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Boosted Top Quarks

The top quark is the heaviest known particle ($m_{top} \simeq 173$ GeV). It is even heavier than the newly discovered Higgs boson ($m_{Higgs} \simeq 125$ GeV). Because of its high mass, the top quark plays an important role in both the Standard Model (in electroweak symmetry breaking, due to its large coupling to the Higgs boson) and in Physics beyond the Standard Model scenarios. At the LHC, due to the large center-of-mass energy of the proton-proton collisions, top quark pairs ($t\bar{t}$) are produced in large amounts. A significant fraction of the top quarks have momenta that are large compared to the top mass and are therefore called "boosted" top quarks.

Reconstruction with the HEPTopTagger

We use the HEPTopTagger (HTT) to reconstruct such boosted top quarks ($P_T>200$ GeV) in the fully hadronic final state.
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Top quark decaying to hadrons. $t\rightarrow b + W(\rightarrow q\bar{q}') = b+q+\bar{q}'$, with $q/q'=u,c,d,s,b$.

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Sketch of a top quark decaying to hadrons, with the top quark at rest (left) or boosted (right).


The HTT uses large jets as inputs, assuming they contain all decay products of the top quark. The substructure of the large jets is then tested for compatibility with a hadronic top decay. We have tested the performance of the HTT with ATLAS data in the lepton+jets final state. Assuming one top quarks decays via $t\rightarrow b + W (\rightarrow \mu\nu)$ we reconstruct events with a muon $\mu$, missing transverse energy (to account for the neutrino) and apply the HTT to the hadronically decaying top quark.
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Top quark candidate mass, reconstructed with the HTT in lepton+jets events. JHEP 06 (2016) 093

The reconstructed top mass (fully hadronic!) and various control distributions are well behaved and agree with the Monte Carlo simulation within uncertainties. Additionally the stability of the HTT with respect to additional proton-proton interactions, referred to as pile-up, has been tested and found to be excellent, by looking at the stability of the reconstructed mass versus the average number of interactions $<\mu>$.
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HTT top quark candidate mass versus average number of interactions $<\mu>$ for events reconstructed with the HTT in the lepton+jets final state. ATLAS-CONF-2013-084

The PI ATLAS group has been leading in using the HTT in ATLAS, e.g. by