Wow, a loop? That would be impressive, what are you defining as a loop? (not a call out, I've just heard it defined differently) I have seen video and heard that 53's, the CH- variety flown by the Navy and Marines and also the Pave Lows can do what basically mimics an aileron roll. The only helos I know of today that can do a "loop" (think over the vertical axis rather than rolling horizontally) as I know it are the Apache and the Lynx flown by the British military I believe. ( I couldn't tell you which branches, someone in the know please jump in if I am mistaken )
Apache doing a loop:
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Lynx doing some acrobatics (roll and a loop)
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