Mono-jet searches for simplified models

[Luca_Mantani]

Hi,
i am studying Simplified Models for the search of Dark Matter at colliders. In particular there are 2 different kind of models that i am studying that are s-channel and t-channel models. Now, there is something that i don't understand related to the mono-jet process.
Here is a paper that shows some of these models: https://arxiv.org/pdf/1603.08002v2.pdf
s-channel models are models in which the new mediator is exchanged in the s-channel when dealing with Dark Matter annihilation, while in t-channel models is exchanged in the t-channel during this process.

Since we are not able to detect DM, we have to look at processes that involves jets in the output.
If you look at page 22 of the paper i linked, there are some diagrams of these kind of processes for the s-channel. All of them involve loops and i don't understand why. Why a process like the ones for the t-channel in page 28 cannot happen? I read that there are difference in the two models in mono-jet production, but i don't understand why there cannot be tree-level scattering in the s-channel one.

Thank you for the help,
Luca

 

[mfb]

I don't know about those specific models, but often the coupling of dark matter to top quarks is stronger - so strong that those loops are more important than tree-level processes involving light quarks. The tree-level top quark process is shown on page 22, but it needs a higher parton energy. Unless the dark matter particle behaves really weird, the same process for light quarks is possible.

 

[Luca_Mantani]

I don't know about those specific models, but often the coupling of dark matter to top quarks is stronger - so strong that those loops are more important than tree-level processes involving light quarks. The tree-level top quark process is shown on page 22, but it needs a higher parton energy. Unless the dark matter particle behaves really weird, the same process for light quarks is possible.

Why isn't that valid even for the t-channel models? So, you are saying that maybe s-channel models are strongly coupled to the top, while the t-channel are not, so we have a substantial difference in phenomenology between the 2 models?

 

[mfb]

Where would you add top loops in figure 5? The interaction there is always quark with two new particles. On page 22 SM particles produce a single new particle.

 

[Luca_Mantani]

Where would you add top loops in figure 5? The interaction there is always quark with two new particles. On page 22 SM particles produce a single new particle.

No, sure.
Maybe now i got it, we can't use top quarks in the initial state because their parton distribution is too low in the proton and the process would be negligible. So in the first model we got that the interaction is proportional to yukawa couplings, while in the second one not. As a consequence in the first one the diagrams with top loops are very important with respect to the tree level with light quarks. On the other hand, the other can only interact with light quarks. Is that right?

Do you know something about Minimal Flavour Violation? Because they use this principle to set the coupling costants proportional to yukawa couplings in the first model, while in the second one they use the same principle, but this time they set all the coupling constants for each generation of quarks equal. I don't understand why is that.

 

[mfb]

I'm from the experimental side, we have theorists here who know that better.