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Determination of the Top Quark Mass with a Template Method in the All-Hadronic Decay Channel using 2.04 fb-1 of ATLAS Data ATLAS-CONF-2012-030 14 March 2012
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| Abstract | ||
| This note presents a measurement of the the top-quark mass from the all-hadronic ttbar decay channel using 2.04fb^-1 of ATLAS data. Fully hadronically decaying top quark pairs are selected from ATLAS data using two tagged b-quark jets and reconstructed by assigning jets according to their compatibility with the ttbar decay hypothesis. The top-quark mass is obtained by fitting templates to the three-jet mass combination of the selected jet triplets. The templates are obtained from combining a ttbar signal derived from Monte-Carlo simulation with a background contribution obtained from data by mixing jets from different events. Five different top-quark mass values have been simulated to construct a set of five templates which are used in the fit for the top-quark mass. This yields a top-quark mass of m_t = (174.9 +- 2.1 (stat.) +- 3.8 (syst.)) GeV in agreement with previous ATLAS results obtained from the the lepton+jets channel. | ||
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Figure 01: Distribution of the mass χ2 for the tt̄ event reconstruction. The data are shown superimposed on the expectation for tt̄ signal and background. png (61kB) eps (69kB) pdf (7kB) |
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Figure 02: Distribution of mjjb observed in data. The top-quark signal is superimposed on the distribution from multijet background. png (58kB) eps (66kB) pdf (7kB) |
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Figure 03: Templates for the top-quark mass fit: each plot was produced with a different top-quark mass assumed for the Monte-Carlo simulation. The top-quark mass signal has been parametrized by a Gaussian (red curves) while the combinatorial background has been parametrized by a Landau function (green). The black curve is the sum of the Gaussian and the Landau function. png (169kB) eps (156kB) pdf (25kB) |
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Figure 04: Validation of event mixing: comparison of jet transverse momentum and of HT distributions of five-jet events with two b-tagged jets measured in data (points) with the distributions obtained by combining a four-jet event with two b-tagged jets and jets of lower transverse momentum from another five-jet event (4+1 mixing). png (145kB) eps (204kB) pdf (34kB) |
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Figure 05: Validation of event mixing: comparison of jet transverse momenta distributions for measured six-jet events (data points) with the distributions obtained by combining a five-jet event with the jets of lower transverse momentum from another inclusive six-jet event (5+1 mixing). Both the data sample and the mixed jets event sample have no b-tagged jets. png (151kB) eps (210kB) pdf (35kB) |
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Figure 06: The distribution obtained from measured six-jet events having no b-tagged jets with the distributions obtained by combining a five-jet event and the jets of lower transverse momentum from another inclusive six-jet event. png (68kB) eps (68kB) pdf (9kB) |
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Figure 07: Validation of event mixing: comparison of number of jets, aplanarity, centrality, and of HT distributions for measured six-jet events having no b-tagged jets with the distributions obtained by combining a five-jet event and the jets of lower transverse momentum from another inclusive six-jet event. png (103kB) eps (135kB) pdf (21kB) |
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Figure 08: Fit of top-quark mass: The black histogram is the sum of contributions from the Monte-Carlo simulation derived signal for a top-quark mass of 174.9 GeV (red) and the background estimate from the 5+1 mixing sample (green). png (63kB) eps (58kB) pdf (8kB) |
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| Auxiliary figures and tables | ||
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Figure
09: Superposition of top-quark mass templates for four different top-quark masses. The templates have been used for the likelihood fit of the top-quark mass. The histograms represent the result from the Monte-Carlo simulation while the smooth lines are a parametrization using the sum of a Gaussian function for the top-quark signal plus a Landau function for the combinatorial background. png (89kB) eps (67kB) pdf (8kB) |
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10: Comparison of jet transverse momenta for the six leading jets in data (points with statistical error bars) with simulated top-quark signal (solid histogram) on top of 5+1-jet mixing background (green shaded histogram). png (106kB) eps (177kB) pdf (20kB) |
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11: Comparison of spatial distance between b-quark jets, aplanarity, centrality, and HT variable in data (points with statistical error bars) with simulated top-quark signal (solid histogram) on top of 5+1-jet mixing background (green shaded histogram). png (104kB) eps (146kB) pdf (17kB) |
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