Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

52 CHAPTER 4: Reconstructing and Selecting Physics Eventcalled generally background and should be removed in order not to enter the analysisand spoil the physics results.The description of the configuration of the full signal event in this study is the subjectof the next section.4.2.1 The Signal Signature in the DetectorTop quark decays almost exclusively to a W boson and a b quark which subsequentlyhadronizes and produces a jet in the detectort(¯t) → W + (W − ) + b(¯b).Hence the decay products of a pair of top quarks only depend on the various W bosondecay channels as listed in Table 4.1 [94, 95].W decay modeBranching RatioW → lν e ; l = e, µ, τ. ∼ 33%W → q¯q; q¯q = u ¯d, c¯s. ∼ 66%Table 4.1: Various W decay channels and their branching ratio.If both of the Ws coming out of the top quarks decay into leptons and their correspondingneutrinos, it is called a full leptonic decay mode of t¯t system. If only one ofthe Ws decays leptonically and the other goes to hadrons, this makes a semi-leptonicdecay mode of a top quark pair. Otherwise, a full hadronic t¯t decay mode correspondsto the situation when both of the W bosons decay hadronically. The branching ratiosof the different decay modes of t¯t event are summarized in Table 4.2.t¯t decay modeBranching RatioFull leptonicSemi leptonicFull Hadronict¯t → W + bW −¯b → l + l − b¯b + E missT ∼ 11%t¯t → W + bW −¯b → lq¯qb¯b + EmissT ∼ 45%t¯t → W + bW −¯b → q¯qq¯qb¯b ∼ 44%Table 4.2: Various t¯t decay channels and their branching ratio.