I am PhD student in biomedical signal processing area. Before starting my PhD in January 2015, I was researcher in a research group in which I had the opportunity to know one of the best researchers in my area of interest who is also a member of the best group in the world. After starting my PhD I decided to continue in the same area but with a different group. Now I want to get in contact with the researcher, I mentioned above, and ask for collaboration.
What should be the content of my email regarding the researcher's relations with my previous group? Should I explain the about the reason's of changing group or not?