Rivet analyses

title: ZEUS_2005_I676091

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++ // -- 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);

} ```