Measurement of the Top Quark Mass and ppbar -> ttbar Cross Section in the All-Hadronic Mode with the CDFII Detector

TL;DR

The study measures the top-quark mass $M_{\rm top}$ and the $t\bar{t}$ cross section $\sigma_{t\bar{t}}$ in the all-hadronic decay channel using the CDF II detector, employing neural-network selection and $b$-tagging to suppress overwhelming QCD background. A novel in situ jet energy scale (JES) calibration is performed by fitting both $m_t^{\rm rec}$ and $m_W^{\rm rec}$ distributions in a two-parameter likelihood, enabling simultaneous determination of $M_{\rm top}$ and JES. The results yield $M_{\rm top}=174.8\pm 2.4\,({\rm stat}+{\rm JES})^{+1.2}_{-1.0}\,\mathrm{GeV}/c^2$ and $\sigma_{t\bar{t}}=7.2\pm 0.5\,({\rm stat})\pm 1.0\,({\rm syst})\pm 0.4\,({\rm lum})$ pb, in agreement with SM predictions and other channels. This work demonstrates a precise all-hadronic measurement by combining detailed event reconstruction, data-driven background estimation, and rigorous calibration, contributing to the global consistency checks of the top-quark sector.

Abstract

We present a measurement of the top quark mass and of the top-antitop pair production cross section using p-pbar data collected with the CDFII detector at the Tevatron Collider at the Fermi National Accelerator Laboratory and corresponding to an integrated luminosity of 2.9 fb-1. We select events with six or more jets satisfying a number of kinematical requirements imposed by means of a neural network algorithm. At least one of these jets must originate from a b quark, as identified by the reconstruction of a secondary vertex inside the jet. The mass measurement is based on a likelihood fit incorporating reconstructed mass distributions representative of signal and background, where the absolute jet energy scale (JES) is measured simultaneously with the top quark mass. The measurement yields a value of 174.8 +- 2.4(stat+JES) ^{+1.2}_{-1.0}(syst) GeV/c^2, where the uncertainty from the absolute jet energy scale is evaluated together with the statistical uncertainty. The procedure measures also the amount of signal from which we derive a cross section, sigma_{ttbar} = 7.2 +- 0.5(stat) +- 1.0 (syst) +- 0.4 (lum) pb, for the measured values of top quark mass and JES.

Figures (11)

• Figure 1: Geometric average of the $\eta$ scaled moments ($\langle M^{s}_{\eta} \rangle$, upper plot) and of the $\phi$ scaled moments ($\langle M^{s}_{\phi} \rangle$, lower plot) for QCD multijet (solid histogram) and simulated $\hbox{$t\bar{t}$}$ (dashed histogram) events with $6\le N_{\mathrm{jets}}\le 8$.
• Figure 2: Neural-network output $N_{\mathrm{out}}$ for QCD multijet (solid histogram) and simulated $\hbox{$t\bar{t}$}$ (dashed histogram) events with $6\le N_{\mathrm{jets}}\le 8$. Histograms are normalized to unity. The neural-network implementation that we use in the TMultiLayer-Perceptron produces an output which is not strictly bound between 0 and 1.
• Figure 3: Histograms of the reconstructed top quark mass $m_{t}^{\mathrm{rec}}$ for 1-tag events, upper plot, and $\ge 2$-tag events, lower plot, are shown in a control region defined by $0.75 \le N_{\mathrm{out}} < 0.85$. Along with the data are plotted the expected background and the signal contribution for $M_{\mathrm{top}}=175$ GeV/$c^{2}$ and the default value $\Delta\mathrm{JES} = 0$, normalized to the difference between the data and the background. The value of the purely statistical $\chi^{2}$ probability is reported on each plot.
• Figure 4: Histograms of the reconstructed $W$ mass $m_{W}^{\mathrm{rec}}$ for 1-tag events, upper plot, and $\ge 2$-tag events, lower plot, are shown in a control region defined by $0.75 \le N_{\mathrm{out}} < 0.85$. Along with the data are plotted the expected background and the signal contribution for $M_{\mathrm{top}}=175$ GeV/$c^{2}$ and the default value $\Delta\mathrm{JES} = 0$, normalized to the difference between the data and the background. The value of the purely statistical $\chi^{2}$ probability is reported on each plot.
• Figure 5: Histograms and corresponding fitted probability density functions for the signal $m_{t}^{\mathrm{rec}}$ in $\ge 2$-tag events for a constant $\Delta\mathrm{JES}$ value ($\Delta\mathrm{JES}=0$), varying the input top quark mass (upper plot) and for a constant $M_{\mathrm{top}}$ value (175 GeV/$c^2$), varying the input jet energy scale (lower plot).