Rivet analyses

Differential cross-sections of Z/γ* → e+e− vs rapidity and ϕ*

Experiment: LHCb (LHC)

Inspire ID: 1208102

Status: VALIDATED

Authors: - Ana Elena Dumitriu - Marek Sirendi - David Ward - Alex Grecu

References: - J. High Energy Phys. 02 (2013) 106 - DOI: 10.1007/JHEP02(2013)106 - arXiv: 1212.4620

Beams: p+ p+

Beam energies: (3500.0, 3500.0)GeV

Run details: - $\mathrm{Z}/\gamma^* \to e^{+}e^{-}_$ decays with di-lepton invariant mass  > 40 GeV/c2 produced in pp collisions at $\sqrt{s}=7$ TeV.

Measurement of the pp → Z0 cross-section in the Z/γ → e+e− mode at $\sqrt{s} = 7$ TeV. Daughter electrons are required to have pT > 20 GeV/c, 2 < η < 4.5 and the dielectron invariant mass in range 60-120 GeV/c2. The cross-section is given as a function of Z rapidity and an angular variable (ϕ*) closely related to Z transverse momentum (derived from the lepton pseudorapidity and azimuthal angle differences). For event generators implementing cross-section QCD corrections only at LO the distributions are normalized to the cross-section measured in data 76.0 ± 0.8 ± 2.0 ± 2.6 ± 0.4 pb, where the first uncertainty is statistical, the second is systematic, the third is due to luminosity uncertainty and the fourth to FSR corrections.

Source code:LHCB_2012_I1208102.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/DileptonFinder.hh"

namespace Rivet {


  /// Differential cross-sections of $\mathrm{Z}/\gamma^* \to e^{+}e^{-}$ vs rapidity and $\phi^*$
  class LHCB_2012_I1208102 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(LHCB_2012_I1208102);


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

    /// Book histograms
    void init() {
      DileptonFinder zeefinder(91.2*GeV, 0.1, Cuts::etaIn(2.0, 4.5) && Cuts::pT > 20*GeV &&
                               Cuts::abspid == PID::ELECTRON, Cuts::massIn(60*GeV, 120*GeV));
      declare(zeefinder, "ZeeFinder");

      book(_h_sigma_vs_y,   2, 1, 1);
      book(_h_sigma_vs_phi, 3, 1, 1);
    }


    /// Do the analysis
    void analyze(const Event& e) {
      const DileptonFinder& zeefinder = apply<DileptonFinder>(e, "ZeeFinder");
      if (zeefinder.empty()) vetoEvent;
      if (zeefinder.bosons().size() > 1)
        MSG_WARNING("Found multiple (" << zeefinder.bosons().size() << ") Z -> e+ e- decays!");

      // Z momenta
      const FourMomentum zee = zeefinder.bosons()[0].momentum();

      if (zeefinder.constituents().size() < 2) vetoEvent;

      const Particle pozitron = zeefinder.constituents()[0];
      const Particle electron = zeefinder.constituents()[1];

      // Calculation of the angular variable
      const double diffphi = deltaPhi(pozitron, electron);
      const double diffpsd = deltaEta(pozitron, electron);
      const double accphi = M_PI - diffphi;
      const double angular = tan(accphi/2) / cosh(diffpsd/2);

      // Fill histograms
      _h_sigma_vs_y->fill(zee.rapidity());
      _h_sigma_vs_phi->fill(angular);
    }


    /// Finalize
    void finalize() {
      const double xs = crossSection()/picobarn;
      scale(_h_sigma_vs_y, xs/sumOfWeights());
      scale(_h_sigma_vs_phi, xs/sumOfWeights());
    }

    /// @}


  private:

    /// @name Histograms
    /// @{
    Histo1DPtr _h_sigma_vs_y, _h_sigma_vs_phi;
    /// @}

  };


  RIVET_DECLARE_PLUGIN(LHCB_2012_I1208102);

}