Rivet analyses
ttbar (to l+jets) differential cross sections at 8 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1404878
Status: VALIDATED
Authors: - Francesco La Ruffa - Christian Gutschow
References: - Expt page: ATLAS-TOPQ-2015-06 - Submitted to EPJC - arXiv: 1511.04716
Beams: p+ p+
Beam energies: (4000.0, 4000.0)GeV
Run details: - non-all-hadronic ttbar production at 8 TeV
Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, tt̄ system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV. The observables have been chosen to emphasize the tt̄ production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb−1, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.
Source
code:ATLAS_2015_I1404878.cc
// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Projections/IdentifiedFinalState.hh"
#include "Rivet/Projections/PromptFinalState.hh"
#include "Rivet/Projections/LeptonFinder.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/VisibleFinalState.hh"
namespace Rivet {
/// ttbar (to l+jets) differential cross sections at 8 TeV
class ATLAS_2015_I1404878 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2015_I1404878);
void init() {
// Eta ranges
Cut eta_full = (Cuts::abseta < 4.2) & (Cuts::pT >= 1.0*MeV);
Cut lep_cuts = (Cuts::abseta < 2.5) && (Cuts::pT > 25*GeV);
// All final state particles
FinalState fs(eta_full);
// Get photons to dress leptons
IdentifiedFinalState photons(fs);
photons.acceptIdPair(PID::PHOTON);
// Projection to find the electrons
IdentifiedFinalState el_id(fs);
el_id.acceptIdPair(PID::ELECTRON);
PromptFinalState electrons(el_id);
electrons.acceptTauDecays(true);
declare(electrons, "electrons");
LeptonFinder dressedelectrons(electrons, photons, 0.1, lep_cuts);
declare(dressedelectrons, "dressedelectrons");
LeptonFinder ewdressedelectrons(electrons, photons, 0.1, eta_full);
declare(ewdressedelectrons, "ewdressedelectrons");
// Projection to find the muons
IdentifiedFinalState mu_id(fs);
mu_id.acceptIdPair(PID::MUON);
PromptFinalState muons(mu_id);
muons.acceptTauDecays(true);
declare(muons, "muons");
LeptonFinder dressedmuons(muons, photons, 0.1, lep_cuts);
declare(dressedmuons, "dressedmuons");
LeptonFinder ewdressedmuons(muons, photons, 0.1, eta_full);
declare(ewdressedmuons, "ewdressedmuons");
// Projection to find neutrinos
IdentifiedFinalState nu_id;
nu_id.acceptNeutrinos();
PromptFinalState neutrinos(nu_id);
neutrinos.acceptTauDecays(true);
// get MET from generic invisibles
VetoedFinalState inv_fs(fs);
inv_fs.addVetoOnThisFinalState(VisibleFinalState(fs));
declare(inv_fs, "InvisibleFS");
// Jet clustering.
VetoedFinalState vfs;
vfs.addVetoOnThisFinalState(ewdressedelectrons);
vfs.addVetoOnThisFinalState(ewdressedmuons);
vfs.addVetoOnThisFinalState(neutrinos);
FastJets jets(vfs, JetAlg::ANTIKT, 0.4, JetMuons::ALL, JetInvisibles::DECAY);
declare(jets, "jets");
// Histogram booking
book(_h["massttbar"] , 1, 1, 1);
book(_h["massttbar_norm"] , 2, 1, 1);
book(_h["ptttbar"] , 3, 1, 1);
book(_h["ptttbar_norm"] , 4, 1, 1);
book(_h["absrapttbar"] , 5, 1, 1);
book(_h["absrapttbar_norm"] , 6, 1, 1);
book(_h["ptpseudotophadron"] , 7, 1, 1);
book(_h["ptpseudotophadron_norm"] , 8, 1, 1);
book(_h["absrappseudotophadron"] , 9, 1, 1);
book(_h["absrappseudotophadron_norm"] ,10, 1, 1);
book(_h["absPout"] ,11, 1, 1);
book(_h["absPout_norm"] ,12, 1, 1);
book(_h["dPhittbar"] ,13, 1, 1);
book(_h["dPhittbar_norm"] ,14, 1, 1);
book(_h["HTttbar"] ,15, 1, 1);
book(_h["HTttbar_norm"] ,16, 1, 1);
book(_h["Yboost"] ,17, 1, 1);
book(_h["Yboost_norm"] ,18, 1, 1);
book(_h["chittbar"] ,19, 1, 1);
book(_h["chittbar_norm"] ,20, 1, 1);
book(_h["RWt"] ,21, 1, 1);
book(_h["RWt_norm"] ,22, 1, 1);
}
void analyze(const Event& event) {
// Get the selected objects, using the projections.
DressedLeptons electrons = apply<LeptonFinder>(event, "dressedelectrons").dressedLeptons();
DressedLeptons muons = apply<LeptonFinder>(event, "dressedmuons").dressedLeptons();
const Jets& jets = apply<FastJets>(event, "jets").jetsByPt(Cuts::pT > 25*GeV && Cuts::abseta < 2.5);
const FinalState& ifs = apply<FinalState>(event, "InvisibleFS");
// Calculate MET
FourMomentum met;
for (const Particle& p : ifs.particles()) met += p.momentum();
// Count the number of b-tags
Jets bjets, lightjets;
for (const Jet& jet : jets){
bool b_tagged = jet.bTags(Cuts::pT > 5*GeV).size();
if ( b_tagged && bjets.size() < 2 ) bjets += jet;
else lightjets += jet;
}
bool single_electron = (electrons.size() == 1) && (muons.empty());
bool single_muon = (muons.size() == 1) && (electrons.empty());
DressedLepton* lepton = nullptr;
if (single_electron) lepton = &electrons[0];
else if (single_muon) lepton = &muons[0];
if(!single_electron && !single_muon) vetoEvent;
if (jets.size() < 4 || bjets.size() < 2) vetoEvent;
FourMomentum pbjet1; // Momentum of bjet1
FourMomentum pbjet2; // Momentum of bjet2
if ( deltaR(bjets[0], *lepton) <= deltaR(bjets[1], *lepton) ) {
pbjet1 = bjets[0].momentum();
pbjet2 = bjets[1].momentum();
} else {
pbjet1 = bjets[1].momentum();
pbjet2 = bjets[0].momentum();
}
double bestWmass = 1000.0*TeV;
double mWPDG = 80.399*GeV;
int Wj1index = -1, Wj2index = -1;
for (unsigned int i = 0; i < (lightjets.size() - 1); ++i) {
for (unsigned int j = i + 1; j < lightjets.size(); ++j) {
double wmass = (lightjets[i].momentum() + lightjets[j].momentum()).mass();
if (fabs(wmass - mWPDG) < fabs(bestWmass - mWPDG)) {
bestWmass = wmass;
Wj1index = i;
Wj2index = j;
}
}
}
// Compute hadronic W boson
FourMomentum pWhadron = lightjets[Wj1index].momentum() + lightjets[Wj2index].momentum();
double pz = _computeneutrinoz(lepton->momentum(), met);
FourMomentum ppseudoneutrino( sqrt(sqr(met.px()) + sqr(met.py()) + sqr(pz)), met.px(), met.py(), pz);
// Compute leptonic, hadronic, combined pseudo-top
FourMomentum ppseudotoplepton = lepton->momentum() + ppseudoneutrino + pbjet1;
FourMomentum ppseudotophadron = pbjet2 + pWhadron;
FourMomentum pttbar = ppseudotoplepton + ppseudotophadron;
Vector3 z_versor(0,0,1);
Vector3 vpseudotophadron = ppseudotophadron.vector3();
Vector3 vpseudotoplepton = ppseudotoplepton.vector3();
// Observables
double ystar = 0.5 * deltaRap(ppseudotophadron, ppseudotoplepton);
double chi_ttbar = exp(2 * fabs(ystar));
double deltaPhi_ttbar = deltaPhi(ppseudotoplepton,ppseudotophadron);
double HT_ttbar = ppseudotophadron.pt() + ppseudotoplepton.pt();
double Yboost = 0.5 * fabs(ppseudotophadron.rapidity() + ppseudotoplepton.rapidity());
double R_Wt = pWhadron.pt() / ppseudotophadron.pt();
double absPout = fabs(vpseudotophadron.dot((vpseudotoplepton.cross(z_versor))/(vpseudotoplepton.cross(z_versor).mod())));
// absolute cross sections
_h["ptpseudotophadron"]->fill( ppseudotophadron.pt()); //pT of pseudo top hadron
_h["ptttbar"]->fill( pttbar.pt()); //fill pT of ttbar in combined channel
_h["absrappseudotophadron"]->fill(ppseudotophadron.absrap());
_h["absrapttbar"]->fill( pttbar.absrap());
_h["massttbar"]->fill( pttbar.mass());
_h["absPout"]->fill( absPout);
_h["chittbar"]->fill( chi_ttbar);
_h["dPhittbar"]->fill( deltaPhi_ttbar);
_h["HTttbar"]->fill( HT_ttbar);
_h["Yboost"]->fill( Yboost);
_h["RWt"]->fill( R_Wt);
// normalised cross sections
_h["ptpseudotophadron_norm"]->fill( ppseudotophadron.pt()); //pT of pseudo top hadron
_h["ptttbar_norm"]->fill( pttbar.pt()); //fill pT of ttbar in combined channel
_h["absrappseudotophadron_norm"]->fill(ppseudotophadron.absrap());
_h["absrapttbar_norm"]->fill( pttbar.absrap());
_h["massttbar_norm"]->fill( pttbar.mass());
_h["absPout_norm"]->fill( absPout);
_h["chittbar_norm"]->fill( chi_ttbar);
_h["dPhittbar_norm"]->fill( deltaPhi_ttbar);
_h["HTttbar_norm"]->fill( HT_ttbar);
_h["Yboost_norm"]->fill( Yboost);
_h["RWt_norm"]->fill( R_Wt);
}
void finalize() {
// Normalize to cross-section
const double sf = crossSection()/picobarn / sumOfWeights();
for (auto& k_h : _h) {
scale(k_h.second, sf);
if (k_h.first.find("_norm") != string::npos) normalize(k_h.second);
}
}
private:
// Compute z component of neutrino momentum given lepton and met
double _computeneutrinoz(const FourMomentum& lepton, FourMomentum& met) const {
double m_W = 80.399; // in GeV, given in the paper
double k = (( sqr( m_W ) - sqr( lepton.mass() ) ) / 2 ) + (lepton.px() * met.px() + lepton.py() * met.py());
double a = sqr ( lepton.E() )- sqr ( lepton.pz() );
double b = -2*k*lepton.pz();
double c = sqr( lepton.E() ) * sqr( met.pT() ) - sqr( k );
double discriminant = sqr(b) - 4 * a * c;
double quad[2] = { (- b - sqrt(discriminant)) / (2 * a), (- b + sqrt(discriminant)) / (2 * a) }; //two possible quadratic solns
double pzneutrino;
if (discriminant < 0) { // if the discriminant is negative:
pzneutrino = - b / (2 * a);
} else { // if the discriminant is positive, take the soln with smallest absolute value
pzneutrino = (fabs(quad[0]) < fabs(quad[1])) ? quad[0] : quad[1];
}
return pzneutrino;
}
/// @todo Replace with central version
double _mT(const FourMomentum &l, FourMomentum &nu) const {
return sqrt( 2 * l.pT() * nu.pT() * (1 - cos(deltaPhi(l, nu))) );
}
/// @name Objects that are used by the event selection decisions
map<string, Histo1DPtr> _h;
};
RIVET_DECLARE_PLUGIN(ATLAS_2015_I1404878);
}