Getting started with ttbar resonance

ttbar resonance analysis searches for new physics in the top anti-top final state. top (anti-top) quark decays into a W boson and a b quark in almost 99% cases. The W boson further decays hadronically into two quarks with around 70% probability (branching ratio). In 10% cases the W boson decays into a lepton and a neutrino. In the analysis, electron and muon final state is with considered. Hence the total branching ratio (Br) is around 20% (leptonic decay). Two major ttbar final states are:

  • all-hadronic (50% Br): both the top (hence the W) decays hadronically. Total six quarks in the final state form six jets.

  • semi-leptonic (30% Br): One top quark decays hadronically and the other one leptonically. Total four jets in the final state. Also known as lepton + jet final state.

Feynman diagrams below show the two dominant ttbar process.

all-had Feynman diagram lepton+jets Feynman diagram
Feynman diagram: all-had (left) and lepton+jet (right) final state

 

Lepton + jet final state

Top decay products can be very collimated along the top momentum or can be spread out depending on the top momentum. The two cases give rise to two top decay topology: boosted and resolved.

Note

boost is the Lorentz boost

  • Boosted topology: In boosted topology the top decay products are very close to each other. In general they are so high that the hardronic top decay products come very close (overlaps) to each other creating a large energy deposit on the calorimeter. This gets reconstructed a large radius jet (R=1.0). On the leptonic side also the decay products can come very close to each other creating an overlapping calorimeter signal for an electron and a jet.

  • Resolved topology: One the other hand in resolved topology, the top quark momentum is comparatively less. Hence the decay products do not overlap and are generally get reconstructed individually. So, in the resolved topology, it is expected to see at least four jets: one from leptonic top decay and another three from the hadronic side.

ttbar final states in boosted and resolved regime as shown below:
ttbar_finalstate  

More on Resolved topology

In the resolved topology there will be at least four jets coming from the the four partons (quarks) and a lepton and MET. In reality there are many more jets coming from different radiations. So, almost always it is expected to have more than four jets in an event. As a result it becomes non-trivial to choose four jets coming from top decay. Currently a ChiSq based algorithm is used to reconstruct the top/anti-top systems. This is described in the next section.