Rivet analyses

Z forward-backward asymmetry

Experiment: ATLAS (LHC)

Inspire ID: 1351916

Status: VALIDATED

Authors: - Christian Gutschow

References: - Expt page: ATLAS-STDM-2011-34 - arXiv: 1503.03709 - JHEP 09 (2015) 049 - DOI: 10.1007/JHEP09(2015)049

Beams: p+ p+

Beam energies: (3500.0, 3500.0)GeV

Run details: - Inclusive Z in the electron channel

This paper presents measurements from the ATLAS experiment of the forward-backward asymmetry in the reaction pp → Z/γ^* → ℓ⁺ℓ⁻, with ℓ being electrons or muons, and the extraction of the effective weak mixing angle. The results are based on the full set of data collected in 2011 in pp collisions at the LHC at $\sqrt{s}$ = 7 TeV, corresponding to an integrated luminosity of 4.8 fb⁻¹. The measured asymmetry values are found to be in agreement with the corresponding Standard Model predictions. The combination of the muon and electron channels yields a value of the effective weak mixing angle of sin²θ_eff^lept = 0.2308$0.0005(stat.)0.0006(syst.)$0.0009(PDF), where the first uncertainty corresponds to data statistics, the second to systematic effects and the third to knowledge of the parton density functions. This result agrees with the current world average from the Particle Data Group fit.

Source code:`ATLAS_2015_I1351916.cc`

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

namespace Rivet {


  class ATLAS_2015_I1351916 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2015_I1351916);

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

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

      _mode = 0;
      if ( getOption("LMODE") == "EL" ) _mode = 1;
      else if ( getOption("LMODE") == "MU" ) _mode = 2;

      LeptonFinder elecs(0.1, Cuts::pT > 25*GeV && Cuts::abseta < 4.9 && Cuts::abspid == PID::ELECTRON);
      declare(elecs, "elecs");

      LeptonFinder muons(0.1, Cuts::pT > 20*GeV && Cuts::abseta < 2.47 && Cuts::abspid == PID::MUON);
      declare(muons, "muons");

      // Book dummy histograms for heterogeneous merging
      if (_mode == 0 || _mode == 1) {
        const Estimate1D& ref_el = refData(2, 1, 2);
        book(_h["el_NCC_pos"], "_el_ncc_pos", ref_el.xEdges());
        book(_h["el_NCC_neg"], "_el_ncc_neg", ref_el.xEdges());
        book(_s["el_CC"], 2, 1, 2);

        // electron-channel only
        const Estimate1D& ref_cf = refData(3, 1, 2);
        book(_h["el_NCF_pos"], "_el_ncf_pos", ref_cf.xEdges());
        book(_h["el_NCF_neg"], "_el_ncf_neg", ref_cf.xEdges());
        book(_s["el_CF"], 3, 1, 2);
      }
      if (_mode == 0 || _mode == 2) {
        const Estimate1D& ref_mu = refData(4, 1, 2);
        book(_h["mu_NCC_pos"], "_mu_ncc_pos", ref_mu.xEdges());
        book(_h["mu_NCC_neg"], "_mu_ncc_neg", ref_mu.xEdges());
        book(_s["mu_CC"], 4, 1, 2);
      }
    }


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

      // Get and cut on dressed leptons
      const DressedLeptons& elecs = apply<LeptonFinder>(event, "elecs").dressedLeptons();
      const DressedLeptons& muons = apply<LeptonFinder>(event, "muons").dressedLeptons();

      if (_mode == 0 || _mode == 1)  fillHistos(elecs, true);
      if (_mode == 0 || _mode == 2)  fillHistos(muons, false);
    }

    void fillHistos(const DressedLeptons& leptons, const bool doCF) {

      if (leptons.size() != 2)  return; // require exactly two leptons
      if (leptons[0].charge3() * leptons[1].charge3() > 0) return; // require opposite charge

      // Identify lepton vs antilepton
      const Particle& lpos = leptons[(leptons[0].charge3() > 0) ? 0 : 1];
      const Particle& lneg = leptons[(leptons[0].charge3() < 0) ? 0 : 1];

      string label;
      if (doCF) { // electron channel
        label = "el_N";
        const double eta1 = lpos.abseta();
        const double eta2 = lneg.abseta();
        if ( (eta1 < 2.47 && inRange(eta2, 2.5, 4.9)) || (eta2 < 2.47 && inRange(eta1, 2.5, 4.9)) ) {
          label += "CF"; // central-forward
        }
        else if (eta1 < 2.47 && eta2 < 2.47) {
          label += "CC"; // central-central
        }
        else  return; // ain't no forward-forward
      }
      else { // muon channel
        label = "mu_NCC"; // only central-central for muons
     }

      const double cosThetaStar = cosCollinsSoper(lneg, lpos);
      const double mll = (lpos.mom() + lneg.mom()).mass();
      label += cosThetaStar < 0.0?  "_neg" : "_pos";
      _h[label]->fill(mll/GeV);
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      const double sf = crossSectionPerEvent() / picobarn;
      scale(_h, sf);
      if (_mode == 0 || _mode == 1) {
        asymm(_h["el_NCC_pos"], _h["el_NCC_neg"], _s["el_CC"]);
        asymm(_h["el_NCF_pos"], _h["el_NCF_neg"], _s["el_CF"]);
      }
      if (_mode == 0 || _mode == 2) {
        asymm(_h["mu_NCC_pos"], _h["mu_NCC_neg"], _s["mu_CC"]);
      }
    }


    // Cosine of the decay angle in the Collins-Soper frame
    double cosCollinsSoper(const FourMomentum& l1, const FourMomentum& l2) {
      const FourMomentum ll = l1 + l2;
      const double nom  = (l1.E() + l1.pz()) * (l2.E() - l2.pz()) - (l1.E() - l1.pz()) * (l2.E() + l2.pz());
      const double denom = ll.mass() * sqrt( sqr(ll.mass()) + sqr(ll.pt()) );
      return sign(ll.pz()) * safediv(nom, denom); // protect against division by zero, you never know...
    }

    /// @}


  private:

    /// Histograms
    map<string, Histo1DPtr> _h;
    /// Asymmetries
    map<string, Estimate1DPtr> _s;

    // option to steer the lepton channel
    size_t _mode;

  };


  RIVET_DECLARE_PLUGIN(ATLAS_2015_I1351916);

}

Rivet analyses

Z forward-backward asymmetry

Source code:ATLAS_2015_I1351916.cc

Source code:`ATLAS_2015_I1351916.cc`