df103_NanoAODHiggsAnalysis.C

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/// \file
/// \ingroup tutorial_dataframe
/// \notebook -draw
/// This tutorial is a simplified but yet complex example of an analysis reconstructing
/// the Higgs boson decaying to two Z bosons from events with four leptons. The data
/// and simulated events are taken from CERN OpenData representing a subset of the data
/// recorded in 2012 with the CMS detector at the LHC. The tutorials follows the Higgs
/// to four leptons analysis published on CERN Open Data portal
/// ([10.7483/OPENDATA.CMS.JKB8.RR42](http://opendata.cern.ch/record/5500)).
/// The resulting plots show the invariant mass of the selected four lepton systems
/// in different decay modes (four muons, four electrons and two of each kind)
/// and in a combined plot indicating the decay of the Higgs boson with a mass
/// of about 125 GeV.
///
/// The following steps are performed for each sample with data and simulated events
/// in order to reconstruct the Higgs boson from the selected muons and electrons:
/// 1. Select interesting events with multiple cuts on event properties, e.g.,
///    number of leptons, kinematics of the leptons and quality of the tracks.
/// 2. Reconstruct two Z bosons of which only one on the mass shell from the selected events and apply additional cuts
///    on the reconstructed objects.
/// 3. Reconstruct the Higgs boson from the remaining Z boson candidates and calculate
///    its invariant mass.
///
/// \macro_image
/// \macro_code
/// \macro_output
///
/// \date October 2018
/// \author Stefan Wunsch (KIT, CERN)

#include "ROOT/RDataFrame.hxx"
#include "ROOT/RVec.hxx"
#include "ROOT/RDF/RInterface.hxx"
#include "TCanvas.h"
#include "TH1D.h"
#include "TLatex.h"
#include "TLegend.h"
#include "Math/Vector4Dfwd.h"
#include "TStyle.h"

using namespace ROOT::VecOps;
using RNode = ROOT::RDF::RNode;
using rvec_f = const RVec<float> &;
using rvec_i = const RVec<int> &;

const auto z_mass = 91.2;

// Select interesting events with four muons
RNode selection_4mu(RNode df)
{
   auto df_ge4m = df.Filter("nMuon>=4", "At least four muons");
   auto df_iso = df_ge4m.Filter("All(abs(Muon_pfRelIso04_all)<0.40)", "Require good isolation");
   auto df_kin = df_iso.Filter("All(Muon_pt>5) && All(abs(Muon_eta)<2.4)", "Good muon kinematics");
   auto df_ip3d = df_kin.Define("Muon_ip3d", "sqrt(Muon_dxy*Muon_dxy + Muon_dz*Muon_dz)");
   auto df_sip3d = df_ip3d.Define("Muon_sip3d", "Muon_ip3d/sqrt(Muon_dxyErr*Muon_dxyErr + Muon_dzErr*Muon_dzErr)");
   auto df_pv = df_sip3d.Filter("All(Muon_sip3d<4) && All(abs(Muon_dxy)<0.5) && All(abs(Muon_dz)<1.0)",
                                "Track close to primary vertex with small uncertainty");
   auto df_2p2n = df_pv.Filter("nMuon==4 && Sum(Muon_charge==1)==2 && Sum(Muon_charge==-1)==2",
                               "Two positive and two negative muons");
   return df_2p2n;
}

// Select interesting events with four electrons
RNode selection_4el(RNode df)
{
   auto df_ge4el = df.Filter("nElectron>=4", "At least our electrons");
   auto df_iso = df_ge4el.Filter("All(abs(Electron_pfRelIso03_all)<0.40)", "Require good isolation");
   auto df_kin = df_iso.Filter("All(Electron_pt>7) && All(abs(Electron_eta)<2.5)", "Good Electron kinematics");
   auto df_ip3d = df_kin.Define("Electron_ip3d", "sqrt(Electron_dxy*Electron_dxy + Electron_dz*Electron_dz)");
   auto df_sip3d = df_ip3d.Define("Electron_sip3d",
                                  "Electron_ip3d/sqrt(Electron_dxyErr*Electron_dxyErr + Electron_dzErr*Electron_dzErr)");
   auto df_pv = df_sip3d.Filter("All(Electron_sip3d<4) && All(abs(Electron_dxy)<0.5) && "
                                "All(abs(Electron_dz)<1.0)",
                                "Track close to primary vertex with small uncertainty");
   auto df_2p2n = df_pv.Filter("nElectron==4 && Sum(Electron_charge==1)==2 && Sum(Electron_charge==-1)==2",
                               "Two positive and two negative electrons");
   return df_2p2n;
}

// Select interesting events with two electrons and two muons
RNode selection_2el2mu(RNode df)
{
   auto df_ge2el2mu = df.Filter("nElectron>=2 && nMuon>=2", "At least two electrons and two muons");
   auto df_eta = df_ge2el2mu.Filter("All(abs(Electron_eta)<2.5) && All(abs(Muon_eta)<2.4)", "Eta cuts");
   auto pt_cuts = [](rvec_f mu_pt, rvec_f el_pt) {
      auto mu_pt_sorted = Reverse(Sort(mu_pt));
      if (mu_pt_sorted[0] > 20 && mu_pt_sorted[1] > 10) {
         return true;
      }
      auto el_pt_sorted = Reverse(Sort(el_pt));
      if (el_pt_sorted[0] > 20 && el_pt_sorted[1] > 10) {
         return true;
      }
      return false;
   };
   auto df_pt = df_eta.Filter(pt_cuts, {"Muon_pt", "Electron_pt"}, "Pt cuts");
   auto dr_cuts = [](rvec_f mu_eta, rvec_f mu_phi, rvec_f el_eta, rvec_f el_phi) {
      auto mu_dr = DeltaR(mu_eta[0], mu_eta[1], mu_phi[0], mu_phi[1]);
      auto el_dr = DeltaR(el_eta[0], el_eta[1], el_phi[0], el_phi[1]);
      if (mu_dr < 0.02 || el_dr < 0.02) {
         return false;
      }
      return true;
   };
   auto df_dr = df_pt.Filter(dr_cuts, {"Muon_eta", "Muon_phi", "Electron_eta", "Electron_phi"}, "Dr cuts");
   auto df_iso = df_dr.Filter("All(abs(Electron_pfRelIso03_all)<0.40) && All(abs(Muon_pfRelIso04_all)<0.40)",
                              "Require good isolation");
   auto df_el_ip3d = df_iso.Define("Electron_ip3d_el", "sqrt(Electron_dxy*Electron_dxy + Electron_dz*Electron_dz)");
   auto df_el_sip3d = df_el_ip3d.Define("Electron_sip3d_el",
                                        "Electron_ip3d_el/sqrt(Electron_dxyErr*Electron_dxyErr + "
                                        "Electron_dzErr*Electron_dzErr)");
   auto df_el_track = df_el_sip3d.Filter("All(Electron_sip3d_el<4) && All(abs(Electron_dxy)<0.5) && All(abs(Electron_dz)<1.0)",
                                         "Electron track close to primary vertex with small uncertainty");
   auto df_mu_ip3d = df_el_track.Define("Muon_ip3d_mu", "sqrt(Muon_dxy*Muon_dxy + Muon_dz*Muon_dz)");
   auto df_mu_sip3d = df_mu_ip3d.Define("Muon_sip3d_mu",
                                        "Muon_ip3d_mu/sqrt(Muon_dxyErr*Muon_dxyErr + Muon_dzErr*Muon_dzErr)");
   auto df_mu_track = df_mu_sip3d.Filter("All(Muon_sip3d_mu<4) && All(abs(Muon_dxy)<0.5) && All(abs(Muon_dz)<1.0)",
                                         "Muon track close to primary vertex with small uncertainty");
   auto df_2p2n = df_mu_track.Filter("Sum(Electron_charge)==0 && Sum(Muon_charge)==0",
                                     "Two opposite charged electron and muon pairs");
   return df_2p2n;
}

// Reconstruct two Z candidates from four leptons of the same kind
RVec<RVec<size_t>> reco_zz_to_4l(rvec_f pt, rvec_f eta, rvec_f phi, rvec_f mass, rvec_i charge)
{
   RVec<RVec<size_t>> idx(2);
   idx[0].reserve(2); idx[1].reserve(2);

   // Find first lepton pair with invariant mass closest to Z mass
   auto idx_cmb = Combinations(pt, 2);
   auto best_mass = -1;
   size_t best_i1 = 0; size_t best_i2 = 0;
   for (size_t i = 0; i < idx_cmb[0].size(); i++) {
      const auto i1 = idx_cmb[0][i];
      const auto i2 = idx_cmb[1][i];
      if (charge[i1] != charge[i2]) {
         ROOT::Math::PtEtaPhiMVector p1(pt[i1], eta[i1], phi[i1], mass[i1]);
         ROOT::Math::PtEtaPhiMVector p2(pt[i2], eta[i2], phi[i2], mass[i2]);
         const auto this_mass = (p1 + p2).M();
         if (std::abs(z_mass - this_mass) < std::abs(z_mass - best_mass)) {
            best_mass = this_mass;
            best_i1 = i1;
            best_i2 = i2;
         }
      }
   }
   idx[0].emplace_back(best_i1);
   idx[0].emplace_back(best_i2);

   // Reconstruct second Z from remaining lepton pair
   for (size_t i = 0; i < 4; i++) {
      if (i != best_i1 && i != best_i2) {
         idx[1].emplace_back(i);
      }
   }

   // Return indices of the pairs building two Z bosons
   return idx;
}

// Compute Z masses from four leptons of the same kind and sort ascending in distance to Z mass
RVec<float> compute_z_masses_4l(const RVec<RVec<size_t>> &idx, rvec_f pt, rvec_f eta, rvec_f phi, rvec_f mass)
{
   RVec<float> z_masses(2);
   for (size_t i = 0; i < 2; i++) {
      const auto i1 = idx[i][0]; const auto i2 = idx[i][1];
      ROOT::Math::PtEtaPhiMVector p1(pt[i1], eta[i1], phi[i1], mass[i1]);
      ROOT::Math::PtEtaPhiMVector p2(pt[i2], eta[i2], phi[i2], mass[i2]);
      z_masses[i] = (p1 + p2).M();
   }
   if (std::abs(z_masses[0] - z_mass) < std::abs(z_masses[1] - z_mass)) {
      return z_masses;
   } else {
      return Reverse(z_masses);
   }
}

// Compute mass of Higgs from four leptons of the same kind
float compute_higgs_mass_4l(const RVec<RVec<size_t>> &idx, rvec_f pt, rvec_f eta, rvec_f phi, rvec_f mass)
{
   const auto i1 = idx[0][0]; const auto i2 = idx[0][1];
   const auto i3 = idx[1][0]; const auto i4 = idx[1][1];
   ROOT::Math::PtEtaPhiMVector p1(pt[i1], eta[i1], phi[i1], mass[i1]);
   ROOT::Math::PtEtaPhiMVector p2(pt[i2], eta[i2], phi[i2], mass[i2]);
   ROOT::Math::PtEtaPhiMVector p3(pt[i3], eta[i3], phi[i3], mass[i3]);
   ROOT::Math::PtEtaPhiMVector p4(pt[i4], eta[i4], phi[i4], mass[i4]);
   return (p1 + p2 + p3 + p4).M();
}

// Apply selection on reconstructed Z candidates
RNode filter_z_candidates(RNode df)
{
   auto df_z1_cut = df.Filter("Z_mass[0] > 40 && Z_mass[0] < 120", "Mass of first Z candidate in [40, 120]");
   auto df_z2_cut = df_z1_cut.Filter("Z_mass[1] > 12 && Z_mass[1] < 120", "Mass of second Z candidate in [12, 120]");
   return df_z2_cut;
}

// Reconstruct Higgs from four muons
RNode reco_higgs_to_4mu(RNode df)
{
   // Filter interesting events
   auto df_base = selection_4mu(df);

   // Reconstruct Z systems
   auto df_z_idx =
      df_base.Define("Z_idx", reco_zz_to_4l, {"Muon_pt", "Muon_eta", "Muon_phi", "Muon_mass", "Muon_charge"});

   // Cut on distance between muons building Z systems
   auto filter_z_dr = [](const RVec<RVec<size_t>> &idx, rvec_f eta, rvec_f phi) {
      for (size_t i = 0; i < 2; i++) {
         const auto i1 = idx[i][0];
         const auto i2 = idx[i][1];
         const auto dr = DeltaR(eta[i1], eta[i2], phi[i1], phi[i2]);
         if (dr < 0.02) {
            return false;
         }
      }
      return true;
   };
   auto df_z_dr =
      df_z_idx.Filter(filter_z_dr, {"Z_idx", "Muon_eta", "Muon_phi"}, "Delta R separation of muons building Z system");

   // Compute masses of Z systems
   auto df_z_mass =
      df_z_dr.Define("Z_mass", compute_z_masses_4l, {"Z_idx", "Muon_pt", "Muon_eta", "Muon_phi", "Muon_mass"});

   // Cut on mass of Z candidates
   auto df_z_cut = filter_z_candidates(df_z_mass);

   // Reconstruct H mass
   auto df_h_mass =
      df_z_cut.Define("H_mass", compute_higgs_mass_4l, {"Z_idx", "Muon_pt", "Muon_eta", "Muon_phi", "Muon_mass"});

   return df_h_mass;
}

// Reconstruct Higgs from four electrons
RNode reco_higgs_to_4el(RNode df)
{
   // Filter interesting events
   auto df_base = selection_4el(df);

   // Reconstruct Z systems
   auto df_z_idx = df_base.Define("Z_idx", reco_zz_to_4l,
                                  {"Electron_pt", "Electron_eta", "Electron_phi", "Electron_mass", "Electron_charge"});

   // Cut on distance between Electrons building Z systems
   auto filter_z_dr = [](const RVec<RVec<size_t>> &idx, rvec_f eta, rvec_f phi) {
      for (size_t i = 0; i < 2; i++) {
         const auto i1 = idx[i][0];
         const auto i2 = idx[i][1];
         const auto dr = DeltaR(eta[i1], eta[i2], phi[i1], phi[i2]);
         if (dr < 0.02) {
            return false;
         }
      }
      return true;
   };
   auto df_z_dr = df_z_idx.Filter(filter_z_dr, {"Z_idx", "Electron_eta", "Electron_phi"},
                                  "Delta R separation of Electrons building Z system");

   // Compute masses of Z systems
   auto df_z_mass = df_z_dr.Define("Z_mass", compute_z_masses_4l,
                                   {"Z_idx", "Electron_pt", "Electron_eta", "Electron_phi", "Electron_mass"});

   // Cut on mass of Z candidates
   auto df_z_cut = filter_z_candidates(df_z_mass);

   // Reconstruct H mass
   auto df_h_mass = df_z_cut.Define("H_mass", compute_higgs_mass_4l,
                                    {"Z_idx", "Electron_pt", "Electron_eta", "Electron_phi", "Electron_mass"});

   return df_h_mass;
}

// Compute mass of two Z candidates from two electrons and two muons and sort ascending in distance to Z mass
RVec<float> compute_z_masses_2el2mu(rvec_f el_pt, rvec_f el_eta, rvec_f el_phi, rvec_f el_mass, rvec_f mu_pt,
                                  rvec_f mu_eta, rvec_f mu_phi, rvec_f mu_mass)
{
   ROOT::Math::PtEtaPhiMVector p1(mu_pt[0], mu_eta[0], mu_phi[0], mu_mass[0]);
   ROOT::Math::PtEtaPhiMVector p2(mu_pt[1], mu_eta[1], mu_phi[1], mu_mass[1]);
   ROOT::Math::PtEtaPhiMVector p3(el_pt[0], el_eta[0], el_phi[0], el_mass[0]);
   ROOT::Math::PtEtaPhiMVector p4(el_pt[1], el_eta[1], el_phi[1], el_mass[1]);
   auto mu_z = (p1 + p2).M();
   auto el_z = (p3 + p4).M();
   RVec<float> z_masses(2);
   if (std::abs(mu_z - z_mass) < std::abs(el_z - z_mass)) {
      z_masses[0] = mu_z;
      z_masses[1] = el_z;
   } else {
      z_masses[0] = el_z;
      z_masses[1] = mu_z;
   }
   return z_masses;
}

// Compute Higgs mass from two electrons and two muons
float compute_higgs_mass_2el2mu(rvec_f el_pt, rvec_f el_eta, rvec_f el_phi, rvec_f el_mass, rvec_f mu_pt, rvec_f mu_eta,
                                rvec_f mu_phi, rvec_f mu_mass)
{
   ROOT::Math::PtEtaPhiMVector p1(mu_pt[0], mu_eta[0], mu_phi[0], mu_mass[0]);
   ROOT::Math::PtEtaPhiMVector p2(mu_pt[1], mu_eta[1], mu_phi[1], mu_mass[1]);
   ROOT::Math::PtEtaPhiMVector p3(el_pt[0], el_eta[0], el_phi[0], el_mass[0]);
   ROOT::Math::PtEtaPhiMVector p4(el_pt[1], el_eta[1], el_phi[1], el_mass[1]);
   return (p1 + p2 + p3 + p4).M();
}

// Reconstruct Higgs from two electrons and two muons
RNode reco_higgs_to_2el2mu(RNode df)
{
   // Filter interesting events
   auto df_base = selection_2el2mu(df);

   // Compute masses of Z systems
   auto df_z_mass =
      df_base.Define("Z_mass", compute_z_masses_2el2mu, {"Electron_pt", "Electron_eta", "Electron_phi", "Electron_mass",
                                                       "Muon_pt", "Muon_eta", "Muon_phi", "Muon_mass"});

   // Cut on mass of Z candidates
   auto df_z_cut = filter_z_candidates(df_z_mass);

   // Reconstruct H mass
   auto df_h_mass = df_z_cut.Define(
      "H_mass", compute_higgs_mass_2el2mu,
      {"Electron_pt", "Electron_eta", "Electron_phi", "Electron_mass", "Muon_pt", "Muon_eta", "Muon_phi", "Muon_mass"});

   return df_h_mass;
}

// Plot invariant mass for signal and background processes from simulated events
// overlay the measured data.
template <typename T>
void plot(T sig, T bkg, T data, const std::string &x_label, const std::string &filename)
{
   // Canvas and general style options
   gStyle->SetOptStat(0);
   gStyle->SetTextFont(42);
   auto c = new TCanvas("c", "", 800, 700);
   c->SetLeftMargin(0.15);

   // Get signal and background histograms and stack them to show Higgs signal
   // on top of the background process
   auto h_sig = *sig;
   auto h_bkg = *bkg;
   auto h_cmb = *(TH1D*)(sig->Clone());
   h_cmb.Add(&h_bkg);
   h_cmb.SetTitle("");
   h_cmb.GetXaxis()->SetTitle(x_label.c_str());
   h_cmb.GetXaxis()->SetTitleSize(0.04);
   h_cmb.GetYaxis()->SetTitle("N_{Events}");
   h_cmb.GetYaxis()->SetTitleSize(0.04);
   h_cmb.SetLineColor(kRed);
   h_cmb.SetLineWidth(2);
   h_cmb.SetMaximum(18);

   h_bkg.SetLineWidth(2);
   h_bkg.SetFillStyle(1001);
   h_bkg.SetLineColor(kBlack);
   h_bkg.SetFillColor(kAzure - 9);

   // Get histogram of data points
   auto h_data = *data;
   h_data.SetLineWidth(1);
   h_data.SetMarkerStyle(20);
   h_data.SetMarkerSize(1.0);
   h_data.SetMarkerColor(kBlack);
   h_data.SetLineColor(kBlack);

   // Draw histograms
   h_cmb.DrawClone("HIST");
   h_bkg.DrawClone("HIST SAME");
   h_data.DrawClone("PE1 SAME");

   // Add legend
   TLegend legend(0.62, 0.70, 0.82, 0.88);
   legend.SetFillColor(0);
   legend.SetBorderSize(0);
   legend.SetTextSize(0.03);
   legend.AddEntry(&h_data, "Data", "PE1");
   legend.AddEntry(&h_bkg, "ZZ", "f");
   legend.AddEntry(&h_cmb, "m_{H} = 125 GeV", "f");
   legend.Draw();

   // Add header
   TLatex cms_label;
   cms_label.SetTextSize(0.04);
   cms_label.DrawLatexNDC(0.16, 0.92, "#bf{CMS Open Data}");
   TLatex header;
   header.SetTextSize(0.03);
   header.DrawLatexNDC(0.63, 0.92, "#sqrt{s} = 8 TeV, L_{int} = 11.6 fb^{-1}");

   // Save plot
   c->SaveAs(filename.c_str());
}

void df103_NanoAODHiggsAnalysis()
{
   // Enable multi-threading
   ROOT::EnableImplicitMT();

   // Create dataframes for signal, background and data samples

   // Signal: Higgs -> 4 leptons
   ROOT::RDataFrame df_sig_4l("Events",
                              "root://eospublic.cern.ch//eos/root-eos/cms_opendata_2012_nanoaod/SMHiggsToZZTo4L.root");

   // Background: ZZ -> 4 leptons
   // Note that additional background processes from the original paper with minor contribution were left out for this
   // tutorial.
   ROOT::RDataFrame df_bkg_4mu("Events",
                               "root://eospublic.cern.ch//eos/root-eos/cms_opendata_2012_nanoaod/ZZTo4mu.root");
   ROOT::RDataFrame df_bkg_4el("Events",
                               "root://eospublic.cern.ch//eos/root-eos/cms_opendata_2012_nanoaod/ZZTo4e.root");
   ROOT::RDataFrame df_bkg_2el2mu("Events",
                                  "root://eospublic.cern.ch//eos/root-eos/cms_opendata_2012_nanoaod/ZZTo2e2mu.root");

   // CMS data taken in 2012 (11.6 fb^-1 integrated luminosity)
   ROOT::RDataFrame df_data_doublemu(
      "Events", {"root://eospublic.cern.ch//eos/root-eos/cms_opendata_2012_nanoaod/Run2012B_DoubleMuParked.root",
                 "root://eospublic.cern.ch//eos/root-eos/cms_opendata_2012_nanoaod/Run2012C_DoubleMuParked.root"});
   ROOT::RDataFrame df_data_doubleel(
      "Events", {"root://eospublic.cern.ch//eos/root-eos/cms_opendata_2012_nanoaod/Run2012B_DoubleElectron.root",
                 "root://eospublic.cern.ch//eos/root-eos/cms_opendata_2012_nanoaod/Run2012C_DoubleElectron.root"});

   // Reconstruct Higgs to 4 muons
   auto df_sig_4mu_reco = reco_higgs_to_4mu(df_sig_4l);
   const auto luminosity = 11580.0;            // Integrated luminosity of the data samples
   const auto xsec_SMHiggsToZZTo4L = 0.0065;   // H->4l: Standard Model cross-section
   const auto nevt_SMHiggsToZZTo4L = 299973.0; // H->4l: Number of simulated events
   const auto nbins = 36;                      // Number of bins for the invariant mass spectrum
   auto df_h_sig_4mu = df_sig_4mu_reco
         .Define("weight", [&]() { return luminosity * xsec_SMHiggsToZZTo4L / nevt_SMHiggsToZZTo4L; }, {})
         .Histo1D({"h_sig_4mu", "", nbins, 70, 180}, "H_mass", "weight");

   const auto scale_ZZTo4l = 1.386;     // ZZ->4mu: Scale factor for ZZ to four leptons
   const auto xsec_ZZTo4mu = 0.077;     // ZZ->4mu: Standard Model cross-section
   const auto nevt_ZZTo4mu = 1499064.0; // ZZ->4mu: Number of simulated events
   auto df_bkg_4mu_reco = reco_higgs_to_4mu(df_bkg_4mu);
   auto df_h_bkg_4mu = df_bkg_4mu_reco
         .Define("weight", [&]() { return luminosity * xsec_ZZTo4mu * scale_ZZTo4l / nevt_ZZTo4mu; }, {})
         .Histo1D({"h_bkg_4mu", "", nbins, 70, 180}, "H_mass", "weight");

   auto df_data_4mu_reco = reco_higgs_to_4mu(df_data_doublemu);
   auto df_h_data_4mu = df_data_4mu_reco
         .Define("weight", []() { return 1.0; }, {})
         .Histo1D({"h_data_4mu", "", nbins, 70, 180}, "H_mass", "weight");

   // Reconstruct Higgs to 4 electrons
   auto df_sig_4el_reco = reco_higgs_to_4el(df_sig_4l);
   auto df_h_sig_4el = df_sig_4el_reco
         .Define("weight", [&]() { return luminosity * xsec_SMHiggsToZZTo4L / nevt_SMHiggsToZZTo4L; }, {})
         .Histo1D({"h_sig_4el", "", nbins, 70, 180}, "H_mass", "weight");

   const auto xsec_ZZTo4el = xsec_ZZTo4mu; // ZZ->4el: Standard Model cross-section
   const auto nevt_ZZTo4el = 1499093.0;    // ZZ->4el: Number of simulated events
   auto df_bkg_4el_reco = reco_higgs_to_4el(df_bkg_4el);
   auto df_h_bkg_4el = df_bkg_4el_reco
         .Define("weight", [&]() { return luminosity * xsec_ZZTo4el * scale_ZZTo4l / nevt_ZZTo4el; }, {})
         .Histo1D({"h_bkg_4el", "", nbins, 70, 180}, "H_mass", "weight");

   auto df_data_4el_reco = reco_higgs_to_4el(df_data_doubleel);
   auto df_h_data_4el = df_data_4el_reco.Define("weight", []() { return 1.0; }, {})
                           .Histo1D({"h_data_4el", "", nbins, 70, 180}, "H_mass", "weight");

   // Reconstruct Higgs to 2 electrons and 2 muons
   auto df_sig_2el2mu_reco = reco_higgs_to_2el2mu(df_sig_4l);
   auto df_h_sig_2el2mu = df_sig_2el2mu_reco
         .Define("weight", [&]() { return luminosity * xsec_SMHiggsToZZTo4L / nevt_SMHiggsToZZTo4L; }, {})
         .Histo1D({"h_sig_2el2mu", "", nbins, 70, 180}, "H_mass", "weight");

   const auto xsec_ZZTo2el2mu = 0.18;      // ZZ->2el2mu: Standard Model cross-section
   const auto nevt_ZZTo2el2mu = 1497445.0; // ZZ->2el2mu: Number of simulated events
   auto df_bkg_2el2mu_reco = reco_higgs_to_2el2mu(df_bkg_2el2mu);
   auto df_h_bkg_2el2mu = df_bkg_2el2mu_reco
         .Define("weight", [&]() { return luminosity * xsec_ZZTo2el2mu * scale_ZZTo4l / nevt_ZZTo2el2mu; }, {})
         .Histo1D({"h_bkg_2el2mu", "", nbins, 70, 180}, "H_mass", "weight");

   auto df_data_2el2mu_reco = reco_higgs_to_2el2mu(df_data_doublemu);
   auto df_h_data_2el2mu = df_data_2el2mu_reco.Define("weight", []() { return 1.0; }, {})
                              .Histo1D({"h_data_2el2mu_doublemu", "", nbins, 70, 180}, "H_mass", "weight");

   // Produce histograms for different channels and make plots
   plot(df_h_sig_4mu, df_h_bkg_4mu, df_h_data_4mu, "m_{4#mu} (GeV)", "higgs_4mu.pdf");
   plot(df_h_sig_4el, df_h_bkg_4el, df_h_data_4el, "m_{4e} (GeV)", "higgs_4el.pdf");
   plot(df_h_sig_2el2mu, df_h_bkg_2el2mu, df_h_data_2el2mu, "m_{2e2#mu} (GeV)", "higgs_2el2mu.pdf");

   // Combine channels for final plot
   auto h_data_4l = df_h_data_4mu.GetPtr();
   h_data_4l->Add(df_h_data_4el.GetPtr());
   h_data_4l->Add(df_h_data_2el2mu.GetPtr());
   auto h_sig_4l = df_h_sig_4mu.GetPtr();
   h_sig_4l->Add(df_h_sig_4el.GetPtr());
   h_sig_4l->Add(df_h_sig_2el2mu.GetPtr());
   auto h_bkg_4l = df_h_bkg_4mu.GetPtr();
   h_bkg_4l->Add(df_h_bkg_4el.GetPtr());
   h_bkg_4l->Add(df_h_bkg_2el2mu.GetPtr());
   plot(h_sig_4l, h_bkg_4l, h_data_4l, "m_{4l} (GeV)", "higgs_4l.pdf");
}

int main()
{
   df103_NanoAODHiggsAnalysis();
}