In this thesis, we present the first exclusive observation of the $\ttbar \rightarrow$ hadronic $\tau$ + jets decay channel. Using these events, we measure the $\ttbar$ pair production cross section and the top quark mass in 2.2 $\fb$ of data collected with the Collider Detector at Fermilab (CDF). The Tevatron accelerator at Fermilab provides collisions of protons and anti-protons at a center-of-mass energy of $\sqrt{s}= 1.96 ~\TeV$ and is one of only two accelerators in the world with enough energy to produce top quarks. With a branching fraction of nearly 10\%, the hadronic $\tau$ + jets decay channel is the third largest $\ttbar$ decay mode, and it has only been minimally explored. This the first measurement of the $\ttbar$ pair production cross section in this decay channel at CDF and the first measurement of the top quark mass in this decay channel in the world. The analysis introduces a new method to recover the total momentum of the $\nu$ produced in the $\tau$ decay and an artificial neural network to reduce the contribution from the largest background source, QCD multijet background. The $\ttbar$ pair production cross section is extracted by minimizing a negative log likelihood function which compares the number of observed events to the number of expected events for a given $\ttbar$ cross section. The top quark mass is extracted by minimizing a negative log likelihood function built from signal and background probabilities which are based on the matrix elements for $\ttbar$ production and decay and $W$ + 4 parton production, respectively. Using events selected with exactly 1 hadronically decaying $\tau$, exactly 4 jets with at least 1 identified as having originated from a $b$ quark, and large missing transverse energy, we measure the $\ttbar$ pair production cross section to be $8.8 \pm 3.3 ~(\mathrm{stat.}) \pm 2.2 ~(\mathrm{syst.})$ pb and the top quark mass to be $172.7 \pm 9.3 ~(\mathrm{stat.}) \pm 3.7 ~(\mathrm{syst.})$ \GeV. We find both values to be in good agreement with previous measurements in other $\ttbar$ decay channels, and the cross section to be consistent with next-to-leading order theoretical predictions.


Posted to /cdf/www/pub/thesis/cdf10672_hare_thesis.pdf
    http://www-cdf.fnal.gov/thesis/cdf10672_hare_thesis.pdf

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