Rivet analyses referenceCMS_2016_I1430892Measurements of ttbar charge asymmetry using dilepton final states in pp collisions at sqrt(s) = 8 TeVExperiment: CMS (LHC) Inspire ID: 1430892 Status: VALIDATED Authors:
Beam energies: (4000.0, 4000.0) GeV Run details:
$\textbf{Abstract:}$ 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/DressedLeptons.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 DressedLeptons dressed_electrons(photons, electrons, 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 DressedLeptons dressed_muons(photons, muons, 0.1);
42 declare(dressed_muons, "DressedMuons");
43
44 // Parton-level top quarks
45 declare(PartonicTops(PartonicTops::DecayMode::E_MU, false), "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 const double weight = 1.0;
86
87 // Use particle-level leptons for the first histogram
88 const DressedLeptons& dressed_electrons = applyProjection<DressedLeptons>(event, "DressedElectrons");
89 const DressedLeptons& dressed_muons = applyProjection<DressedLeptons>(event, "DressedMuons");
90
91 const vector<DressedLepton> dressedels = dressed_electrons.dressedLeptons();
92 const vector<DressedLepton> dressedmus = dressed_muons.dressedLeptons();
93
94 const size_t ndressedel = dressedels.size();
95 const size_t ndressedmu = dressedmus.size();
96
97 // For the particle-level histogram, require exactly one electron and exactly one muon, to select the ttbar->emu channel.
98 // Note this means ttbar->emu events with additional PromptFinalState dilepton pairs from the shower are vetoed - for PYTHIA8,
99 // this affects ~0.5% of events, so the effect is well below the level of sensitivity of the measured distribution.
100 if ( ndressedel == 1 && ndressedmu == 1 ) {
101
102 const int electrontouse = 0, muontouse = 0;
103
104 // Opposite-charge leptons only
105 if ( sameSign(dressedels[electrontouse], dressedmus[muontouse]) ) {
106 MSG_INFO("Error, e and mu have same charge, skipping event");
107 } else {
108 // Get the four-momenta of the positively- and negatively-charged leptons
109 FourMomentum lepPlus = dressedels[electrontouse].charge() > 0 ? dressedels[electrontouse] : dressedmus[muontouse];
110 FourMomentum lepMinus = dressedels[electrontouse].charge() > 0 ? dressedmus[muontouse] : dressedels[electrontouse];
111
112 // Now calculate the variable
113 double dabseta_temp = lepPlus.abseta() - lepMinus.abseta();
114
115 fillWithUFOF( _h_dabsetadressedleptons, dabseta_temp, weight );
116 }
117
118 }
119
120
121 // The remaining variables use parton-level information.
122
123 // Get the leptonically decaying tops
124 const Particles leptonicpartontops = apply<ParticleFinder>(event, "LeptonicPartonTops").particlesByPt();
125 Particles chargedleptons;
126
127 unsigned int ntrueleptonictops = 0;
128 bool oppositesign = false;
129
130 if ( leptonicpartontops.size() == 2 ) {
131 for (size_t k = 0; k < leptonicpartontops.size(); ++k) {
132
133 // Get the lepton
134 const Particle lepTop = leptonicpartontops[k];
135 const auto isPromptChargedLepton = [](const Particle& p){return (isChargedLepton(p) && isPrompt(p, false, false));};
136 Particles lepton_candidates = lepTop.allDescendants(firstParticleWith(isPromptChargedLepton), false);
137 if ( lepton_candidates.size() < 1 ) MSG_WARNING("error, PartonicTops::DecayMode::E_MU top quark had no daughter lepton candidate, skipping event.");
138
139 // In some cases there is no lepton from the W decay but only leptons from the decay of a radiated gamma.
140 // These hadronic PartonicTops are currently being mistakenly selected by PartonicTops::DecayMode::E_MU (as of April 2017), and need to be rejected.
141 // PartonicTops::DecayMode::E_MU is being fixed in Rivet, and when it is the veto below should do nothing.
142 /// @todo Should no longer be necessary -- remove
143 bool istrueleptonictop = false;
144 for (size_t i = 0; i < lepton_candidates.size(); ++i) {
145 const Particle& lepton_candidate = lepton_candidates[i];
146 if ( lepton_candidate.hasParent(PID::PHOTON) ) {
147 MSG_DEBUG("Found gamma parent, top: " << k+1 << " of " << leptonicpartontops.size() << " , lepton: " << i+1 << " of " << lepton_candidates.size());
148 continue;
149 }
150 if ( !istrueleptonictop && sameSign(lepTop,lepton_candidate) ) {
151 chargedleptons.push_back(lepton_candidate);
152 istrueleptonictop = true;
153 }
154 else MSG_WARNING("Error, found extra prompt charged lepton from top decay (and without gamma parent), ignoring it.");
155 }
156 if ( istrueleptonictop ) ++ntrueleptonictops;
157 }
158 }
159
160 if ( ntrueleptonictops == 2 ) {
161 oppositesign = !( sameSign(chargedleptons[0],chargedleptons[1]) );
162 if ( !oppositesign ) MSG_WARNING("error, same charge tops, skipping event.");
163 }
164
165 if ( ntrueleptonictops == 2 && oppositesign ) {
166
167 // Get the four-momenta of the positively- and negatively-charged leptons
168 const FourMomentum lepPlus = chargedleptons[0].charge() > 0 ? chargedleptons[0] : chargedleptons[1];
169 const FourMomentum lepMinus = chargedleptons[0].charge() > 0 ? chargedleptons[1] : chargedleptons[0];
170
171 const double dabseta_temp = lepPlus.abseta() - lepMinus.abseta();
172
173 // Get the four-momenta of the positively- and negatively-charged tops
174 const FourMomentum topPlus_p4 = leptonicpartontops[0].pid() > 0 ? leptonicpartontops[0] : leptonicpartontops[1];
175 const FourMomentum topMinus_p4 = leptonicpartontops[0].pid() > 0 ? leptonicpartontops[1] : leptonicpartontops[0];
176
177 const FourMomentum ttbar_p4 = topPlus_p4 + topMinus_p4;
178
179 const double tt_mass_temp = ttbar_p4.mass();
180 const double tt_absrapidity_temp = ttbar_p4.absrapidity();
181 const double tt_pT_temp = ttbar_p4.pT();
182 const double dabsrapidity_temp = topPlus_p4.absrapidity() - topMinus_p4.absrapidity();
183
184 // Fill parton-level histos
185 fillWithUFOF( _h_dabseta, dabseta_temp, weight );
186 fillWithUFOF( _h_dabsrapidity, dabsrapidity_temp, weight );
187
188 // Now fill the same variables in the 2D and profile histos vs ttbar invariant mass, pT, and absolute rapidity
189 for (int i_var = 0; i_var < 3; ++i_var) {
190 double var;
191 if ( i_var == 0 ) {
192 var = tt_mass_temp;
193 } else if ( i_var == 1 ) {
194 var = tt_pT_temp;
195 } else {
196 var = tt_absrapidity_temp;
197 }
198
199 fillWithUFOF( _h_dabsrapidity_var[i_var], dabsrapidity_temp, var, weight );
200 fillWithUFOF( _h_dabseta_var[i_var], dabseta_temp, var, weight );
201
202 fillWithUFOF( _h_dabsrapidity_profile[i_var], dabsrapidity_temp, var, weight, (_h_dabsrapidity->xMax() + _h_dabsrapidity->xMin())/2. );
203 fillWithUFOF( _h_dabseta_profile[i_var], dabseta_temp, var, weight, (_h_dabseta->xMax() + _h_dabseta->xMin())/2. );
204 }
205
206 }
207
208 }
209
210
211 /// Normalise histograms to unit area
212 void finalize() {
213
214 normalize(_h_dabsetadressedleptons);
215
216 normalize(_h_dabseta);
217 normalize(_h_dabsrapidity);
218
219 for (int i_var = 0; i_var < 3; ++i_var) {
220 normalize(_h_dabsrapidity_var[i_var]);
221 normalize(_h_dabseta_var[i_var]);
222 }
223
224 }
225
226
227 private:
228
229 Histo1DPtr _h_dabsetadressedleptons, _h_dabseta, _h_dabsrapidity;
230 Histo2DPtr _h_dabseta_var[3], _h_dabsrapidity_var[3];
231 Profile1DPtr _h_dabseta_profile[3], _h_dabsrapidity_profile[3];
232
233 const vector<double> _bins_tt_mass = {300., 430., 530., 1200.};
234 const vector<double> _bins_tt_pT = {0., 41., 92., 300.};
235 const vector<double> _bins_tt_absrapidity = {0., 0.34, 0.75, 1.5};
236 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.};
237 const vector<double> _bins_dabsrapidity = {-2., -44./60., -20./60., 0., 20./60., 44./60., 2.};
238
239 void fillWithUFOF(Histo1DPtr h, double x, double w) {
240 h->fill(std::max(std::min(x, h->xMax()-1e-9),h->xMin()+1e-9), w);
241 }
242
243 void fillWithUFOF(Histo2DPtr h, double x, double y, double w) {
244 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), w);
245 }
246
247 void fillWithUFOF(Profile1DPtr h, double x, double y, double w, double c) {
248 h->fill(std::max(std::min(y, h->xMax()-1e-9),h->xMin()+1e-9), float(x > c) - float(x < c), w);
249 }
250
251
252 };
253
254
255 // The hook for the plugin system
256 RIVET_DECLARE_PLUGIN(CMS_2016_I1430892);
257
258
259}
|