IS the $ttH$ an actual particle? If so, how can it form? I though the life of the top quark was way to short to form a particle.
If it is not a particle, what is meant by coupling.
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In this entry one finds the Feynman diagrams of the triple vertex :
The experimental measurements confirm the existence of these diagramsm and thus the ttH coupling .
The results of statistically independent searches for Higgs bosons produced in conjunction with a top quark-antiquark pair and decaying to pairs of W bosons, Z bosons, photons, τ leptons, or bottom quark jets are combined to maximize sensitivity. An excess of events is observed, with a significance of 5.2 standard deviations, over the expectation from the background-only hypothesis.
So it is not a particle, but a triple vertex that is referred, and the coupling is the value of the strength of the Htt vertex in the Feynman diagram. ( there are many vertices in these diagrams with different couplings gtH, gtt, ggg and when one goes to caluclating the proton protn full crossection to ttH, many more enter the calculation).
In the standard model, the Higgs decay probability grows with the mass of the particle in the pair it could decay into. That is why there is such a large b-bbar channel . The top mass is larger than the Higgs mass so it cannot decay directly, and the coupling to top can only be measured indirectly in the manner above. The measurement confirms the behavior expected from the standard model.
"Coupling" in this sense just means an interaction. The Higgs can interact with top quarks on account of their mass, and a $ttH$ interaction is of particular interest because the large top quark mass means that the interaction is quite strong.
In some cases, $ttH$ might refer to a specific interaction in which the Higgs and a top anti-top pair are the final products.