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CMS_2016_I1430892

Measurements of ttbar charge asymmetry using dilepton final states in pp collisions at sqrt(s) = 8 TeV
Experiment: CMS (LHC)
Inspire ID: 1430892
Status: VALIDATED
Authors:
  • Jacob Linacre
References:
  • Phys. Lett. B 760 (2016) 365
  • DOI:10.1016/j.physletb.2016.07.006
  • arXiv: 1603.06221
  • https://hepdata.net/record/ins1430892
Beams: p+ p+
Beam energies: (4000.0, 4000.0) GeV
Run details:
  • Dilepton ttbar events at $\sqrt{s}=8 \text{TeV}$, where the leptons are prompt elecrons or muons (not from tau). No other cuts. All but one of the variables require top quarks in the event record.

$\textbf{Abstract:}$ The charge asymmetry in $\mathrm{t\bar{t}}$ events is measured using dilepton final states produced in pp collisions at the LHC at $\sqrt{s}=8\:$TeV. The data sample, collected with the CMS detector, corresponds to an integrated luminosity of 19.5 $\mathrm{fb^{-1}}$. The measurements are performed using events with two oppositely charged leptons (electrons or muons) and two or more jets, where at least one of the jets is identified as originating from a bottom quark. The charge asymmetry is measured from differences in kinematic distributions, unfolded to the parton level, of positively and negatively charged top quarks and leptons. The $\mathrm{t\bar{t}}$ and leptonic charge asymmetries are found to be 0.011 +/- 0.011 (stat) +/- 0.007 (syst) and 0.003 +/- 0.006 (stat) +/- 0.003 (syst), respectively. These results, as well as charge asymmetry measurements made as a function of $\mathrm{t\bar{t}}$ system kinematic properties, are in agreement with predictions of the standard model. $\textbf{Particle-level addition to Rivet routine:}$ While the analysis was performed at the parton-level only, $\Delta|\eta_{\ell}|$ is a purely leptonic variable and it has been checked that the results of the analysis would have been essentially unchanged had it been defined at particle-level using dressed leptons instead of using the parton-level top quark daughter leptons. We therefore include both particle- and parton-level versions of this distribution in the Rivet routine, with the former identified in the plot title. For same-flavour dilepton final states, the particle-level definition in the full phase space is problematic because the two leptons can come from fully-hadronic $\mathrm{t\bar{t}}$ plus a dilepton pair from radiation. Such pairs have invariant mass $M_{\ell\ell}\sim 0$ and produce a peak near zero in the $\Delta|\eta_{\ell}|$ distribution. We therefore select only the $\mathrm{t\bar{t}}\to e\mu$ final state, by requiring exactly one electron and exactly one muon. Note this means $\mathrm{t\bar{t}}\to e\mu$ events with additional dilepton pairs from radiation are vetoed. For PYTHIA8 this amounts to 0.5% of $\mathrm{t\bar{t}}\to e\mu$ events - well below the level of sensitivity of the measured distribution. $\textbf{Histograms and covariance matrices:}$ The error bars in the measured distributions should not be used for fitting because there are significant correlations between bins. The covariance matrices for the statistical and systematic uncertainties in each distribution can be found in hepdata. The single-differential cross sections in hepdata are normalised to unit area (i.e. the integral is equal to one), while the double-differential cross sections in hepdata are normalised to the sum of entries (such that the sum of all bin heights is equal to one). This should be taken into account when comparing the measured distributions to the Rivet results and when using the covariance matrices. $\textbf{Underflow and overflow bins:}$ The lower and upper $\Delta|y_\mathrm{t}|$ and $\Delta|\eta_{\ell}|$ bins (starting and ending at -2 and +2, respectively) contain underflow and overflow events, i.e. the complete distribution from -infinity to +infinity is covered. Similarly, the upper $M_\mathrm{t\bar{t}}$, $p_\mathrm{T}^\mathrm{t\bar{t}}$, and $\left|y_\mathrm{t\bar{t}}\right|$ bins contain overflow events up to +infinity.

Source code: CMS_2016_I1430892.cc
  1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/FinalState.hh"
  4#include "Rivet/Projections/IdentifiedFinalState.hh"
  5#include "Rivet/Projections/PromptFinalState.hh"
  6#include "Rivet/Projections/LeptonFinder.hh"
  7#include "Rivet/Projections/PartonicTops.hh"
  8
  9namespace Rivet {
 10
 11
 12  /// CMS 8 TeV dilepton channel ttbar charge asymmetry analysis
 13  class CMS_2016_I1430892 : public Analysis {
 14  public:
 15
 16    /// Constructor
 17    RIVET_DEFAULT_ANALYSIS_CTOR(CMS_2016_I1430892);
 18
 19
 20    /// Book histograms and initialise projections
 21    void init() {
 22
 23      // Complete final state
 24      FinalState fs;
 25
 26      // Projection for dressed electrons and muons
 27      IdentifiedFinalState photons(fs);
 28      photons.acceptIdPair(PID::PHOTON);
 29
 30      IdentifiedFinalState el_id(fs);
 31      el_id.acceptIdPair(PID::ELECTRON);
 32      PromptFinalState electrons(el_id);
 33      declare(electrons, "Electrons");
 34      LeptonFinder dressed_electrons(electrons, photons, 0.1);
 35      declare(dressed_electrons, "DressedElectrons");
 36
 37      IdentifiedFinalState mu_id(fs);
 38      mu_id.acceptIdPair(PID::MUON);
 39      PromptFinalState muons(mu_id);
 40      declare(muons, "Muons");
 41      LeptonFinder dressed_muons(muons, photons, 0.1);
 42      declare(dressed_muons, "DressedMuons");
 43
 44      // Parton-level top quarks
 45      declare(PartonicTops(TopDecay::E_MU, PromptEMuFromTau::NO), "LeptonicPartonTops");
 46
 47
 48      // Booking of histograms
 49
 50      // This histogram is independent of the parton-level information, and is an
 51      // addition to the original analysis. It is compared to the same data as
 52      // the parton-level delta_abseta histogram d05-x01-y01.
 53      book(_h_dabsetadressedleptons, "d00-x01-y01", _bins_dabseta);
 54
 55      // The remaining histos use parton-level information
 56      book(_h_dabseta, "d05-x01-y01", _bins_dabseta);
 57      book(_h_dabsrapidity, "d02-x01-y01", _bins_dabsrapidity);
 58
 59      // 2D histos
 60      book(_h_dabsrapidity_var[0], "d11-x01-y01", _bins_dabsrapidity, _bins_tt_mass);
 61      book(_h_dabseta_var[0], "d17-x01-y01", _bins_dabseta, _bins_tt_mass);
 62
 63      book(_h_dabsrapidity_var[1], "d23-x01-y01", _bins_dabsrapidity, _bins_tt_pT);
 64      book(_h_dabseta_var[1], "d29-x01-y01", _bins_dabseta, _bins_tt_pT);
 65
 66      book(_h_dabsrapidity_var[2], "d35-x01-y01", _bins_dabsrapidity, _bins_tt_absrapidity);
 67      book(_h_dabseta_var[2], "d41-x01-y01", _bins_dabseta, _bins_tt_absrapidity);
 68
 69      // Profile histos for asymmetries
 70      book(_h_dabsrapidity_profile[0], "d08-x01-y01", _bins_tt_mass);
 71      book(_h_dabseta_profile[0], "d14-x01-y01", _bins_tt_mass);
 72
 73      book(_h_dabsrapidity_profile[1], "d20-x01-y01", _bins_tt_pT);
 74      book(_h_dabseta_profile[1], "d26-x01-y01", _bins_tt_pT);
 75
 76      book(_h_dabsrapidity_profile[2], "d32-x01-y01", _bins_tt_absrapidity);
 77      book(_h_dabseta_profile[2], "d38-x01-y01", _bins_tt_absrapidity);
 78
 79    }
 80
 81
 82    /// Perform the per-event analysis
 83    void analyze(const Event& event) {
 84
 85
 86      // Use particle-level leptons for the first histogram
 87      const LeptonFinder& dressed_electrons = apply<LeptonFinder>(event, "DressedElectrons");
 88      const LeptonFinder& dressed_muons = apply<LeptonFinder>(event, "DressedMuons");
 89
 90      const DressedLeptons dressedels = dressed_electrons.dressedLeptons();
 91      const DressedLeptons dressedmus = dressed_muons.dressedLeptons();
 92
 93      const size_t ndressedel = dressedels.size();
 94      const size_t ndressedmu = dressedmus.size();
 95
 96      // For the particle-level histogram, require exactly one electron and exactly one muon, to select the ttbar->emu channel.
 97      // Note this means ttbar->emu events with additional PromptFinalState dilepton pairs from the shower are vetoed - for PYTHIA8,
 98      // this affects ~0.5% of events, so the effect is well below the level of sensitivity of the measured distribution.
 99      if ( ndressedel == 1 && ndressedmu == 1 ) {
100
101        const int electrontouse = 0, muontouse = 0;
102
103        // Opposite-charge leptons only
104        if ( sameSign(dressedels[electrontouse], dressedmus[muontouse]) ) {
105          MSG_INFO("Error, e and mu have same charge, skipping event");
106        } else {
107          // Get the four-momenta of the positively- and negatively-charged leptons
108          FourMomentum lepPlus = dressedels[electrontouse].charge() > 0 ? dressedels[electrontouse] : dressedmus[muontouse];
109          FourMomentum lepMinus = dressedels[electrontouse].charge() > 0 ? dressedmus[muontouse] : dressedels[electrontouse];
110
111          // Now calculate the variable
112          double dabseta_temp = lepPlus.abseta() - lepMinus.abseta();
113
114          fillWithUFOF( _h_dabsetadressedleptons, dabseta_temp);
115        }
116
117      }
118
119
120      // The remaining variables use parton-level information.
121
122      // Get the leptonically decaying tops
123      const Particles leptonicpartontops = apply<ParticleFinder>(event, "LeptonicPartonTops").particlesByPt();
124      Particles chargedleptons;
125
126      unsigned int ntrueleptonictops = 0;
127      bool oppositesign = false;
128
129      if ( leptonicpartontops.size() == 2 ) {
130        for (size_t k = 0; k < leptonicpartontops.size(); ++k) {
131
132          // Get the lepton
133          const Particle lepTop = leptonicpartontops[k];
134          const auto isPromptChargedLepton = [](const Particle& p){return (isChargedLepton(p) && isPrompt(p, false, false));};
135          Particles lepton_candidates = lepTop.allDescendants(firstParticleWith(isPromptChargedLepton), false);
136          if ( lepton_candidates.size() < 1 ) MSG_WARNING("error, TopDecay::E_MU top quark had no daughter lepton candidate, skipping event.");
137
138          // In some cases there is no lepton from the W decay but only leptons from the decay of a radiated gamma.
139          // These hadronic PartonicTops are currently being mistakenly selected by TopDecay::E_MU (as of April 2017), and need to be rejected.
140          // TopDecay::E_MU is being fixed in Rivet, and when it is the veto below should do nothing.
141          /// @todo Should no longer be necessary -- remove
142          bool istrueleptonictop = false;
143          for (size_t i = 0; i < lepton_candidates.size(); ++i) {
144            const Particle& lepton_candidate = lepton_candidates[i];
145            if ( lepton_candidate.hasParentWith(Cuts::pid == PID::PHOTON) ) {
146              MSG_DEBUG("Found gamma parent, top: " << k+1 << " of " << leptonicpartontops.size() << " , lepton: " << i+1 << " of " << lepton_candidates.size());
147              continue;
148            }
149            if ( !istrueleptonictop && sameSign(lepTop,lepton_candidate) ) {
150              chargedleptons.push_back(lepton_candidate);
151              istrueleptonictop = true;
152            }
153            else MSG_WARNING("Error, found extra prompt charged lepton from top decay (and without gamma parent), ignoring it.");
154          }
155          if ( istrueleptonictop ) ++ntrueleptonictops;
156        }
157      }
158
159      if ( ntrueleptonictops == 2 ) {
160        oppositesign = !( sameSign(chargedleptons[0],chargedleptons[1]) );
161        if ( !oppositesign ) MSG_WARNING("error, same charge tops, skipping event.");
162      }
163
164      if ( ntrueleptonictops == 2 && oppositesign ) {
165
166        // Get the four-momenta of the positively- and negatively-charged leptons
167        const FourMomentum lepPlus = chargedleptons[0].charge() > 0 ? chargedleptons[0] : chargedleptons[1];
168        const FourMomentum lepMinus = chargedleptons[0].charge() > 0 ? chargedleptons[1] : chargedleptons[0];
169
170        const double dabseta_temp = lepPlus.abseta() - lepMinus.abseta();
171
172        // Get the four-momenta of the positively- and negatively-charged tops
173        const FourMomentum topPlus_p4 = leptonicpartontops[0].pid() > 0 ? leptonicpartontops[0] : leptonicpartontops[1];
174        const FourMomentum topMinus_p4 = leptonicpartontops[0].pid() > 0 ? leptonicpartontops[1] : leptonicpartontops[0];
175
176        const FourMomentum ttbar_p4 = topPlus_p4 + topMinus_p4;
177
178        const double tt_mass_temp = ttbar_p4.mass();
179        const double tt_absrapidity_temp = ttbar_p4.absrapidity();
180        const double tt_pT_temp = ttbar_p4.pT();
181        const double dabsrapidity_temp = topPlus_p4.absrapidity() - topMinus_p4.absrapidity();
182
183        // Fill parton-level histos
184        fillWithUFOF( _h_dabseta, dabseta_temp);
185        fillWithUFOF( _h_dabsrapidity, dabsrapidity_temp);
186
187        // Now fill the same variables in the 2D and profile histos vs ttbar invariant mass, pT, and absolute rapidity
188        for (int i_var = 0; i_var < 3; ++i_var) {
189          double var;
190          if ( i_var == 0 ) {
191            var = tt_mass_temp;
192          } else if ( i_var == 1 ) {
193            var = tt_pT_temp;
194          } else {
195            var = tt_absrapidity_temp;
196          }
197
198          fillWithUFOF( _h_dabsrapidity_var[i_var], dabsrapidity_temp, var);
199          fillWithUFOF( _h_dabseta_var[i_var], dabseta_temp, var);
200
201          fillWithUFOF( _h_dabsrapidity_profile[i_var], dabsrapidity_temp, var, (_h_dabsrapidity->xMax() + _h_dabsrapidity->xMin())/2. );
202          fillWithUFOF( _h_dabseta_profile[i_var], dabseta_temp, var, (_h_dabseta->xMax() + _h_dabseta->xMin())/2. );
203        }
204
205      }
206
207    }
208
209
210    /// Normalise histograms to unit area
211    void finalize() {
212
213      normalize(_h_dabsetadressedleptons);
214
215      normalize(_h_dabseta);
216      normalize(_h_dabsrapidity);
217
218      for (int i_var = 0; i_var < 3; ++i_var) {
219        normalize(_h_dabsrapidity_var[i_var]);
220        normalize(_h_dabseta_var[i_var]);
221      }
222
223    }
224
225
226  private:
227
228    Histo1DPtr _h_dabsetadressedleptons, _h_dabseta, _h_dabsrapidity;
229    Histo2DPtr _h_dabseta_var[3], _h_dabsrapidity_var[3];
230    Profile1DPtr _h_dabseta_profile[3], _h_dabsrapidity_profile[3];
231
232    const vector<double> _bins_tt_mass = {300., 430., 530., 1200.};
233    const vector<double> _bins_tt_pT = {0., 41., 92., 300.};
234    const vector<double> _bins_tt_absrapidity = {0., 0.34, 0.75, 1.5};
235    const vector<double> _bins_dabseta = { -2., -68./60., -48./60., -32./60., -20./60., -8./60., 0., 8./60., 20./60., 32./60., 48./60., 68./60., 2.};
236    const vector<double> _bins_dabsrapidity = {-2., -44./60., -20./60., 0., 20./60., 44./60., 2.};
237
238    void fillWithUFOF(Histo1DPtr h, double x) {
239      h->fill(std::max(std::min(x, h->xMax()-1e-9),h->xMin()+1e-9));
240    }
241
242    void fillWithUFOF(Histo2DPtr h, double x, double y) {
243      h->fill(std::max(std::min(x, h->xMax()-1e-9),h->xMin()+1e-9), std::max(std::min(y, h->yMax()-1e-9),h->yMin()+1e-9));
244    }
245
246    void fillWithUFOF(Profile1DPtr h, double x, double y, double c) {
247      h->fill(std::max(std::min(y, h->xMax()-1e-9),h->xMin()+1e-9), float(x > c) - float(x < c));
248    }
249
250
251  };
252
253
254  RIVET_DECLARE_PLUGIN(CMS_2016_I1430892);
255
256
257}