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ZEUS_1997_I450085

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 $-1.375 < |\eta| < 1.875$; Pseudorapidity difference between jets $|\eta| < 0.5$;

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
  1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/Beam.hh"
  4#include "Rivet/Projections/DISKinematics.hh"
  5#include "Rivet/Projections/FastJets.hh"
  6
  7namespace Rivet {
  8	// @brief ZEUS dijet photoproduction in different xgamma regions
  9  
 10	class ZEUS_1997_I450085 : public Analysis {
 11	public:
 12
 13		// Constructor
 14		RIVET_DEFAULT_ANALYSIS_CTOR(ZEUS_1997_I450085);
 15		
 16		// Initialization, books projections and histograms
 17		void init() {
 18			
 19			// Projections
 20			// checking recombination scheme and radius checked with original code from M.Wing
 21			FinalState fs;
 22			declare(FastJets(fs, fastjet::JetAlgorithm::kt_algorithm, fastjet::RecombinationScheme::Et_scheme, 1.0), "Jets");
 23			declare(DISKinematics(), "Kinematics");
 24
 25			//Histograms
 26			// Table 1
 27			book(_h_etabar_all[0], 1, 1, 1);
 28			book(_h_etabar_all[1], 2, 1, 1);
 29			book(_h_etabar_all[2], 3, 1, 1);
 30			book(_h_etabar_all[3], 4, 1, 1);
 31			// Table 6
 32			book(_h_etabar[1][0], 21, 1, 1);
 33			book(_h_etabar[1][1], 22, 1, 1);
 34			book(_h_etabar[1][2], 23, 1, 1);
 35			book(_h_etabar[1][3], 24, 1, 1);
 36			// Table 7
 37			book(_h_etabar[0][0], 25, 1, 1);
 38			book(_h_etabar[0][1], 26, 1, 1);
 39			book(_h_etabar[0][2], 27, 1, 1);
 40			book(_h_etabar[0][3], 28, 1, 1);
 41
 42		}
 43		// Analysis
 44		void analyze(const Event & event) {
 45
 46			// Determine kinematics, including event orientation since ZEUS coord system is for +z = proton direction
 47			const DISKinematics & kin = apply<DISKinematics>(event, "Kinematics");
 48		    if (kin.failed()) vetoEvent;
 49			const int orientation = kin.orientation();
 50
 51			// Q2 and inelasticity cuts
 52			if (kin.Q2() > 4 * GeV2) vetoEvent;
 53			if (!inRange(kin.y(), 0.2, 0.8)) vetoEvent;
 54
 55			// Jet calculation
 56			const Jets jets = apply<FastJets>(event, "Jets") \
 57				.jets(Cuts::Et > 6 * GeV && Cuts::etaIn(-1.375 * orientation, 1.875 * orientation), cmpMomByEt);
 58			MSG_DEBUG("Jet multiplicity = " << jets.size());
 59			//Dijet event selection
 60			if (jets.size() < 2) vetoEvent;
 61			const Jet & j1 = jets[0];
 62		    const Jet & j2 = jets[1];
 63	
 64			//Jet eta, average eta and eta difference calculation
 65			const double eta1 = orientation * j1.eta(), eta2 = orientation * j2.eta();
 66			const double etabar = (eta1 + eta2) / 2;
 67			const double etadiff = eta1 - eta2;
 68
 69			//Cut in pseudorapidity difference
 70			if (abs(etadiff) > 0.5) vetoEvent;
 71
 72			// Calculation of x_gamma^obs
 73			/// note Assuming Ee is the lab frame positron momentum, not in proton rest frame cf.
 74		    const double xyobs = (j1.Et() * exp(-eta1) + j2.Et() * exp(-eta2)) / (2 * kin.y() * kin.beamLepton().E());
 75			const size_t i_xyobs = (xyobs < 0.75) ? 0 : 1;
 76
 77			//Classify events according to minimum jet energies
 78			size_t iE_min = 0;
 79			if (j1.Et() > 8 * GeV && j2.Et() > 8 * GeV)
 80				iE_min = 1;
 81			if (j1.Et() > 11 * GeV && j2.Et() > 11 * GeV)
 82				iE_min = 2;
 83			if (j1.Et() > 15 * GeV && j2.Et() > 15 * GeV)
 84				iE_min = 3;
 85
 86			//Fill histograms
 87			for (size_t isel = 0; isel <= iE_min; ++isel) {
 88
 89				//T1
 90				_h_etabar_all[isel]->fill(etabar);
 91				//T6, T7
 92				if (xyobs < 0.3) vetoEvent;
 93				_h_etabar[i_xyobs][isel]->fill(etabar);
 94			}
 95			
 96
 97		}
 98		
 99		// Finalize
100		void finalize() {
101			const double sf = crossSection() / nanobarn / sumOfWeights();
102			for (size_t ix = 0; ix < 2; ++ix) {
103				for (auto& h : _h_etabar[ix]) scale(h, sf);
104
105			}
106			for (auto& h : _h_etabar_all) scale(h, sf);
107
108		}
109
110
111	private:
112		//name Histograms
113		Histo1DPtr _h_etabar_all[4], _h_etabar[2][4];
114
115	};
116	
117	RIVET_DECLARE_PLUGIN(ZEUS_1997_I450085);
118
119}