Plagiarism between a Physical Review Letters paper and a previous arXiv preprint
(too long for a comment)

I would contact the authors of the suspected plagiarism and demand an explanation in the politest possible way. Try not to imply that you're certain your paper was plagiarized, but ask instead if, by citing your paper, whether or not they are aware that your conclusions are so similar. I would also add that you have sent the paper to the same journal, which being a PRL should be rather obvious due to the length limitations.

If you are sure their submission date to PRL comes after you submitted your pre-print to ArXiv, and that they were also aware of it, then you have proof that you were the first to make those results public. I believe if the authors do not respond well to your email you have good chances of contacting the PRL editorial board and, using your pre-print as proof, claim that you were plagiarized.

The best case scenario is to be able to solve your problems with the authors of the other paper, but to be honest I'm not sure most human beings would respond well to this situation. Maybe you'll have to get in touch with the board. If I were you, I would definitely not leave things as be, and would insist until the matter was solved (even if legal measures are needed).