Multi-statelevel detection
You're looking for cluster detection. In your case, it's very basic: there's no more than one dimension along which to detect clusters.
It's probably overkill, but 1-dimensional k-means is probably what you want.
Matlab has good documentation on k-means. (Also, seriously, all this is wikipedia or textbook knowledge, there's very many free resources. You very rarely start reading research papers when you enter a new field.)