Rivet analyses

Scaled momentum distributions for K_S⁰ and Λ/Λ̄ 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^±p deep inelastic scattering with p at 920~GeV, e^± at 27.5 GeV $\sqrt{s} = 318~\GeV$

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 330pb⁻¹. The evolution of these distributions with the photon virtuality, Q², was studied in the kinematic region 10 < Q² < 40000GeV² 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_S⁰ and Λ/Λ̄ strange hadrons.

Source code:`ZEUS_2011_I945935.cc`

// -*- 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);

}

Rivet analyses

Scaled momentum distributions for KS0 and Λ/Λ̄ in DIS at HERA

Source code:ZEUS_2011_I945935.cc

Scaled momentum distributions for K_S⁰ and Λ/Λ̄ in DIS at HERA

Source code:`ZEUS_2011_I945935.cc`