I Higgs, top production

One small remark, since the main Q was already answered:

Well you can have the formation of single tops too...[formation via weak interaction]
And for pp colliders, you have ~2 times more single tops than antitops...


Also what's the difference in the ttbar decays you posted?

 

The diagram is exactly the same as the one you drew with the lepton final states... Remember that the W boson can decay to leptons+neutrinos, but it can also decay to q+q' (one up-type and one down-type quarks, eg [itex]W^- \rightarrow \bar{c}d[/itex])

 

Your fermion flows are not correct. You should take care in which way the arrows of your fermion lines go. In the standard model, the fermion flow never changes direction.

 

also if you want to be extremely scholastic, writing [itex]q\bar{q}[/itex] on the diagrams can be misleading... the reason they write [itex]\bar{t}t\rightarrow b \bar{b} q\bar{q} l \nu[/itex] is to make it readable (in fact read the quarks as jets)... In the diagram you should make sure you prime the quarks coming from the Ws, in explicitely [itex]W\rightarrow q \bar{q}~'[/itex], because the two quarks are not each others' antiparticle (eg you don't have [itex]W \rightarrow u \bar{u}[/itex]).

 

Ws don't need an arrow...
the arrows for antiparticles [as you plot the diagrams] should look to the left (so for example some W product(s)-find them out- and [itex]\bar{t}\bar{b}[/itex] are wrong).
the arrows for particles to the right.

 

To start with, all of your tbar arrows are in the wrong direction.

 

Arrows don't necessariy show the flow of the interaction. They are there corresponding to some mathematical quantity, as are the Feynman Diagrams... they roughly tell you what kind of Dirac spinors you are using. In some interpretation, the antiparticles move "backwards in time", although that's an interpretation and shouldn't be taken literally in the same way as the Feynman diagrams shouldn't be literally taken as the physical interaction, so in that view it's natural to draw them like that...[in order to have a flow]

well in your plot there are 2 arrows moving out from a "vertex".... [have a look at ttbar]... of course the tops don't come from the same vertex, but still.... the idea is again the one I mentioned, particles -> antiparticles <-.

A decay is pretty simple an interaction that looks like this:
[itex]1 \rightarrow 2+3+...+N[/itex]
An interaction though should have at least 2 particles in the initial state....
Although a decay is an interaction too [eg the particle 1 decays via an interaction to the particles 2,3...N) ...