Rivet analyses

title: ZEUS_2004_I649041

Dijet dependence on photon virtuality in ep

Experiment: ZEUS (HERA)

Inspire ID: 649041

Status: VALIDATED

Authors: - Zain Mahmood

References: - arXiv: 0404033 - Eur.Phys.J.C 35 (2004) 487-500

Beams: p+ e-, p+ e+, e+ p+, e- p+

Beam energies: (820.0, 27.5); (820.0, 27.5); (27.5, 820.0); (27.5, 820.0)GeV

Run details: - 27.5 GeV positrons colliding with 820 GeV protons

The dependence of dijet production on the virtuality of the exchanged photon, $Q^2$, has been studied by measuring dijet cross sections in the range $0 \lesssim Q2 < 2000$ GeV$^2$ with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb$^{-1}$. Dijet cross sections were measured for jets with transverse energy $E^\text{jet}\text{T} > 7.5$ and 6.5 GeV and pseudorapidities in the photon-proton centre-of-mass frame in the range $-3 < \eta^\text{jet} < 0$. The variable $x^\text{obs}\gamma$, a measure of the photon momentum entering the hard process, was used to enhance the sensitivity of the measurement to the photon structure. The $Q^2$ dependence of the ratio of low- to high-$x^\text{obs}\gamma$ events was measured. Next-to-leading-order QCD predictions were found to generally underestimate the low-$x^\mathrm{obs}\gamma$ contribution relative to that at high $x^\mathrm{obs}_\gamma$. Monte Carlo models based on leading-logarithmic parton-showers, using a partonic structure for the photon which falls smoothly with increasing $Q^2$, provide a qualitative description of the data.

Source code:ZEUS_2004_I649041.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/Beam.hh"

include "Rivet/Projections/DISFinalState.hh"

include "Rivet/Projections/DISKinematics.hh"

include "Rivet/Projections/FastJets.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief Dijet dependence on photon virtuality in ep class ZEUS_2004_I649041 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ZEUS_2004_I649041);

// Book projections and histograms
void init() {
  // Declare your final-state and jet projections
  declare(FastJets(DISFinalState(DISFrame::HCM), fastjet::JetAlgorithm::kt_algorithm,
                   fastjet::RecombinationScheme::Et_scheme, 1.0),
          "DISFSJets");
  declare(DISKinematics(), "Kinematics");

  // Book single-differential histograms
  book(_h_Q2[0], 1, 1, 1);
  book(_h_Q2[1], 2, 1, 1);
  book(_h_Q2[2], 2, 1, 2);

  // Book double-differential vs. Q² and jet Et
  book(_h_Q2Et[0], 3, 1, 1);
  book(_h_Q2Et[1], 4, 1, 1);
  book(_h_Q2Et[2], 5, 1, 1);
  book(_h_Q2Et[3], 6, 1, 1);
  book(_h_Q2Et[4], 7, 1, 1);
  book(_h_Q2Et[5], 8, 1, 1);
  book(_h_Q2Et[6], 9, 1, 1);

  // Book double-differential vs. Q² and forward-jet eta
  book(_h_Q2etaf[0], 10, 1, 1);
  book(_h_Q2etaf[1], 11, 1, 1);
  book(_h_Q2etaf[2], 12, 1, 1);
  book(_h_Q2etaf[3], 13, 1, 1);
  book(_h_Q2etaf[4], 14, 1, 1);
  book(_h_Q2etaf[5], 15, 1, 1);
  book(_h_Q2etaf[6], 16, 1, 1);
}


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

  // Get the DIS kinematics
  const DISKinematics& kin = apply<DISKinematics>(event, "Kinematics");
  if (kin.failed()) vetoEvent;
  const int orientation = kin.orientation();

  // Define kinematic variables
  double Q2 = kin.Q2();
  double y = kin.y();

  // Set pseudorapidity cuts in the HCM frame based on orientation:
  // If proton is along +z (orientation=+1) we use [-3,0]; if not, flip to [0,3].
  double etamin = -3.0;
  double etamax = 0.0;
  if (orientation < 0) {
    etamin = 0.0;
    etamax = 3.0;
  }

  // Basic Q² and inelasticity cuts
  if (!inRange(Q2, 0.0, 2000.0)) vetoEvent;
  if (!inRange(y, 0.2, 0.55)) vetoEvent;

  // Jet selection: use jets clustered in the HCM frame with the adjusted eta range
  const Jets jets = apply<FastJets>(event, "DISFSJets")
                        .jets(Cuts::Et > 6.5 * GeV && Cuts::etaIn(etamin, etamax), cmpMomByEt);
  MSG_DEBUG("Jet multiplicity = " << jets.size());
  if (jets.size() < 2) vetoEvent;
  const Jet& j1 = jets[0];
  const Jet& j2 = jets[1];

  // Cut on leading jet
  if (j1.Et() < 7.5 * GeV) vetoEvent;

  // Correct the jet pseudorapidities so they always represent the proton direction.
  const double eta1 = orientation * j1.eta();
  const double eta2 = orientation * j2.eta();

  // Calculate xobs according to the paper
  const double xobs = (j1.Et() * exp(-eta1) + j2.Et() * exp(-eta2))
      / (2 * kin.y() * kin.beamLepton().E());


  // Fill single-differential histograms
  _h_Q2[0]->fill(Q2);
  if (xobs < 0.75) _h_Q2[1]->fill(Q2);
  if (xobs > 0.75) _h_Q2[2]->fill(Q2);

  // Fill double-differential histograms: Q² vs. jet E_T.
  if (inRange(Q2, 0.0, 1.0)) _h_Q2Et[0]->fill(j1.Et());
  if (inRange(Q2, 0.1, 0.55)) _h_Q2Et[1]->fill(j1.Et());
  if (inRange(Q2, 1.0, 4.5)) _h_Q2Et[2]->fill(j1.Et());
  if (inRange(Q2, 4.5, 10.5)) _h_Q2Et[3]->fill(j1.Et());
  if (inRange(Q2, 10.5, 49.0)) _h_Q2Et[4]->fill(j1.Et());
  if (inRange(Q2, 49.0, 120.0)) _h_Q2Et[5]->fill(j1.Et());
  if (inRange(Q2, 120.0, 2000.0)) _h_Q2Et[6]->fill(j1.Et());

  // For the forward-jet η histograms: select the jet with the larger corrected pseudorapidity
  const Jet& fjet = (orientation * j1.eta() > orientation * j2.eta() ? j1 : j2);
  double fjetEta = orientation * fjet.eta();

  // Fill double-differential histograms: Q² vs. forward jet η.
  if (inRange(Q2, 0.0, 1.0)) _h_Q2etaf[0]->fill(fjetEta);
  if (inRange(Q2, 0.1, 0.55)) _h_Q2etaf[1]->fill(fjetEta);
  if (inRange(Q2, 1.0, 4.5)) _h_Q2etaf[2]->fill(fjetEta);
  if (inRange(Q2, 4.5, 10.5)) _h_Q2etaf[3]->fill(fjetEta);
  if (inRange(Q2, 10.5, 49.0)) _h_Q2etaf[4]->fill(fjetEta);
  if (inRange(Q2, 49.0, 120.0)) _h_Q2etaf[5]->fill(fjetEta);
  if (inRange(Q2, 120.0, 2000.0)) _h_Q2etaf[6]->fill(fjetEta);
}


// Finalize: scale histograms by cross section, etc.
void finalize() {
  const double sf = crossSection() / picobarn / sumOfWeights();
  scale(_h_Q2, sf);
  scale(_h_Q2Et, sf);
  scale(_h_Q2etaf, sf);
  double q2BinWidths[7] = {1, .45, 3, 6, 38.5, 71, 1880};
  for (unsigned int ix = 0; ix < 7; ++ix) {
    scale(_h_Q2Et[ix], 1. / q2BinWidths[ix]);
    scale(_h_Q2etaf[ix], 1. / q2BinWidths[ix]);
  }
}

private:

/// name histograms
Histo1DPtr _h_Q2[3];
Histo1DPtr _h_Q2Et[7];
Histo1DPtr _h_Q2etaf[7];

};

RIVET_DECLARE_PLUGIN(ZEUS_2004_I649041);

} ```