Measurement of the Top Quark Mass in the All-Hadronic Top-Antitop Decay Channel Using Proton-Proton Collision Data from the ATLAS Experiment at a Centre-of-Mass Energy of 8 TeV

A measurement of the top quark mass ($m_{top}$) is presented using top quark candidates reconstructed from the jets in all-hadronic $t\bar{t}$ decays. The analysis makes use of the full ATLAS dataset collected over the 2012 period at a centre-of-mass energy of $\sqrt{s}=8$ TeV and with a total integrated luminosity of $\int Ldt=20.6$ fb$^{-1}$. A five-jet trigger, together with an offline cut requiring five central jets with a transverse momentum of at least 60 GeV, was used to pre-select candidate signal events. A series of selection cuts are subsequently employed to increase the signal fraction in data events, motivated by both measured data and simulation. These cuts, as well the analytic $\chi^2$ reconstruction algorithm to designate which jets are to be used to reconstruct the top quarks, aim to select those events and candidate top quarks deemed most consistent with an all-hadronic $t\bar{t}$ decay topology. The most discriminating cut involves the output of a b-tagging algorithm employed to identify jets suspected as having been initiated by a bottom-type quark – a key signature of top quark decays. The measurement is made using a one-dimensional template method, in which a series of distributions are produced for an observable sensitive to the top quark mass by using simulated signal samples with varying input $m_{top}$ values. The ratio of the three- to two-jet invariant masses of candidate top quark - $W$ boson pairs is selected as the $m_{top}$-sensitive observable, as it is less susceptible to uncertainties arising from the jet energy scale, thereby reducing the corresponding systematic uncertainty on $m_{top}$. The contributions from backgrounds, including the dominant QCD multi-jet production but also $t\bar{t}$ events with at least one leptonic $W$ boson decay, are estimated using a mix of simulation and data-driven methods. The final measurement is made via a binned $\chi^2$ minimization procedure which takes into account uncertainties in the bin contents from the measured data as well as uncertainties in the parameterization for both signal and background shapes. The final measured value is found to be $m_{top} = 174.29 \ \pm 0.52 \ $(stat)$ \ \pm \ 1.03 \ $(syst) GeV, consistent both with recent LHC measurements in the same and orthogonal decay channels and the current world average. The fraction of background events within the range of the final distribution, measured simultaneously, is found to be $F_{bkgd}= 0.517 \ \pm \ 0.015 \ $(stat)$ \ \pm \ 0.075 \ $(syst).