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ATLAS_2016_I1502620

W and Z inclusive cross sections at 7 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1502620
Status: VALIDATED
Authors:
  • Jan Kretzschmar
  • Christian Gutschow
References: Beams: p+ p+
Beam energies: (3500.0, 3500.0) GeV
Run details:
  • p + p -> Z + X ( Z -> e e )

High-precision measurements by the ATLAS Collaboration are presented of inclusive $W^+\to\ell^+$, $W^-\to\ell^-$, $Z/\gamma^\ast\to\ell\ell$ ($\ell=e,\mu$) Drell-Yan production cross sections at the LHC. The data were collected in proton-proton collisions at $\sqrt{s}=7$ TeV with an integrated luminosity of 4.6 fb${}^{-1}$. Differential $W^+$ and $W^-$ cross sections are measured in a lepton pseudorapidity range $|\eta_\ell| < 2.5$. Differential $Z/\gamma^\ast$ cross sections are measured as a function of the absolute dilepton rapidity, for $|y_{\ell\ell}| < 3.6$, for three intervals of dilepton mass, $m_{\ell\ell}$, extending from 46 to 150 GeV. The integrated and differential electron- and muon-channel cross sections are combined and compared to theoretical predictions using recent sets of parton distribution functions. The data, together with the final inclusive $e^\pm p$ scattering cross-section data from H1 and ZEUS, are interpreted in a next-to-next-to-leading-order QCD analysis, and a new set of parton distribution functions, ATLAS-epWZ16, is obtained. The ratio of strange-to-light sea-quark densities in the proton is determined more accurately than in previous determinations based on collider data only, and is established to be close to unity in the sensitivity range of the data. A new measurement of the CKM matrix element $|V_{cs}|$ is also provided. N.B.: Use :LMODE to choose specify the signature decay channel directly. Default, run everything, assuming all decay modes are generated Contains Z (W): fill only Z (W) signature histgrams Contains EL (MU): assume only electron (muon) decay modes are being generated.

Source code: ATLAS_2016_I1502620.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/WFinder.hh"
#include "Rivet/Projections/ZFinder.hh"

namespace Rivet {

  /// @brief inclusive W/Z cross sections at 7 TeV
  class ATLAS_2016_I1502620 : public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(ATLAS_2016_I1502620);
    //@}


    /// @name Analysis methods
    //@{

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

      // Get options from the new option system
      // run everything
      _mode = 0;
      _runZ = true;
      _runW = true;
      if ( getOption("LMODE") == "EL" || 
	   getOption("LMODE") == "ZEL" ||
	   getOption("LMODE") == "WEL" ) 
	_mode = 1;
      if ( getOption("LMODE") == "MU" || 
	   getOption("LMODE") == "ZMU" ||
	   getOption("LMODE") == "WMU" ) 
	_mode = 2;
      if ( getOption("LMODE") == "Z" || 
	   getOption("LMODE") == "ZEL" || 
	   getOption("LMODE") == "ZMU" ) 
	_runW = false;
      if ( getOption("LMODE") == "W" || 
	   getOption("LMODE") == "WEL" || 
	   getOption("LMODE") == "WMU" ) 
	_runZ = false;




      ///Initialise and register projections here
      const FinalState fs;

      Cut Wcuts = Cuts::pT >= 25*GeV; // minimum lepton pT
      Cut Zcuts = Cuts::pT >= 20.0*GeV;

      WFinder wfinder_edressed(fs, Wcuts, PID::ELECTRON, 40*GeV, 13*TeV, 25*GeV, 0.1, 
				 WFinder::CLUSTERNODECAY, WFinder::NOTRACK, WFinder::TRANSMASS);
      declare(wfinder_edressed, "WFinder_edressed");

      ZFinder zfindere(fs, Zcuts, PID::ELECTRON, 46.0*GeV, 150*GeV, 0.1, ZFinder::CLUSTERNODECAY, ZFinder::NOTRACK);
      declare(zfindere, "ZFindere");

      WFinder wfinder_mdressed(fs, Wcuts, PID::MUON, 40*GeV, 13*TeV, 25*GeV, 0.1, 
				 WFinder::CLUSTERNODECAY, WFinder::NOTRACK, WFinder::TRANSMASS);
      declare(wfinder_mdressed, "WFinder_mdressed");

      ZFinder zfinderm(fs, Zcuts, PID::MUON, 46.0*GeV, 150*GeV, 0.1, ZFinder::CLUSTERNODECAY, ZFinder::NOTRACK);
      declare(zfinderm, "ZFinderm");


      /// Book histograms here      
      if (_runW) {
	_h_Wp_eta = bookHisto1D(   9, 1, 1);
	_h_Wm_eta = bookHisto1D(  10, 1, 1);
	_h_W_asym = bookScatter2D(35, 1, 1);
      }

      if (_runZ) {
	_h_Zcenlow_y_dressed   = bookHisto1D(11, 1, 1);
	_h_Zcenpeak_y_dressed  = bookHisto1D(12, 1, 1);
	_h_Zcenhigh_y_dressed  = bookHisto1D(13, 1, 1);
	_h_Zfwdpeak_y_dressed  = bookHisto1D(14, 1, 1);
	_h_Zfwdhigh_y_dressed  = bookHisto1D(15, 1, 1);
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      
      // W stuff 
      const WFinder& wfindere = apply<WFinder>(event, "WFinder_edressed");	     
      const WFinder& wfinderm = apply<WFinder>(event, "WFinder_mdressed");	     
      
      if (wfindere.bosons().size()+wfinderm.bosons().size() == 1 && _runW) {

        const double weight = event.weight();

	Particle lep;
	if (_mode !=2 && wfindere.bosons().size() == 1 ) {
	  lep = wfindere.constituentLeptons()[0];
	}
	else if (_mode !=1 && wfinderm.bosons().size() == 1 ) {
	  lep = wfinderm.constituentLeptons()[0];
	}
	if (lep.charge3() > 0)  _h_Wp_eta->fill(lep.abseta(), weight);
	else                    _h_Wm_eta->fill(lep.abseta(), weight);
	
      }

      // now the Z stuff. 
      const ZFinder& zfindere = apply<ZFinder>(event, "ZFindere");
      const ZFinder& zfinderm = apply<ZFinder>(event, "ZFinderm");
      

      // must be one and only one candidate.
      if (zfindere.bosons().size()+zfinderm.bosons().size() == 1 && _runZ) {

	const double weight = event.weight();
	
	Particle Zboson;
	ParticleVector leptons;
	
	// candidate is e+e-
	if (_mode != 2 && zfindere.bosons().size() == 1 ) {
	  
	  Zboson = zfindere.boson();
	  leptons = zfindere.constituents();
	}  

	// candidate is mu+mu-
        else if (_mode !=1 && zfinderm.bosons().size() == 1 ) {
	  
	  Zboson = zfinderm.boson();
	  leptons = zfinderm.constituents();
	  
	}
	const double zrap  = Zboson.absrap();
	const double zmass = Zboson.mass();
	const double eta1 = leptons[0].abseta();
	const double eta2 = leptons[1].abseta();
		
	// separation into central/forward and three mass bins
	if (eta1 < 2.5 && eta2 < 2.5) {
	  if (zmass < 66.0*GeV)        _h_Zcenlow_y_dressed->fill(zrap, weight);
	  else if (zmass < 116.0*GeV)  _h_Zcenpeak_y_dressed->fill(zrap, weight);
	  else                         _h_Zcenhigh_y_dressed->fill(zrap, weight);
	} 
	else if ((eta1 < 2.5 && 2.5 < eta2 && eta2 < 4.9) || (eta2 < 2.5 && 2.5 < eta1 && eta1 < 4.9)) {
	  if (zmass < 66.0*GeV)   vetoEvent;
	  if (zmass < 116.0*GeV)  _h_Zfwdpeak_y_dressed->fill(zrap, weight);
	  else                    _h_Zfwdhigh_y_dressed->fill(zrap, weight);
	}
      }
      
    }

    /// Normalise histograms etc., after the run
    void finalize() {

      // Construct asymmetry: (dsig+/deta - dsig-/deta) / (dsig+/deta + dsig-/deta)
      //divide(*_h_Wp_eta - *_h_Wm_eta, *_h_Wp_eta + *_h_Wm_eta, _h_W_asym);
      if (_runW) {
	for (size_t i = 0; i < _h_Wp_eta->numBins(); ++i) {
	  YODA::HistoBin1D& bp = _h_Wp_eta->bin(i);
	  YODA::HistoBin1D& bm = _h_Wm_eta->bin(i);
	  const double sum  = bp.height() + bm.height();
	  //const double xerr = 0.5 * bp.xWidth();
	  double val = 0., yerr = 0.;

	  if (sum) {
	    const double pos2  = bp.height() * bp.height();
	    const double min2  = bm.height() * bm.height();
	    const double errp2 = bp.heightErr() * bp.heightErr();
	    const double errm2 = bm.heightErr() * bm.heightErr();
	    val = (bp.height() - bm.height()) / sum;
	    yerr = 2. * sqrt(errm2 * pos2 + errp2 * min2) / (sum * sum);
	  }
	  _h_W_asym->addPoint(bp.midpoint(), val, 0.5*bp.xWidth(), yerr);
	}
      }

      // Print summary info
      const double xs_pb(crossSection() / picobarn);
      const double sumw(sumOfWeights());
      MSG_DEBUG( "Cross-section/pb     : " << xs_pb       );
      MSG_DEBUG( "Sum of weights       : " << sumw        );
      MSG_DEBUG( "nEvents              : " << numEvents() );

      ///  Normalise, scale and otherwise manipulate histograms here
      double lfac = 1.0;
      // If we have been running on both electrons and muons, need to divide by two to
      // get the xsec for one flavour
      if (_mode == 0) lfac = 0.5;
      const double sf = lfac * 0.5 * xs_pb / sumw; // 0.5 accounts for rapidity bin width

      if (_runW){
	scale(_h_Wp_eta, sf);
	scale(_h_Wm_eta, sf);
      }

      if (_runZ){
	scale(_h_Zcenlow_y_dressed, sf);
	scale(_h_Zcenpeak_y_dressed, sf);
	scale(_h_Zcenhigh_y_dressed, sf);
	scale(_h_Zfwdpeak_y_dressed, sf);
	scale(_h_Zfwdhigh_y_dressed, sf);
      }
    }

    //@}


  protected:
    size_t _mode;
    bool _runZ, _runW;

  private:

    /// @name Histograms
    //@{
    Histo1DPtr _h_Wp_eta, _h_Wm_eta;
    Scatter2DPtr _h_W_asym;

    Histo1DPtr _h_Zcenlow_y_dressed;
    Histo1DPtr _h_Zcenpeak_y_dressed;
    Histo1DPtr _h_Zcenhigh_y_dressed;
    Histo1DPtr _h_Zfwdpeak_y_dressed;
    Histo1DPtr _h_Zfwdhigh_y_dressed;
    //@}

  };

  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(ATLAS_2016_I1502620);


}

// END END END