I'm not certain there is an actual paper yet; I suspect from some of the phrasings that it was merely an announcement that their program succeeded, but a formal paper will take a while.

I did find this reddit post by Dr. Mann [1] where he says:

> We were just in the process of debugging and optimizing the code when our new example was found. Because we are in the early stages of the computational experiments, we were surprised to find this example so quickly. We are hopeful of finding more new examples as we proceed.

[1] https://www.reddit.com/r/math/comments/3fe347/15th_pentagon/...