) scale(_h_K, fact);
  for (unsigned int ih = 3; ih < 5; ++ih) {
    CounterPtr count = ih == 3 ? _cPi : _cK;
    double sigma = count->val() * fact;
    double error = count->err() * fact;
    // hist for axis
    Estimate1DPtr cross;
    book(cross, ih, 1, 1);
    for (auto& b : cross->bins()) {
      if (inRange(sqrtS(), b.xMin(), b.xMax())) {
        b.set(sigma, error);
      }
    }
  }
}

///@}


/// @name Histograms
///@{
Histo1DPtr _h_Pi, _h_K;
CounterPtr _cPi, _cK;
///@}

};

RIVET_DECLARE_PLUGIN(ALEPH_2003_I626022);

} ```