I have been searching for information on how to compute (or at least partially compute) the propagator of a string given an initial and a final loop in bosonic string theory. I have found several pioneering papers (Alvarez seems to be the first big name), but it appears the pioneers hit several problems (integrating out the parametrizations of initial and final loops for one).
I can't find any more recent papers, however, or even review articles presenting results in a more readable way. As a result I am not sure I have the full picture or even if I correctly understand what I read so far.
Can anybody help me find a good paper explaining the state of the art? Or just explain the state of the art here.