Rivet analyses

title: H1_1999_I481112

Measurement of D* meson cross-sections at HERA and determination of the gluon density in the proton using NLO QCD

Experiment: H1 (HERA)

Inspire ID: 481112

Status: VALIDATED

Authors: - Luca Marsili - Hannes Jung

References: - Nucl.Phys.B 545 (1999) 21 - DOI: 10.1016/S0550-3213(99)00119-4 - arXiv: hep-ex/9812023 - DESY-98-204

Beams: e+ p+, p+ e+

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

Run details: none listed

With the H1 detector at the $ep$ collider HERA, $D^$ meson production cross sections have been measured in deep inelastic scattering with four-momentum transfers $Q^2>2$ GeV$^2$ and in photoproduction at energies around $W_{\gamma p} \sim 88$ GeV and 194 GeV. Next-to-Leading Order QCD calculations are found to describe the differential cross sections within theoretical and experimental uncertainties. Using these calculations, the NLO gluon momentum distribution in the proton, $x_g g(x_g)$, has been extracted in the momentum fraction range $7.5 \times 10^{-4}< x_g <4 \times 10^{-2}$ at average scales $mu^2 =25$ to 50 GeV$^2$. The gluon momentum fraction $x_g$ has been obtained from the measured kinematics of the scattered electron and the $D^$ meson in the final state.

Source code:H1_1999_I481112.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DISKinematics.hh"

include "Rivet/Projections/FastJets.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Measurement of D* meson cross-sections at HERA class H1_1999_I481112 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(H1_1999_I481112);


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


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

  // Initialise and register projections

  // The basic final-state projection:
  // all final-state particles within
  // the given eta acceptance
  const FinalState fs(Cuts::abseta < 1.5);
  declare(fs, "fs");
  // The final-state particles declared above are clustered using FastJet with

  //Initialize quantities needed for cuts
  declare(DISKinematics(), "Kinematics");
  declare(UnstableParticles(), "DStars");


  Histo1DPtr dummy;

  book(_h["211"], 2, 1, 1);
  book(_h["311"], 3, 1, 1);
  book(_h["411"], 4, 1, 1);
  book(_h["511"], 5, 1, 1);
  book(_h["611"], 6, 1, 1);
  book(_h["rap194"], 7, 1, 1);
  book(_h["pt194"], 8, 1, 1);
  book(_h["rap88"], 9, 1, 1);
  book(_h["pt88"], 10, 1, 1);
  book(_hpt, {2.5, 3.5, 5.0, 10.0}, {"d11-x01-y01", "d11-x01-y02", "d11-x01-y03"});
  book(_h["1211"], 12, 1, 1);
  book(_h["1212"], 12, 1, 2);
  book(_h["1311"], 13, 1, 1);
}


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


  const DISKinematics& dk = apply<DISKinematics>(event, "Kinematics");


  bool isDIS = false;
  bool ETAG44 = false;
  bool ETAG33 = false;

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

  if (Q2 > 2 && Q2 < 100 && y > 0.05 && y < 0.7) isDIS = true;
  if (Q2 < 0.009 && y > 0.02 && y < 0.32) ETAG44 = true;
  if (Q2 < 0.01 && y > 0.29 && y < 0.62) ETAG33 = true;
  if (isDIS == false && ETAG44 == false && ETAG33 == false) vetoEvent;


  //Creating array of D*
  Cut cuts = isDIS ? (Cuts::pT > 1.5 * GeV && Cuts::abseta < 1.5)
                   : (Cuts::pT > 2 * GeV && Cuts::absrap < 1.5);
  Particles unstables = apply<ParticleFinder>(event, "DStars").particles(cuts);
  const Particles dstars = select(unstables, [](const Particle& p) { return p.abspid() == PID::DSPLUS; });

  if (dstars.empty()) vetoEvent;
  MSG_DEBUG("D*" << dstars.size());

  const Particle& dstar = dstars.front();
  // boosting the system
  const LorentzTransform hcmboost = dk.boostHCM();
  const FourMomentum hcmMom = hcmboost.transform(dstar.momentum());

  //discriminate between dis and photoprod, and between ETA33 and ETA 44

  //kinematics quantities
  const double E = dstar.E();
  const double p_z = dstar.pz();
  //std::cout<<"y: "<<y<<endl;
  if (y < 0.02) vetoEvent;
  const double m2 = 2.25; // charm mass^2
  const double E_e = dk.beamLepton().E();
  const double z = (E - p_z) / (2 * y * E_e);
  if (z > 1) {
    MSG_DEBUG("Momentum fraction greater than unity! This should not happen. Vetoing event.");
    vetoEvent;
  }

  const double M2 = (1.44 * hcmMom.pT2() + m2) / (z * (1 - z));
  const double x_g = (M2 + Q2) / (y * dk.s());

  const double y_capp = dstar.rapidity();
  const double W = sqrt(dk.W2());

  //perform the cuts
  if (isDIS == true) {
    _h["211"]->fill(dstar.pT());
    _h["411"]->fill(dstar.eta());
    _h["511"]->fill(Q2);
    _h["611"]->fill(log10(x_g));
    //boosting to the hcm frame
    _h["311"]->fill(hcmMom.pT());
  }
  if (ETAG33 == true && abs(y_capp) < 1.5 && dstar.pT() > 2.5 * GeV) {

    _h["rap194"]->fill(y_capp);
    _h["pt194"]->fill(dstar.pT());
    _hpt->fill(dstar.pT(), y_capp);

    if (W > 173 && W < 273) {
      _h["1211"]->fill(log10(x_g));
    }
    if (W > 130 && W < 230) {
      _h["1212"]->fill(log10(x_g));
    }
  }
  if (ETAG44 == true && abs(y_capp) < 1.5 && dstar.pT() > 2) {

    _h["pt88"]->fill(dstar.pT());
    _h["rap88"]->fill(y_capp);
    _h["1311"]->fill(log10(x_g));
  }
}

/// Normalise histograms etc., after the run
void finalize() {
  // conversion factors from ep to gamma p xsections (as given in publication)
  const double F_etag33 = 0.0128;
  const double F_etag44 = 0.0838;

  double norm = crossSection() / nanobarn / sumW();
  scale(_h["211"], norm);

  scale(_h["311"], norm);
  scale(_h["411"], norm);
  scale(_h["511"], norm);
  scale(_h["611"], norm);
  double norm_mub = crossSection() / microbarn / sumW();

  scale(_h["rap194"], norm_mub / F_etag33);
  scale(_h["pt194"], norm_mub / F_etag33);
  scale(_h["rap88"], norm_mub / F_etag44);
  scale(_h["pt88"], norm_mub / F_etag44);

  scale(_hpt, norm / F_etag33);
  scale(_h["1211"], norm_mub / F_etag33);
  scale(_h["1212"], norm_mub / F_etag33);
  scale(_h["1311"], norm_mub / F_etag44);
}

///@}


/// @name Histograms
///@{
map<string, Histo1DPtr> _h;
Histo1DGroupPtr _hpt;
///@}

};

RIVET_DECLARE_PLUGIN(H1_1999_I481112);

} ```