Rivet analyses

Dijet photoproduction analysis

Experiment: ZEUS (HERA Run I)

Inspire ID: 450085

Status: VALIDATED

Authors: - Juan Jose Juan Castella - Jon Butterworth - Matthew Wing

References: - EEur. Phys. J C 1, (1998) 109-122 - DESY-97-196 - hep-ex/9710018

Beams: p+ e+

Beam energies: (820.0, 27.5)GeV

Run details: - 820 GeV protons colliding with 27.5 GeV positrons; Direct and resolved photoproduction of dijet events; Both jets E_T > 6 GeV; Jet pseudorapidity −1.375 < |η| < 1.875; Pseudorapidity difference between jets |η| < 0.5;

Dijet cross sections are presented using photoproduction data obtained with the ZEUS detector during 1994. These measurements represent an extension of previous results, as the higher statistics allow cross sections to be measured at higher jet transverse energy (ETJ). Jets are identified in the hadronic final state, and the cross sections compared to complete next-to-leading order QCD calculations. Agreement with these calculations is seen for the pseudorapidity dependence of the direct photon events with ETJ > 6 GeV and of the resolved photon events with ETJ > 11 GeV. Calculated cross sections for resolved photon processes with 6 GeV < ETJ < 11 GeV lie below the data. The paper also studied reconstructing the jets with two different types of legacy cone algorithm (EUCELL and PUCELL). Histograms for these are not generated by this rivet routine as focus was on the the KTCLUS data.

Source code:`ZEUS_1997_I450085.cc`

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/Beam.hh"
#include "Rivet/Projections/DISKinematics.hh"
#include "Rivet/Projections/FastJets.hh"

namespace Rivet {
    // @brief ZEUS dijet photoproduction in different xgamma regions
  
    class ZEUS_1997_I450085 : public Analysis {
    public:

        // Constructor
        RIVET_DEFAULT_ANALYSIS_CTOR(ZEUS_1997_I450085);
        
        // Initialization, books projections and histograms
        void init() {
            
            // Projections
            // checking recombination scheme and radius checked with original code from M.Wing
            FinalState fs;
            declare(FastJets(fs, fastjet::JetAlgorithm::kt_algorithm, fastjet::RecombinationScheme::Et_scheme, 1.0), "Jets");
            declare(DISKinematics(), "Kinematics");

            //Histograms
            // Table 1
            book(_h_etabar_all[0], 1, 1, 1);
            book(_h_etabar_all[1], 2, 1, 1);
            book(_h_etabar_all[2], 3, 1, 1);
            book(_h_etabar_all[3], 4, 1, 1);
            // Table 6
            book(_h_etabar[1][0], 21, 1, 1);
            book(_h_etabar[1][1], 22, 1, 1);
            book(_h_etabar[1][2], 23, 1, 1);
            book(_h_etabar[1][3], 24, 1, 1);
            // Table 7
            book(_h_etabar[0][0], 25, 1, 1);
            book(_h_etabar[0][1], 26, 1, 1);
            book(_h_etabar[0][2], 27, 1, 1);
            book(_h_etabar[0][3], 28, 1, 1);

        }
        // Analysis
        void analyze(const Event & event) {

            // Determine kinematics, including event orientation since ZEUS coord system is for +z = proton direction
            const DISKinematics & kin = apply<DISKinematics>(event, "Kinematics");
            if (kin.failed()) vetoEvent;
            const int orientation = kin.orientation();

            // Q2 and inelasticity cuts
            if (kin.Q2() > 4 * GeV2) vetoEvent;
            if (!inRange(kin.y(), 0.2, 0.8)) vetoEvent;

            // Jet calculation
            const Jets jets = apply<FastJets>(event, "Jets") \
                .jets(Cuts::Et > 6 * GeV && Cuts::etaIn(-1.375 * orientation, 1.875 * orientation), cmpMomByEt);
            MSG_DEBUG("Jet multiplicity = " << jets.size());
            //Dijet event selection
            if (jets.size() < 2) vetoEvent;
            const Jet & j1 = jets[0];
            const Jet & j2 = jets[1];
    
            //Jet eta, average eta and eta difference calculation
            const double eta1 = orientation * j1.eta(), eta2 = orientation * j2.eta();
            const double etabar = (eta1 + eta2) / 2;
            const double etadiff = eta1 - eta2;

            //Cut in pseudorapidity difference
            if (abs(etadiff) > 0.5) vetoEvent;

            // Calculation of x_gamma^obs
            /// note Assuming Ee is the lab frame positron momentum, not in proton rest frame cf.
            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) ? 0 : 1;

            //Classify events according to minimum jet energies
            size_t iE_min = 0;
            if (j1.Et() > 8 * GeV && j2.Et() > 8 * GeV)
                iE_min = 1;
            if (j1.Et() > 11 * GeV && j2.Et() > 11 * GeV)
                iE_min = 2;
            if (j1.Et() > 15 * GeV && j2.Et() > 15 * GeV)
                iE_min = 3;

            //Fill histograms
            for (size_t isel = 0; isel <= iE_min; ++isel) {

                //T1
                _h_etabar_all[isel]->fill(etabar);
                //T6, T7
                if (xyobs < 0.3) vetoEvent;
                _h_etabar[i_xyobs][isel]->fill(etabar);
            }
            

        }
        
        // Finalize
        void finalize() {
            const double sf = crossSection() / nanobarn / sumOfWeights();
            for (size_t ix = 0; ix < 2; ++ix) {
                for (auto& h : _h_etabar[ix]) scale(h, sf);

            }
            for (auto& h : _h_etabar_all) scale(h, sf);

        }


    private:
        //name Histograms
        Histo1DPtr _h_etabar_all[4], _h_etabar[2][4];

    };
    
    RIVET_DECLARE_PLUGIN(ZEUS_1997_I450085);

}

Rivet analyses

Dijet photoproduction analysis

Source code:ZEUS_1997_I450085.cc

Source code:`ZEUS_1997_I450085.cc`