Rivet analyses

Multi-jet production in NC DIS

Experiment: ZEUS (HERA)

Inspire ID: 676091

Status: VALIDATED

Authors: - Christian Gutschow

References: - Eur.Phys.J.C 44 (2005) 183-193 - arXiv: hep-ex/0502007

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

Beam energies: (920.0, 27.5); (27.5, 920.0); (920.0, 27.5); (27.5, 920.0)GeV

Run details: - NC DIS events

Multijet production rates in neutral current deep inelastic scattering have been measured in the range of exchanged boson virtualities $10 < Q^2 < 5000$ GeV$^2$. The data were taken at the $ep$ collider HERA with centre-of-mass energy sqrt(s) = 318 GeV using the ZEUS detector and correspond to an integrated luminosity of 82.2 pb$^{-1}$. Jets were identified in the Breit frame using the $k_\text{T}$ cluster algorithm in the longitudinally invariant inclusive mode. Measurements of differential dijet and trijet cross sections are presented as functions of jet transverse energy $E_\text{T,B}^\text{jet}$, pseudorapidity $\eta_\text{LAB}^\text{jet}$ and $Q^2$ with $E_\text{T,B}^\text{jet} > 5$ GeV and $-1 < \eta_\text{LAB}^\text{jet} < 2.5$. Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant $\alpha_\text{s}(M_Z)$, determined from the ratio of the trijet to dijet cross sections, is $\alpha_\text{s}(M_Z) = 0.1179 \pm 0.0013$(stat.) $^{+0.0028}{-0.0046}$(exp.) $^{+0.0064}{-0.0046}$(th.)

Source code:ZEUS_2005_I676091.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/DISFinalState.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/FinalState.hh"

namespace Rivet {


  /// @brief Multi-jet production in NC DIS
  class ZEUS_2005_I676091 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(ZEUS_2005_I676091);


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

    /// Book histograms and initialise projections before the run
    void init() {

      // The final-state particles are clustered in Breit frame
      // using FastJet with the kT algorithm and a jet-radius parameter of 1.
      const DISFinalState DISfs(DISFrame::BREIT);
      declare(DISfs, "fs");

      FastJets jets(DISfs, JetAlg::KT, 1.0);
      declare(jets, "jets");

      // Book histograms.
      book(_h["Et0"], 1, 1, 1);
      book(_h["Et1"], 2, 1, 1);
      book(_h["Et2"], 3, 1, 1);
      book(_h["Eta0"], 4, 1, 1);
      book(_h["Eta1"], 5, 1, 1);
      book(_h["Eta2"], 6, 1, 1);
      book(_h["2jetQ2"], 7, 1, 1);
      book(_h["3jetQ2"], 8, 1, 1);
      book(_ratio, 9, 1, 1);
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {

      // Lorentz invariant DIS quantities
      const DISKinematics& dis = apply<DISFinalState>(event, "fs").kinematics();
      if (dis.failed()) vetoEvent;

      const double Q2 = dis.Q2();
      const double y = dis.y();

      // Kinematic cuts on virtuality and inelasticity.
      if (!inRange(Q2, 10. * GeV2, 5000. * GeV2)) vetoEvent;
      if (!inRange(y, 0.04, 0.6)) vetoEvent;

      // Lorentz boosts for Breit and lab frames.
      const LorentzTransform breitboost = dis.boostBreit();
      const LorentzTransform labboost = breitboost.inverse();

      // Retrieve clustered jets in Breit frame, sorted by pT.
      Jets alljets = apply<FastJets>(event, "jets").jets(Cuts::Et > 5 * GeV, cmpMomByEt);

      // Cut on Pseurdorapidity in lab frame.
      // 1 if hadron in "conventional" +z direction, -1 if in -z.
      size_t jetcount = 3;
      Jets jets;
      vector<double> etas;
      const int orientation = dis.orientation();
      for (auto& jet : alljets) {
        jet.transformBy(labboost); // boost to lab frame
        if (inRange(jet.eta() * orientation, -1., 2.5)) {
          if (jetcount--) etas.push_back(jet.eta() * orientation);
          jet.transformBy(breitboost);
          jets += jet;
        }
      }
      if (jets.size() < 2) vetoEvent;

      if (jets.size() == 2) {
        if ((jets[0].mom() + jets[1].mom()).mass() <= 25 * GeV) vetoEvent;
      }
      else if ((jets[0].mom() + jets[1].mom() + jets[2].mom()).mass() <= 25 * GeV)
        vetoEvent;

      _h["2jetQ2"]->fill(Q2 / GeV2);

      if (jets.size() < 3) vetoEvent;

      _h["3jetQ2"]->fill(Q2 / GeV2);

      _h["Et0"]->fill(jets[0].Et() / GeV);
      _h["Et1"]->fill(jets[1].Et() / GeV);
      _h["Et2"]->fill(jets[2].Et() / GeV);

      std::sort(etas.begin(), etas.end(), std::greater<double>());
      _h["Eta0"]->fill(etas[0]);
      _h["Eta1"]->fill(etas[1]);
      _h["Eta2"]->fill(etas[2]);
    }

    /// Normalise histograms after the run
    void finalize() {

      scale(_h, crossSection() / picobarn / sumW());

      divide(_h["3jetQ2"], _h["2jetQ2"], _ratio);
    }

    /// @}

  private:

    /// @name Histograms
    /// @{
    map<string, Histo1DPtr> _h;
    Estimate1DPtr _ratio;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(ZEUS_2005_I676091);

}