Massive Gluon search in the top pair lepton+jets channel
using Dynamical Likelihood Method with 1.9 fb-1
Kunitaka Kondo(Waseda), Taichi Kubo(Tsukuba), Junji Naganoma(Waseda)
For the CDF Collaboration


Overview: This study presents the results of a search for the new color-octet particle decaying to the top quark pair, which we call "Massive Gluon (G)". The SM qqbar→ttbar process and the qqbar→G→ttbar interfere in this case. The decay of the top quark is assumed to be the SM decay, so we select the lepton+jet channel with at least 1 secondary vertex b-tagging using 1.9 ± 0.1 fb-1 of data. Dynamical Likelihood Method (DLM) is used to reconstruct the event-by-event top pair invariant mass. The reconstructed data top pair invariant mass distribution agrees with the standard model expectations in the explored mass and width range (400 GeV ≤ Mass ≤ 800 GeV, 0.05 ≤ Γ/M ≤ 0.5). We set the limits on the Massive Gluon strength of coupling.

Link to Public Note

Results:
DescriptionPlotLink
Top quark pair invariant mass:
Top quark pair invariant mass is reconstructed event-by-event using DLM without the production matrix element information.
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Consistency with SM:
The best fitted strengh of coupling by changing the mass and width of Massive Gluon with SM expectations by the pseudo-experiments. The results from data are consistent with the SM expectations within 1.7σ for all the explored parameter space. The non-tt background fluctuation give the high 3σ from the pseudo experiment. The difference of figures is the width of Massive Gluon.
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Upper/Lower Limits:
The upper/lower limits on the strength of coupling with 95% confidence level as a function of mass of Massive Gluon. The difference of figures is the width of Massive Gluon. The yellow regions are excluded. The limits become worse at the larger width.
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Likelihood fit example:
The examples assuming Γ/M=0.1 and M=800 GeV.
The example of the fit (Top left) and -2log[likelihood] (top right). The discrepancy with SM expectation is ~1.7σ (bottom left). The lower/upper limits (bottom right).


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Top quark transverse momenta:
The primary effect of Massive Gluon appears in the top pair invariant mass distribution. The secondary effect will appear in the Pt of the top quarks. The top quark Pt's are reconstructed by the same way as the top pair invariant mass. No significant discrepancy can be seen.
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Likelihood distribution:
Data and MC comparison in log(likehood) distribution as a validation.
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Transfer function validation:
Data and MC comparison in the bottom/light quark and the neutrino resoponse variables by taking the event-by-event maximum like response.
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Maintained by Junji Naganoma
Last modified: Tue Jan 15 11:10:26 JST 2008