Rivet analyses

title: ZEUS_2011_I945935

Scaled momentum distributions for $K^0_S$ and $\Lambda/\bar{\Lambda}$ in DIS at HERA

Experiment: ZEUS (HERA)

Inspire ID: 945935

Status: VALIDATED

Authors: - Andrii Verbytskyi

References: - JHEP 1203 (2012) 020 - DOI: 10.1007/JHEP03(2012)020 - DESY-11-205 - arXiv: 1111.3526

Beams: p+ e+, p+ e-

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

Run details: - $e^{\pm} p$ deep inelastic scattering with $p$ at 920~GeV, $e^{\pm}$ at 27.5 GeV \to $\sqrt{s} = 318~\GeV$

Scaled momentum distributions for the strange hadrons K0s and Lambda/bar Lambda were measured in deep inelastic ep scattering with the ZEUS detector at HERA using an integrated luminosity of $330 pb^{-1}$. The evolution of these distributions with the photon virtuality, $Q^2$, was studied in the kinematic region $10<Q^2<40000 GeV^2$ and $0.001<x<0.75$, where $x$ is the Bjorken scaling variable. Clear scaling violations are observed. Predictions based on different approaches to fragmentation were compared to the measurements. Leading-logarithm parton-shower Monte Carlo calculations interfaced to the Lund string fragmentation model describe the data reasonably well in the whole range measured. Next-to-leading-order QCD calculations based on fragmentation functions, FFs, extracted from $e^+e^-$ data alone, fail to describe the measurements. The calculations based on FFs extracted from a global analysis including $e^+e^-$, $ep$ and $pp$ data give an improved description. The measurements presented in this paper have the potential to further constrain the FFs of quarks, anti-quarks and gluons yielding $K^0_{S}$ and $\Lambda/\bar{ \Lambda}$ strange hadrons.

Source code:ZEUS_2011_I945935.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DISFinalState.hh"

include "Rivet/Projections/DISKinematics.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Scaled momenta of identified particles class ZEUS_2011_I945935 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ZEUS_2011_I945935);


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

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

  const UnstableParticles& labcut = UnstableParticles();
  declare(labcut, "UFS");
  const DISKinematics& diskin = DISKinematics();
  declare(diskin, "Kinematics");
  const DISFinalState& disfsbf = DISFinalState(labcut, DISFrame::BREIT);
  declare(disfsbf, "FSBF");

  for (size_t offset = 0; offset < 5; offset++) {
    book(_h_K0S[offset], 2, 1, offset + 1);
    book(_h_LAMBDA[offset], 5, 1, offset + 1);
  }

  book(_h_K0S[5], 3, 1, 1);
  book(_h_K0S[6], 3, 1, 2);
  book(_h_LAMBDA[5], 6, 1, 1);
  book(_h_LAMBDA[6], 6, 1, 2);
  book(_h_Q2_tmp, "_TMP/N", 7, 0, 7);
}

int getbinQ2v1(const DISKinematics& dk) {
  if (inRange(dk.Q2() / GeV2, 10.0, 40.0) && inRange(dk.x(), 0.001, 0.75)) return 1;
  if (inRange(dk.Q2() / GeV2, 40.0, 160.0) && inRange(dk.x(), 0.001, 0.75)) return 2;
  if (inRange(dk.Q2() / GeV2, 160.0, 640.0) && inRange(dk.x(), 0.001, 0.75)) return 3;
  if (inRange(dk.Q2() / GeV2, 640.0, 2560.0) && inRange(dk.x(), 0.001, 0.75)) return 4;
  if (inRange(dk.Q2() / GeV2, 2650.0, 10240.0) && inRange(dk.x(), 0.001, 0.75)) return 5;
  return -1;
}
int getbinQ2v2(const DISKinematics& dk) {
  if (inRange(dk.Q2() / GeV2, 10.0, 100.0) && inRange(dk.x(), 0.001, 0.75)) return 1;
  if (inRange(dk.Q2() / GeV2, 100.0, 1000.0) && inRange(dk.x(), 0.001, 0.75)) return 2;
  return -1;
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  /// DIS kinematics
  const DISKinematics& dk = apply<DISKinematics>(event, "Kinematics");
  const double q2 = dk.Q2();
  const double x = dk.x();
  const double y = dk.y();

  if (!inRange(q2 / GeV2, 10.0, 40000.0)) vetoEvent;
  if (!inRange(y, 0.04, 0.95)) vetoEvent;
  if (!inRange(x, 0.001, 0.75)) vetoEvent;
  const int ofv1 = getbinQ2v1(dk) - 1;
  const int ofv2 = getbinQ2v2(dk) - 1;
  if (ofv1 < 0) vetoEvent;

  /// @todo Do the event by event analysis here
  _h_Q2_tmp->fill(ofv1);
  if (ofv2 >= 0) _h_Q2_tmp->fill(5 + ofv2);
  const DISFinalState& disfsbf = apply<DISFinalState>(event, "FSBF");

  for (const Particle& p : select(disfsbf.particles(), Cuts::abspid == abs(PID::K0S))) {
    //// Scaled energy.
    if (p.pz() > 0) continue;
    const double energy = p.momentum().vector3().mod();
    const double scaledEnergy = 2.0 * energy / sqrt(q2);
    _h_K0S[ofv1]->fill(scaledEnergy);
    if (ofv2 >= 0) _h_K0S[5 + ofv2]->fill(scaledEnergy);
  }

  for (const Particle& p : select(disfsbf.particles(), Cuts::abspid == abs(PID::LAMBDA))) {
    //// Scaled energy.
    if (p.pz() > 0) continue;
    const double energy = p.momentum().vector3().mod();
    const double scaledEnergy = 2.0 * energy / sqrt(q2);
    _h_LAMBDA[ofv1]->fill(scaledEnergy);
    if (ofv2 >= 0) _h_LAMBDA[5 + ofv2]->fill(scaledEnergy);
  }
}

/// Normalise histograms etc., after the run
void finalize() {
  for (size_t offset = 0; offset < 7; offset++) {
    scale(_h_K0S[offset], 1.0 / _h_Q2_tmp->bin(offset + 1).sumW());
    scale(_h_LAMBDA[offset], 1.0 / _h_Q2_tmp->bin(offset + 1).sumW());
  }
}

Histo1DPtr _h_K0S[7];
Histo1DPtr _h_LAMBDA[7];
Histo1DPtr _h_Q2_tmp;

};

RIVET_DECLARE_PLUGIN(ZEUS_2011_I945935);

} ```