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CMS_2018_I1646260

Search for BSM with 2 soft leptons + MET at 13 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1646260
Status: VALIDATED NOTREENTRY NOHEPDATA
Authors:
  • Andy Buckley
  • Derek Yeung
References:
  • Phys.Lett. B782 (2018) 440-467
  • DOI: 10.1016/j.physletb.2018.05.062
  • CMS-SUS-16-048
  • arXiv: 1801.01846
Beams: p+ p+
Beam energies: (6500.0, 6500.0) GeV
Run details:
  • Low-mass l^+ l^- + pTmiss (l = el, mu)

A search is presented for new physics in events with two low-momentum, oppositely charged leptons (electrons or muons) and missing transverse momentum in proton-proton collisions at a centre-of-mass energy of 13 TeV. The data collected using the CMS detector at the LHC correspond to an integrated luminosity of 35.9/fb. The observed event yields are consistent with the expectations from the Standard Model. The analysis is optimised for study of pair production of charginos and neutralinos with nearly degenerate masses, as expected in natural supersymmetry models with light higgsinos, as well as in terms of the pair production of top squarks, when the lightest neutralino and the top squark have similar masses.

Source code: CMS_2018_I1646260.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/DirectFinalState.hh"
#include "Rivet/Projections/IndirectFinalState.hh"
#include "Rivet/Projections/TauFinder.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/MissingMomentum.hh"
#include "Rivet/Projections/Smearing.hh"
#include "Rivet/Tools/Cutflow.hh"

namespace Rivet {


  /// CMS 2 soft lepton + MET in 36/fb of 13 TeV pp
  class CMS_2018_I1646260 : public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(CMS_2018_I1646260);


    /// @name Analysis methods
    //@{

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

      PromptFinalState electrons(Cuts::abspid == PID::ELECTRON);
      SmearedParticles recoelectrons(electrons, [](const Particle& e) -> double {
          static const vector<double> ptedges = { 5., 10., 15., 20., 25., 30. };
          static const vector<double> etaedges = { 0.0, 0.8, 1.442, 1.556, 2.0, 2.5 };
          static const vector<vector<double>> effs = { { 0.336, 0.344, 0.233, 0.309, 0.243 },
                                                       { 0.412, 0.402, 0.229, 0.359, 0.287 },
                                                       { 0.465, 0.448, 0.250, 0.394, 0.327 },
                                                       { 0.496, 0.476, 0.261, 0.408, 0.341 },
                                                       { 0.503, 0.482, 0.255, 0.418, 0.352 } };
          const int ipt = binIndex(e.pT()/GeV, ptedges);
          const int ieta = binIndex(e.abseta(), etaedges);
          if (ipt < 0 || ieta < 0) return 0;
          return effs[ipt][ieta];
        }, ELECTRON_SMEAR_CMS_RUN2);
      declare(recoelectrons, "Electrons");

      PromptFinalState muons(Cuts::abspid == PID::MUON);
      SmearedParticles recomuons(muons, [](const Particle& m) -> double {
          static const vector<double> ptedges = { 3.5, 10., 15., 20., 25., 30. };
          static const vector<double> etaedges = { 0.0, 0.9, 1.2, 2.1, 2.4 };
          static const vector<vector<double>> effs = { { 0.647, 0.627, 0.610, 0.566 },
                                                       { 0.718, 0.662, 0.660, 0.629 },
                                                       { 0.739, 0.694, 0.678, 0.655 },
                                                       { 0.760, 0.725, 0.685, 0.670 },
                                                       { 0.763, 0.733, 0.723, 0.696 } };
          const int ipt = binIndex(m.pT()/GeV, ptedges);
          const int ieta = binIndex(m.abseta(), etaedges);
          if (ipt < 0 || ieta < 0) return 0;
          return effs[ipt][ieta];
        }, MUON_SMEAR_CMS_RUN2);
      declare(recomuons, "Muons");

      TauFinder taus(TauFinder::DecayMode::LEPTONIC);
      declare(taus, "Taus");

      FastJets jets4(IndirectFinalState(Cuts::abseta < 4.9), FastJets::ANTIKT, 0.4);
      SmearedJets recojets4(jets4, JET_SMEAR_CMS_RUN2, JET_BTAG_EFFS(0.8, 0.1, 0.4));
      declare(recojets4, "Jets");

      // MissingMomentum met(FinalState(Cuts::abseta < 4.9));
      // SmearedMET recomet(met, MET_SMEAR_CMS_RUN2);
      // declare(recomet, "MET");


      // Book SR counters
      for (size_t i = 0; i < 3; ++i) {
        for (size_t j = 0; j < 4; ++j)
          book(_srcounts_ewino[i][j], "sr_ewino_" + toString(i) + "_" + toString(j));
        for (size_t j = 0; j < 3; ++j)
          book(_srcounts_stop[i][j],   "sr_stop_" + toString(i) + "_" + toString(j));
      }

      // Cut-flow setup
      const strings cfnames = {"2mu", "mu+mu-", "pTmumu > 3 GeV", "Mmumu in [4,50] GeV",
                               "Mmumu not in [9,10.5] GeV", "pTmiss in [125, 200] GeV",
                               "mu+pTmiss trigger", "ISR jet", "HT > 100 GeV",
                               "pTmiss/HT in [0.6, 1.4]", "b-tag veto", "Mtautau veto"};
      _cutflows.addCutflow("EW", cfnames+strings{"MT < 70 GeV"});
      _cutflows.addCutflow("St", cfnames);
    }


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

      _cutflows.fillinit();

      // Leptons
      const Particles elecs = apply<ParticleFinder>(event, "Electrons").particlesByPt(Cuts::pT > 5*GeV && Cuts::abseta < 2.5);
      const Particles muons = apply<ParticleFinder>(event, "Muons").particlesByPt(Cuts::pT > 5*GeV && Cuts::pT < 30*GeV && Cuts::abseta < 2.4);
      const Particles leptons = sortByPt(elecs + muons);
      if (leptons.empty()) vetoEvent;
      _nevtMu += 1;

      if (leptons.size() != 2) vetoEvent;
      _cutflows.fill(1);
      if (leptons[0].charge() * leptons[1].charge() >= 0) vetoEvent;
      _cutflows.fill(2);

      // Dilepton cuts
      const FourMomentum pll = leptons[0].mom() + leptons[1].mom();
      if (pll.pT() < 3*GeV) vetoEvent;
      _cutflows.fill(3);
      const bool sameflav = (leptons[0].abspid() == leptons[1].abspid());
      if (sameflav) {
        if (!inRange(pll.mass()/GeV, 4, 50)) vetoEvent;
        _cutflows.fill(4);
        if (inRange(pll.mass()/GeV, 9, 10.5)) vetoEvent;
        _cutflows.fill(5);
      } else {
        _cutflows.fill(4);
        _cutflows.fill(5);
      }

      // Jets
      Jets jets = apply<SmearedJets>(event, "Jets").jetsByPt(Cuts::pT > 25*GeV && Cuts::abseta < 2.4);

      // MET
      FourMomentum p4miss;
      for (const Jet& j : jets) p4miss -= j;
      for (const Particle& e : elecs) p4miss -= e;
      const double ptmiss = p4miss.pT();
      for (const Particle& m : muons) p4miss -= m;
      const double ptmiss_mu = p4miss.pT();
      const bool mumu = (muons.size() == 2);
      if (ptmiss < (mumu ? 125*GeV : 200*GeV)) vetoEvent;
      if (ptmiss_mu < (mumu ? 125*GeV : 200*GeV)) vetoEvent;
      _cutflows.fill(6);

      // mu+pTmiss trigger (65% efficient in low-ETmiss region)
      double triggerSF = 1.0;
      if (mumu && ptmiss < 200*GeV) triggerSF = 0.65;
      _cutflows.fill(7, triggerSF);

      // ISR jet
      if (jets.empty()) vetoEvent;
      _cutflows.fill(8, triggerSF);

      // MET/HT
      const double ht = sum(jets, Kin::pT, 0.0);
      if (ht < 100*GeV) vetoEvent;
      _cutflows.fill(9, triggerSF);
      if (!inRange(ptmiss/ht, 0.6, 1.4)) vetoEvent;
      _cutflows.fill(10, triggerSF);

      // b-jet veto
      if (any(jets, hasBTag(Cuts::pT > 5*GeV))) vetoEvent; //< b-jet veto with ad hoc tagging threshold
      _cutflows.fill(11, triggerSF);

      // Tau veto
      const Particles taus = apply<ParticleFinder>(event, "Taus").particlesByPt();
      if (taus.size() >= 2) {
        const double mtt = (taus[0].mom() + taus[1].mom()).mass();
        if (mtt < 160*GeV) vetoEvent;
      }
      _cutflows.fill(12, triggerSF);

      // EWino SR (ee, mumu)
      if (sameflav) {
        for (const Particle& l : leptons) {
          if (l.abspid() == PID::MUON and l.pT() < 5*GeV) vetoEvent;
          if (mT(l.mom(), p4miss) > 70*GeV) vetoEvent;
        }
        _cutflows["EW"].fill(13, triggerSF);
        static const vector<double> ptmissedges_ewino = {125., 200., 250., DBL_MAX};
        static const vector<double> mlledges_ewino = {4., 9., 10.5, 20., 30., 50.};
        const int iptm = binIndex(ptmiss/GeV, ptmissedges_ewino);
        const int imll = binIndex(pll.mass()/GeV, mlledges_ewino);
        _srcounts_ewino[iptm][imll < 1 ? 0 : imll-1]->fill(triggerSF);

      } else {

        // Stop SR
        if (leptons[0].abspid() == PID::MUON and leptons[0].pT() < 5*GeV) vetoEvent;
        _cutflows["St"].fill(13, triggerSF);
        static const vector<double> ptmissedges_stop = {125., 200., 300., DBL_MAX};
        static const vector<double> ptledges_stop = {5., 12., 20., 30.};
        const int iptm = binIndex(ptmiss/GeV, ptmissedges_stop);
        const int iptl = binIndex(leptons[0].pT()/GeV, ptledges_stop);
        _srcounts_stop[iptm][iptl]->fill(triggerSF);
      }

    }


    /// Normalise histograms etc., after the run
    void finalize() {
      MSG_INFO("Num events with >= 1 muon = " << _nevtMu << " / " << numEvents());

      const double sf = 35.9*crossSection()/femtobarn/sumOfWeights();
      for (size_t i = 0; i < 3; ++i) {
        scale(_srcounts_ewino[i], sf);
        scale(_srcounts_stop[i], sf);
      }
      _cutflows.scale(sf);
      MSG_INFO("CUTFLOWS:\n\n" << _cutflows);
    }

    //@}


    /// @name Histograms
    //@{
    CounterPtr _srcounts_ewino[3][4], _srcounts_stop[3][3];
    //@}

    /// Cut-flows
    int _nevtMu = 0;
    Cutflows _cutflows;


  };


  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(CMS_2018_I1646260);


}