Measurement of the mass difference between top and antitop quarks

Abstract

A measurement of the mass difference between the top and the antitop quark (∆m_t = m_t − m_tbar) is performed using events with a muon or an electron and at least four jets in the final state. The analysis is based on data collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.96 ± 0.11 fb-1, and yields the value of ∆m_t = −0.44 ± 0.46 (stat) ± 0.27 (syst) GeV. This result is consistent with equality of particle and antiparticle masses required by CPT invariance, with a significantly improved precision compared to existing measurements.

Main Document

Detailed record in CDS, and the Fulltext in PDF. (to come soon ...)

Some further explanation about the analysis

In this analysis, ttbar events decaying in the electron+jets and muon+jets channels are used. These are events where one of the two top quarks decays hadronically (t -> bW ->bqqbar) and the other top quark decays in the lepton channel (t -> bW -> b l nu_{l}). The lepton charge allows us to split the data sample in two pieces, where top or antitop quarks decay hadronically. For each event category, the Ideogram method is used to measure the mass of the top quarks and finally both measured masses are subtracted from each other. The Ideogram method was already used before by the CMS experiment (and other experiments) to measure the top quark mass (link1 and link2). The method is modified in this analysis as it incorporates a different kinematic fit and it uses only the hadronic portion of the ttbar decay in the kinematic fit.

Approved Results from TOP-11-019 ( click on plot to get .png )

Result	Abbreviated Caption
	Main top mass difference result.

Approved Data/MC comparison plots from TOP-11-019 ( click on plot to get .pdf )

The simulation is normalized to the number of events observed in data. Overflows are included in the last bins of the distributions. The ratio of the number of observed events in data to the number of events expected from simulation is shown at the bottom of each plot. The error band corresponds to the systematic uncertainties related to jet energy scale, jet energy resolution, background estimation and pileup modeling.

Figure lepton^{+}+jets	Figure lepton^{-}+jets	Abbreviated Caption
		Transverse Momentum of all the selected jets after applying the event selection.
		Number of selected jets after applying the event selection.
		Fitted top quark mass of the jet combination with the lowest chisquare after applying the event selection.
		Minimum chisquare of the kinematic fit of the jet combination with the lowest chisquare after applying the event selection.

Other Approved plots from TOP-11-019 ( click on plot to get .pdf )

Figure	Abbreviated Caption
	Bias on the extracted top quark mass as a function of generated top quark mass for inclusive lepton+jets events in ensembles of pseudo-experiments.
	Width of the pull distribution as a function of generated top quark mass for inclusive lepton+jets events in ensembles of pseudo-experiments.
	Bias on the estimated top quark mass as a function of the generated top quark mass for lepton^{+}+jets events, after the calibration based on inclusive lepton+jets pseudo-experiments.
	Bias on the estimated top quark mass as a function of the generated top quark mass for lepton^{-}+jets events, after the calibration based on inclusive lepton+jets pseudo-experiments.
	Width of the pull distribution as a function of the generated top quark mass for lepton^{+}+jets events, after the calibration based on inclusive lepton+jets pseudo-experiments.
	Width of the pull distribution as a function of the generated top quark mass for lepton^{-}+jets events, after the calibration based on inclusive lepton+jets pseudo-experiments.
	Difference between (a) the measured m_t and the average m_t, and (b) the measured ∆m_t both for data and simulation, as a function of the number of reconstructed collision vertices. (c) Distribution for the number of reconstructed collision vertices in data and in simulation, after reweighting for pileup

Tables from TOP-11-019 ( click on plot to get .png )

Result	Abbreviated Caption
	Number of events following full event selection for μ^{+}+jets, μ^{−}+jets, e^{+}+jets, and e^{−}+jets for data, and as expected from simulation, before any rescaling. Uncertainties are purely statistical and do not include uncertainties on production cross sections, integrated luminosity, detector acceptance, or selection efficiencies.
	Overview of systematic uncertainties on ∆m_t. The total is defined by adding in quadrature the contributions from all sources, by choosing for each the larger of the estimated shift or its statistical uncertainty, as indicated by the bold script.

-- StijnBlyweert - 06-Apr-2012