I read the total equation for the standard model is something like this https://www.symmetrymagazine.org/sites/default/files/images/standard/sml.png
It's quite complicated. My understanding is that the fields representing each particle can be modeled separately in a fairly straightforward* way (to give an incomplete / non physical theory).
Is this correct, and can a model of a simpler universe containing just one type of particle be described in this way? If so, which particle in the Standard Model would be the simplest? I realise this could be a subjective question but I am hoping it may not be. Please assume no existing knowledge of physics (only maths) and no real-world experience, just assess the simplicity from a mathematical perspective.
You don't have to give a justification (unless it is contentious) or the mathematical form of the field, unless you would like to. I hope to address those kind of topics in future.
*Mathematically but not conceptually! It can be argued whether interaction terms are intrinsic or extrinsic to an individual field. For this question I am excluding all interaction terms that involve another field. To be clear, that includes even phenomena like self-interactions mediated by virtual particles of other fields.