Measurement of the top anti-top quark pair production cross-section in the $ \tau $ $ + $ jets channel at $ \sqrt{s} $ = 7 TeV using ATLAS detector.

The Large Hadron Collider (LHC) is a factory for top quarks, which are the heaviest elementary particle discovered so far. Due to its small life time a top quark decays before hadronization, and hence most of its properties are not washed out. Top quark events are often a large background in many beyond-the-Standard Model scenarios. For this reason, a deep knowledge of its properties and behaviour is of crucial importance in searches for new physics at the LHC. This dissertation describes a measurement of the top anti-top ($ t \bar{t} $) pair production cross-section in the tau + jets channel. The measurement is obtained by using 4.7 $fb^{-1}$ data collected in year 2011 by the ATLAS detector from proton-proton (pp) collisions at the center of mass energy $ \sqrt{s} $ = 7 TeV. Selected events are required to have one isolated tau with missing transverse energy and hadronic jets, at least one of which must be originated from a b quark. The measured cross section is 152 $\pm$ 12 (stat.)$ \; $ $ ^{+28}_{-30} $ (syst.) $ \pm $ 1.8 (lumi.)pb, which is in agreement with standard model predictions $ 167^{+17}_{-18} $. Using a data sample of $ 1.67 \; \;\mathrm{fb^{-1} }$ ATLAS collaboration had obtained a cross-section value at $ \sqrt{s} $ = 7 TeV, $\sigma_{t \bar{t}} = 194 \pm 18 (\mathrm{stat.}) \pm 47 (\mathrm{syst.}) \;\; \mathrm{pb}$ at center of mass energy equal to 7 TeV. The result obtained by CMS collaboration at $ \sqrt{s} $= 7 TeV, using a data sample of is $ 3.9 \; \; \mathrm{fb^{-1} }$ is $\sigma_{t \bar{t}} = 152 \pm 12 (\mathrm{stat.}) \pm 32 (\mathrm{syst.}) \pm 3 (\mathrm{lumi.}) \;\; \mathrm{pb}$.