Highlights of top quark measurements in hadronic final states at ATLAS

Measurements of inclusive and differential top quark production cross sections in hadronic final states, including hadronic τ decays, in proton-proton collisions with the ATLAS detector at the Large Hadron Collider are presented at center-of-mass energies of 7, 8 and 13 TeV. The inclusive cross section measurements reach high precision and are compared to other decay modes and the best available theoretical calculations. Differential measurements of the kinematic properties of top quark events are also discussed. These measurements use boosted top quarks, probing our understanding of top quark production in the TeV regime.


Backround estimation
Events where the τ had in the nal state is real; includes single top, W/Z+jets, diboson.
Events where the τ lepton in the nal state is fake (misidentied); dominated by multi-jet processes. τ +jet √ s = 8 TeV: Results Cross-section extraction for each τ had and then combined: Data prediction agreement in the signal region; total uncertainty 12%, agreement with the SM prediction.
tt cross-section in the τ + lepton channel at Final states with an electron or a muon and a hadronically decaying τ lepton; searches for top-quark decays to b-quarks + charged Higgs, decaying to τ + neutrino.

Event Selection
A primary vertex with ≥ 5 tracks (each with p T > 4GeV ); ≥ 1 τ candidate; one isolated high-p T µ or e; ≥ 2 jets with p T > 25 GeV and |η|<2.5; E miss T > 30GeV to reduce the multi-jet background.
Signal extraction discriminants employed which outputs are used to separate hadronic tau from jets; use boosted decision tree (BDT) discriminants.
Same sign (SS) and opposite sign (OS) BDT j distributions.

Background methods
Fit BDT shape with background and signal template (template tting).
Matrix method to extract background after a cut on BDT > 0.7.
good agreement with the numbers obtained by the two methods.

Measuring the tt cross-section
The cross-section is derived from the number of observed OSSS signal events in the ≥ 1 b-tag data sample; the results are given separately for τ 1 (one track candidate) and τ 3 (> one tracks candidate) and then combined.

Systematic uncertainties
detailed studies of high-p T SM processes; searches of anomalies that could be signals for new physics.

Event selection
Event selection primary vertex with ve or more charged tracks; no reconstructed e/µ with p T > 25 GeV; at least 2 large-R jets with p T > 350 GeV and |η| < 2.0 ⇒ leading jet p T > 500 GeV; ≥ 2 small-R jets with p T > 25 GeV and |η| < 2.5; ≥ 2 small-R b-tagged jets ⇒ each associated with just one of the top-tagged large-R jets; Large background ⇒ multi-jet events S bg = 1 2 ( G A + H B ) × C S: signal region; G,A,H,B: regions multi-jet dominated;