What is the experiment in which participants do not know if they are in the experimental or the control group but the experimenters do know which participants are part of which group?

The experiment you are referring to is called a single-blind experiment. In this design, participants are unaware of their group assignments—whether they are receiving the treatment (experimental group) or a placebo (control group)—to prevent bias in their responses. However, the experimenters are aware of which participants are in which group, allowing them to administer the treatment and collect data accordingly. This setup helps to mitigate participant expectations and improve the validity of the results.