Rivet analyses

title: ZEUS_2008_I763404

Diffractive photoproduction of dijets

Experiment: ZEUS (HERA Run II)

Inspire ID: 763404

Status: UNVALIDATED

Authors: - Ilkka Helenius - Christine O. Rasmussen

References: - Eur.Phys.J.C55:177,2008 - DESY 07/161 - arXiv: 0710.1498

Beams: p+ e+

Beam energies: (920.0, 27.5)GeV

Run details: - 920 GeV protons colliding with 27.5 GeV positrons; Diffractive photoproduction of dijets; Leading jet $pT > 7.5$ GeV, second jet $pT > 6.5$ GeV; Jet pseudorapidity $-1.5 < \eta^{jet} < 1.5$

ZEUS analysis for diffractive photoproduction of dijets in proton-positron collisions with beam energies of 920~GeV on 27.5~GeV at the ep collider HERA using an integrated luminosity of 77.2 pb-1. The measurements were made in the kinematic range Q2 < 1 GeV^2, 0.20 < y < 0.85 and xIP < 0.025, where Q2 is the photon virtuality, y is the inelasticity and xIP is the fraction of the proton momentum taken by the diffractive exchange. The two jets with the highest transverse energy, Ejet, were required to satisfy E_jet > 7.5 and 6.5 GeV, respectively, and to lie in the pseudorapidity range -1.5 < η_{jet} < 1.5.

Source code:ZEUS_2008_I763404.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/Beam.hh"

include "Rivet/Projections/DISDiffHadron.hh"

include "Rivet/Projections/DISKinematics.hh"

include "Rivet/Projections/FastJets.hh"

namespace Rivet {

/// @brief ZEUS dijet photoproduction study used in the ZEUS jets PDF fit /// /// This class is a reproduction of the HZTool routine for the ZEUS /// dijet photoproduction paper which was used in the ZEUS jets PDF fit. /// /// @author Ilkka Helenius class ZEUS_2008_I763404 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ZEUS_2008_I763404);

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

// Book projections and histograms
void init() {

  /// @todo Acceptance
  FinalState fs;
  // Final state particles with central tracking detector.
  declare(FastJets(fs, JetAlg::KT, 1.0), "Jets");

  // Projections
  declare(DISKinematics(), "Kinematics");
  declare(DISDiffHadron(), "Hadron");

  book(_h_dsigma_all[0], 1, 1, 1);
  book(_h_dsigma_all[1], 2, 1, 1);
  book(_h_dsigma_all[2], 3, 1, 1);
  book(_h_dsigma_all[3], 4, 1, 1);
  book(_h_dsigma_all[4], 5, 1, 1);
  book(_h_dsigma_all[5], 6, 1, 1);
  book(_h_xgamma, 7, 1, 1);
  book(_h_dsigma_xgamma[0][0], 8, 1, 1);
  book(_h_dsigma_xgamma[0][1], 9, 1, 1);
  book(_h_dsigma_xgamma[0][2], 10, 1, 1);
  book(_h_dsigma_xgamma[0][3], 11, 1, 1);
  book(_h_dsigma_xgamma[1][0], 12, 1, 1);
  book(_h_dsigma_xgamma[1][1], 13, 1, 1);
  book(_h_dsigma_xgamma[1][2], 14, 1, 1);
  book(_h_dsigma_xgamma[1][3], 15, 1, 1);

  nVeto0 = 0;
  nVeto1 = 0;
  nVeto2 = 0;
  nVeto3 = 0;
  nVeto4 = 0;
}

// Do the analysis
void analyze(const Event& event) {

  // Derive the DIS kinematics.
  const DISKinematics& kin = apply<DISKinematics>(event, "Kinematics");

  // Derive the diffractive kinematics (should be used for diffractive only).
  Particle hadronOut;
  Particle hadronIn;
  try {
    const DISDiffHadron& diffhadr = apply<DISDiffHadron>(event, "Hadron");
    hadronOut = diffhadr.out();
    hadronIn = diffhadr.in();
  }
  catch (const Error& e) {
    vetoEvent;
  }

  // Determine event orientation, since coord system is for +z = proton direction
  const int orientation = kin.orientation();

  // Calculate the photon 4-momentum from the incoming and outgoing lepton.
  const FourMomentum qleptonIn = kin.beamLepton().momentum();
  const FourMomentum qleptonOut = kin.scatteredLepton().momentum();
  const FourMomentum qphoton = qleptonIn - qleptonOut;

  // Calculate the Pomeron 4-momentum from the incoming and outgoing hadron
  const FourMomentum pHadOut = hadronOut.momentum();
  const FourMomentum pHadIn = hadronIn.momentum();
  const FourMomentum pPomeron = pHadIn - pHadOut;

  // Q2 and inelasticity cuts
  if (kin.Q2() > 1 * GeV2) vetoEvent;
  ++nVeto0;
  if (!inRange(kin.y(), 0.2, 0.85)) vetoEvent;
  ++nVeto1;

  // Jet selection and veto.
  const Jets jets = apply<FastJets>(event, "Jets")
                        .jets(Cuts::Et > 6.5 * GeV && Cuts::etaIn(-1.5 * orientation, 1.5 * orientation),
                              cmpMomByEt);
  MSG_DEBUG("Jet multiplicity = " << jets.size());
  if (jets.size() < 2) vetoEvent;
  ++nVeto2;
  const Jet& j1 = jets[0];
  const Jet& j2 = jets[1];
  if (j1.Et() < 7.5 * GeV) vetoEvent;
  ++nVeto3;

  // Veto on x_Pomeron.
  const double xPom = (pPomeron * qphoton) / (pHadIn * qphoton);
  if (xPom > 0.025) vetoEvent;
  ++nVeto4;

  // Computation of event-level variables.
  const double eta1 = orientation * j1.eta(), eta2 = orientation * j2.eta();
  const double xyobs = (j1.Et() * exp(-eta1) + j2.Et() * exp(-eta2))
      / (2 * kin.y() * kin.beamLepton().E());
  const size_t i_xyobs = (xyobs < 0.75) ? 1 : 0;
  const double zPobs = (j1.Et() * exp(eta1) + j2.Et() * exp(eta2)) / (2 * xPom * kin.beamHadron().E());
  const double M_X = sqrt((pPomeron + qphoton).mass2());

  // Fill histograms

  _h_dsigma_all[0]->fill(kin.y());
  _h_dsigma_all[1]->fill(M_X);
  _h_dsigma_all[2]->fill(xPom);
  _h_dsigma_all[3]->fill(zPobs);
  _h_dsigma_all[4]->fill(j1.Et());
  _h_dsigma_all[5]->fill(eta1);

  _h_xgamma->fill(xyobs);

  _h_dsigma_xgamma[i_xyobs][0]->fill(kin.y());
  _h_dsigma_xgamma[i_xyobs][1]->fill(M_X);
  _h_dsigma_xgamma[i_xyobs][2]->fill(xPom);
  _h_dsigma_xgamma[i_xyobs][3]->fill(zPobs);
}

// Finalize
void finalize() {
  const double norm = crossSection() / picobarn / sumOfWeights();
  scale(_h_xgamma, norm);
  for (auto& h : _h_dsigma_all) scale(h, norm);
  for (auto& h : _h_dsigma_xgamma[0]) scale(h, norm);
  for (auto& h : _h_dsigma_xgamma[1]) scale(h, norm);

  // Cross section in nb for these observables.
  scale(_h_dsigma_all[2], 1e-3);
  scale(_h_dsigma_xgamma[0][2], 1e-3);
  scale(_h_dsigma_xgamma[1][2], 1e-3);

  double dPHO = nVeto1;

  MSG_INFO("ZEUS_2008_I763403");
  MSG_INFO("Cross section = " << crossSection() / picobarn);
  MSG_INFO("Number of events = " << numEvents() << ", sumW = " << sumOfWeights());
  MSG_INFO("Events passing electron veto1= " << nVeto0 << " (" << nVeto0 / dPHO * 100. << "%)");
  MSG_INFO("Events passing electron veto2= " << nVeto1 << " (" << nVeto1 / dPHO * 100. << "%)");
  MSG_INFO("Events passing jet size veto = " << nVeto2 << " (" << nVeto2 / dPHO * 100. << "%)");
  MSG_INFO("Events passing jet Et veto   = " << nVeto3 << " (" << nVeto3 / dPHO * 100. << "%)");
  MSG_INFO("Events passing xPom veto     = " << nVeto4 << " (" << nVeto4 / dPHO * 100. << "%)");
}

/// @}

private:

/// @name Histograms
/// @{
Histo1DPtr _h_dsigma_all[6];
Histo1DPtr _h_xgamma;
Histo1DPtr _h_dsigma_xgamma[2][4];
/// @}

int nVeto0, nVeto1, nVeto2, nVeto3, nVeto4;

};

RIVET_DECLARE_PLUGIN(ZEUS_2008_I763404);

} ```