I'm a graduate mathematics student and I did my undergrad in applied math. I also took the normal 10 hrs of physics foundations and then a semester of modern physics (basic quantum intro, special relativity, orbit states etc.). I was thinking about pursuing study in areas that would be closely related to mathematical physics and was looking for suggestions of topics that would line up well, particularly in higher abstract algebra topics. I'm not sure which areas are most closely related on the physics side, whether HEP or what, but am still interested in physics even though my proper course of study headed in a different direction. I am planning to go into a PhD program after finishing my masters next year.