group theory (Lie groups in particular), a bit of representation theory, a bit on calculus of variation, to complex analysis and contour integration (from field theory)
Can you tell that in general which discipline is needed for which part of the theory so that I can have a more flexible approach. For example I've heard that 'strong interaction' can be explained with the help of Beta Function.
depending on what kind of research you do, you pretty much need everything that you have learned in the (under)graduate courses: from solving partial differential equations, linear analysis, topology, algebra, asymptotics, integral transforms, group theory, real and complex analysis, statistical mechanics, vector analysis...etc.