Measurement of the Z to TlTh cross-section and search for the standard model vector boson fusion produced Higgs to TlTh at ATLAS

Thesis

This thesis presents a cross-section measurement of the Z → τ_ιτ_h process performed using the ATLAS detector at the Large Hadron Collider at CERN. The data used were collected at a centre-of-mass energy of 7 TeV during 2010 and corresponds to an integrated luminosity of 36 pb⁻¹. This measurement uses the final state where one τ lepton decays leptonically, to an electron or muon, plus neutrinos, and the other τ lepton decays hadronically. The γ*/Z → ιι, ttbar and di-boson background processes are estimated using Monte Carlo simulation, the W → τν Monte Carlo background prediction is estimated using a semi data-driven method. The multijet background is estimated with a fully data-driven ‘ABCD’ method using three exclusive control regions built by inverting uncorrelated requirements on the isolation of the electron or muon and the product of the electric charge of the identified electron or muon and hadronically decayed τ lepton.

The measured cross-sections in each channel are corrected for the individual branching ratios and yield the total cross-section σ(Z -> ττ,m_inv 66 - 116 GeV) = 1142 ± 135.5(stat.) ± 206.2(syst.) ± 40.19(lumi.) ± 3.6(theo.) pb for the electron channel and σ(Z → ττ,m_inv 66 - 116 GeV) = 857.6 ± 81.4(stat.) ± 132.5(syst.) ± 30.19(lumi.) ± 2.8(theo.) pb for the muon channel, where the invariant mass of the Z boson is between 66 and 116 GeV. These are in good agreement with the theoretical prediction of 960 ± 49.5 pb. Performing a measurement of a well-known Standard Model process using the ATLAS detector in this new high energy regime is essential for validating the τ lepton identification in particular, which is essential for any new physics search such as H → τ_ιτ_h.

A cut-based analysis for the Vector Boson Fusion produced Higgs boson decaying to two τ leptons channel is introduced and the possibility of using the more complex analysis technique, the Matrix Element method, to increase sensitivity is discussed.

Authors: Larner, AB

• Publication date: 2011
• DOI: 10.5287/ora-0njyjpryg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: tau leptons; particle physics; Higgs; Z Boson
• Subjects: Particle physics