Rivet analyses

title: CLEOII_1997_I439745

Measurement of the cross section for $\gamma\gamma\to\Lambda^0\bar{\Lambda^0}$

Experiment: CLEOII (CESR)

Inspire ID: 439745

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 56 (1997) R2485-R2489

Beams: e+ e-, 22 22

Beam energies: (5.3, 5.3); (1.2, 1.2); (1.2, 1.2); (1.3, 1.3); (1.4, 1.4); (1.6, 1.6)GeV

Run details: - Either gamma gamma collisions or e+e- with the gamma gamma process

Measurement of the cross section for $\gamma\gamma\to\Lambda^0\bar{\Lambda^0}$. The cross section for $\gamma\gamma$ collisions as a funmction of the $gamma\gamma$ centre-of-mass energy is provided, together with the total cross section for the production in $e^+e^-$ via the $\gamma\gamma$ process.

Source code:CLEOII_1997_I439745.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/Beam.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief gamma gamma -> Lambda0 Lambdabar0 class CLEOII_1997_I439745 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(CLEOII_1997_I439745);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(Beam(), "Beams");
  declare(FinalState(), "FS");
  declare(UnstableParticles(Cuts::abspid == 3122 || Cuts::abspid == 3212), "UFS");
  // gamma gamma collision
  if (beamIDs().first == 22 && beamIDs().second == 22)
    book(_c, "TMP/nLam", refData(1, 1, 1));
  else {
    for (unsigned int ix = 0; ix < 2; ++ix) {
      book(_h_sigma[ix], 2 + ix, 1, 1);
    }
  }
}

void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) {
  for (const Particle& child : p.children()) {
    if (child.children().empty()) {
      --nRes[child.pid()];
      --ncount;
    }
    else {
      findChildren(child, nRes, ncount);
    }
  }
}

bool findScattered(const Particle& beam, double& q2) {
  bool found = false;
  Particle scat = beam;
  while (!scat.children().empty()) {
    found = false;
    for (const Particle& p : scat.children()) {
      if (p.pid() == scat.pid()) {
        scat = p;
        found = true;
        break;
      }
    }
    if (!found) break;
  }
  if (!found) return false;
  q2 = -(beam.mom() - scat.mom()).mass2();
  return true;
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // e+e- collision
  int ne = 0;
  const Beam& beams = apply<Beam>(event, "Beams");
  if (_h_sigma[0]) {
    // find scattered leptons and calc Q2
    double q12 = -1, q22 = -1;
    if (!findScattered(beams.beams().first, q12)) vetoEvent;
    if (!findScattered(beams.beams().second, q22)) vetoEvent;
    ne = 1;
  }
  // check the final state
  const FinalState& fs = apply<FinalState>(event, "FS");
  map<long, int> nCount;
  int ntotal(0);
  for (const Particle& p : fs.particles()) {
    nCount[p.pid()] += 1;
    ++ntotal;
  }
  const FinalState& ufs = apply<FinalState>(event, "UFS");
  Particle Lambda, Lambar;
  // loop over baryon
  bool matched = false;
  for (const Particle& p : ufs.particles()) {
    if (p.children().empty() || p.pid() < 0) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    // and antibaryons
    for (const Particle& p2 : ufs.particles()) {
      if (p2.children().empty() || p2.pid() > 0) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(p2, nRes2, ncount2);
      if (ncount2 != 2 * ne) continue;
      matched = 2;
      for (const auto& val : nRes2) {
        if (abs(val.first) == 11) {
          if (val.second != ne) {
            matched = false;
            break;
          }
        }
        else if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        Lambda = p;
        Lambar = p2;
        break;
      }
    }
  }
  if (!matched) vetoEvent;
  if (_h_sigma[0]) {
    FourMomentum pCMS = Lambda.mom() + Lambar.mom();
    Vector3 axis = pCMS.p3().unit();
    LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(pCMS.betaVec());
    if (abs(boost.transform(Lambda.mom()).p3().unit().dot(axis)) > 0.6) vetoEvent;
    if (abs(boost.transform(Lambar.mom()).p3().unit().dot(axis)) > 0.6) vetoEvent;
    _h_sigma[0]->fill("10.6"s);
    if (Lambda.pid() == 3122 && Lambar.pid() == -3122) _h_sigma[1]->fill("10.6"s);
  }
  else {
    Vector3 axis = beams.beams().first.mom().p3().unit();
    if (abs(axis.dot(Lambda.mom().p3().unit())) > 0.6) vetoEvent;
    if (abs(axis.dot(Lambar.mom().p3().unit())) > 0.6) vetoEvent;
    if (Lambda.pid() == 3122 && Lambar.pid() == -3122) _c->fill(sqrtS() / GeV);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  if (_h_sigma[0]) {
    scale(_h_sigma, crossSection() / picobarn / sumOfWeights());
  }
  else {
    scale(_c, crossSection() / nanobarn / sumOfWeights());
    Estimate1DPtr tmp;
    book(tmp, 1, 1, 1);
    barchart(_c, tmp);
  }
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _c;
BinnedHistoPtr<string> _h_sigma[2];
/// @}

};

RIVET_DECLARE_PLUGIN(CLEOII_1997_I439745);

} ```