TOP-11-031
Constraints on the top quark charge from top pair events

Abstract

The top quark is the electroweak isospin partner of the bottom quark and is expected to have an electric charge of + 2/3 e. We present a measurement that discriminates between two top quark charge hypotheses using the muon+jets final state of top-pair events detected with the CMS experiment. The measurement is performed with proton-proton collisions at sqrt{s} = 7 TeV, corresponding to an integrated luminosity of 4.6 fb-1. We exploit charge correlations between high-pT muons from W boson decays and soft muons from B-hadron decays in b jets to constrain the top-quark charge. A normalised asymmetry of the events categorised with a charge of either +2/3 e or −4/3 e is calculated. The scenario with an exotic top quark charge of −4/3 e would correspond to an asymmetry between the two categories of A = −1. This can be excluded with high significance and the measured asymmetry of Ameas = 0.97 +/- 0.12(stat.) +/- 0.31(sys.) agrees nicely with the standard-model expectation of A = +1.

Document

The document can be found here.

Figures

Object reconstruction and event selection

Figure Caption
both_Cutflow.png The event yield for data and simulated events for each criterion in addition to the standard CMS selection in the ttbar semi-muonic channel. b-jet identification: at least two b-tagged jets (as described in Section 3.1); jet association: quality requirement on invariant masses (as described in Section 3.2); charge assignment: existence of a soft muon within a b jet (as described in Section 4).[Get pdf version]
ptSoftMuon.png Comparison between data and simulation of the transverse momentum relative to the b-jet axis of soft muons. The last bin contains overflow.[Get pdf version]

Assignment of the b-jet charge

Figure Caption
ptLeptonsOfBMesons.png pt_rel of the soft muons on generator level stemming from B- and D-meson decays with respect to the b-quark jet axis that have at least a pt_rel of 4 GeV. [Get pdf version]
EffptLeptonsOfBJets.png Probability of correct charge assignment for reconstructed soft muons as function of the momentum transverse to the b-jet axis. [Get pdf version]
optSoftLeptonPtJetaxisCombinedPAS.png Optimisation of the charge discrimination: The statistical significance of the discrimination performance can be quantified in terms of epsilon D**2 as described in the text. Here, the performance is given in dependence of the minimal requirement on pt_rel for a combined method with the jet-charge observable and the method where only the charge of the muon is used as discriminator.[Get pdf version]
jetChargeb.png The jet-charge observable of jets stemming from b and bbar quarks. There is no requirement on the jets to contain a soft muon. [Get pdf version]

Categorisation and statistical evaluation

Figure Caption
lepside.png Assignment of the charge of the top quark candidates: The two jets with highest b-tag are considered to originate from the top quark decays. The charge q_b is determined for the jet that fulfils all charge requirements. If both jets allow for charge reconstruction the jet with higher b-tag is considered. Two cases are possible. If the charge reconstructed b-jet is associated to the leptonic decay branch (this plot) the top quark charge is calculated according to q_top = q_mu + q_b. If the charge of the b-jet is associated to the hadronic decay branch, the other b-jet is considered to be the one on the leptonic decay branch and having an opposite charge −q_b. [Get pdf version]
hadside.png Assignment of the charge of the top quark candidates: The two jets with highest b-tag are considered to originate from the top quark decays. The charge q_b is determined for the jet that fulfils all charge requirements. If both jets allow for charge reconstruction the jet with higher b-tag is considered. Two cases are possible. If the charge reconstructed b-jet is associated to the leptonic decay branch the top quark charge is calculated according to q_top = q_mu + q_b. If the charge of the b-jet is associated to the hadronic decay branch, the other b-jet is considered to be the one on the leptonic decay branch and having an opposite charge −q_b (this plot). [Get pdf version]
ScenariosLumi.png Associated charge of the top-quark candidates and predicted distribution for simulated SM (q_top=2/3 e) events. MC is normalised to luminosity. The shaded area corresponds to the statistical error of the simulated events. [Get pdf version]
TopChargeMeasurement.png Probability function of the asymmetry obtained from pseudo experiments for the SM and the exotic hypothesis. The vertical line corresponds to the measured asymmetry. [Get pdf version]
AlphaBeta.png Type I (alpha) and Type II (beta) error rate for the hypothesis test, including all systematic uncertainties. [Get pdf version]
Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf AlphaBeta.pdf r4 r3 r2 r1 manage 16.2 K 2012-03-08 - 18:21 YvonneKuessel Type I and Type II error rate for the hypothesis test, including all systematic uncertainties.
PNGpng AlphaBeta.png r4 r3 r2 r1 manage 24.0 K 2012-03-08 - 18:49 YvonneKuessel  
PDFpdf EffptLeptonsOfBJets.pdf r3 r2 r1 manage 16.1 K 2012-03-08 - 18:23 YvonneKuessel Efficiency of correct charge assignment for generated/reconstructed soft muons.
PNGpng EffptLeptonsOfBJets.png r3 r2 r1 manage 77.5 K 2012-03-08 - 19:15 BenediktHegner  
PDFpdf ScenariosLumi.pdf r4 r3 r2 r1 manage 13.9 K 2012-03-08 - 18:25 YvonneKuessel Associated charge of the top-quark candidates and predicted distribution for simulated SM.
PNGpng ScenariosLumi.png r4 r3 r2 r1 manage 84.3 K 2012-03-08 - 19:16 BenediktHegner  
PDFpdf TopChargeMeasurement.pdf r4 r3 r2 r1 manage 15.6 K 2012-03-08 - 18:29 YvonneKuessel Probability function of the asymmetry obtained from pseudo experiments for the SM and the exotic hypothesis.
PNGpng TopChargeMeasurement.png r4 r3 r2 r1 manage 32.7 K 2012-03-08 - 18:50 YvonneKuessel  
PDFpdf both_Cutflow.pdf r4 r3 r2 r1 manage 15.1 K 2012-03-08 - 18:31 YvonneKuessel The event yield for data and simulated events for each criterion in addition to the standard CMS selection in the ttbar semi-muonic channel
PNGpng both_Cutflow.png r4 r3 r2 r1 manage 95.5 K 2012-03-08 - 19:16 BenediktHegner  
PDFpdf hadside.pdf r2 r1 manage 31.3 K 2012-03-08 - 18:40 YvonneKuessel Assignment procedure of the charge of the top quark candidates (hadronic side).
PNGpng hadside.png r2 r1 manage 62.8 K 2012-03-08 - 19:17 BenediktHegner  
PDFpdf jetChargeb.pdf r3 r2 r1 manage 14.1 K 2012-03-08 - 18:32 YvonneKuessel Jet charge of b-jets and anti-b-jets.
PNGpng jetChargeb.png r2 r1 manage 74.9 K 2012-03-08 - 19:17 BenediktHegner  
PDFpdf lepside.pdf r2 r1 manage 31.3 K 2012-03-08 - 18:43 YvonneKuessel Assignment procedure of the charge of the top quark candidates (leptonic side).
PNGpng lepside.png r2 r1 manage 62.1 K 2012-03-08 - 19:17 BenediktHegner  
PDFpdf optSoftLeptonPtJetaxisCombinedPAS.pdf r3 r2 r1 manage 13.9 K 2012-03-08 - 18:33 YvonneKuessel Power of charge discrimination as a function of pt_rel.
PNGpng optSoftLeptonPtJetaxisCombinedPAS.png r2 r1 manage 79.7 K 2012-03-08 - 19:18 BenediktHegner  
PDFpdf ptLeptonsOfBMesons.pdf r3 r2 r1 manage 15.4 K 2012-03-08 - 18:34 YvonneKuessel pt_rel of the soft muons on generator level stemming from B- and D-meson decays with respect to the b-quark axis that have at least a pt_rel of 4 GeV.
PNGpng ptLeptonsOfBMesons.png r4 r3 r2 r1 manage 107.5 K 2012-03-08 - 19:18 BenediktHegner  
PDFpdf ptSoftMuon.pdf r2 r1 manage 17.9 K 2012-03-08 - 18:36 YvonneKuessel  
PNGpng ptSoftMuon.png r4 r3 r2 r1 manage 106.5 K 2012-03-08 - 19:18 BenediktHegner  
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Topic revision: r9 - 2013-12-04 - HeikoGeenen
 
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