In AtiyahBott's paper on YangMills equations on Riemann surfaces, a special case of what they do is to prove that Unitary YangMills connections over a R.S $M$ are in bijective correspondence with unitary representations of a central extension of the fundamental group. To this end, given a representation, they construct a unitary bundle as follows (and therein lies my confusion): Take a degree one $U(1)$ bundle $Q$. Now, pull this bundle back to the universal cover $\tilde{M}$. The new bundle is a $U(1) \times \pi_1(M)$ bundle over $M$. Now apparently, one can "lift the representation of the central extension and thus form a unitary bundle" (with a connection). Why can one do this?
