Top Quark Pair Production at a 500 GeV CLIC Collider

The mass of the top quark is studied in fully hadronic and semi leptonic decays of top & anti-top pairs using event samples from signal and background (SM) processes.

The semi-leptonic decay:

e+ e- ---> t tbar ---> q qbar bbar l v b

The fully-hadronic decay:

e+ e- ---> t tbar ---> q qbar bbar q qbar b

All possible decay modes of top anti-top pairs were generated. The mass of the top quark was taken to be 174.0 GeV and the cross-section 1.37 GeV.

The signal process studied in the note was e+ e- ---> t tbar . Since the top quark almost always decays into a W boson and a b quark, the signal events can therefore be grouped into different classes according to the decay of the W boson. The three channels which can be classified are the fully hadronic, semi leptonic, and fully leptonic channel (which was rejected in the note)

Data Analysis

Following from the signal event, the Lepton Finder classified all events as fully hadronic, semi leptonic and fully leptonic. The fully hadronic and semi leptonic events were clustered into four or six jets respectively. The lepton finder code can be found under:

less TTBarAnalysis/Selection-Rejection-Classification/src/TTbarLeptonVeto.cc

Following jet clustering, Flavour Tagging (b tagging) identified the different jets. Flavour tagging was performed with LCFI Flavour Tagging package. Information on the b tagging can be found under:

less TTBarAnalysis/Selection-Rejection-Classification/src/TTbarBTagSelection.cc

After the identification of b jets and the classification of events from the W boson decay, the next step of the analysis is the pairing of W candidates and b jets into top quark candidates. Information on the kinematic fit, using the MarlinKinFit package, can be found:

less TTBarAnalysis/Selection-Rejection-Classification/src/TTbarWbCombinationSelection.cc

less TTBarAnalysis/Selection-Rejection-Classification/src/TTbarWbCombinationSelection4Jets.cc

Higgs mass measurements

For the Higgsstrahlung process at 350 GeV and 500 GeV, need lepton identification to distinguish between muon and electron from the recoil Z in a model-independent decay: The signal selection procedure should consist of an examination of the particles available in the collection of PandoraPFA particle flow objects (PFOs).

For 500 GeV Hzqq analysis, need flavour tagging to tag the jets likely associated with b quarks. Also require a kinematic fit to divide which jet goes to the Z or H.

For 500 GeV Hmumu event, this signal sample comprised both Higgsstrahlung and WW fusion. Need to use a jet reconstruction algorithm to force each event into two jets, then can search for b quark decays of Higgs.