What you mean with  training participants?

I try to investigate how the psychological presence of employees, that are taking part in a job training, can be increased in order to maximize the training output of a company

Lots of studies covering training effectiveness done for the military; e.g., learning while under stress, amount of stress, types of stress, etc.  There is probably at least a few on how to keep groups interested/awake and engaged while sleep deprived.  Airborne school cadre teach using primer words and actions that require a specific response or action from the students; helps to bring a wandering mind back to the situation at hand.  I believe this is a form of "gamifying/gamification" which are terms definitely worth researching for your goal of maximizing training output.