It’d obliterate what ever particle(s) were unfortunate enough to be in the way. They’d turn in to a shower of other particles, like sticking your head in a particle acelerator, but all at once.

It’s highly unlikely all of the energy would go in to you, so it’d amount to a narrow beam of ionizing radiation. Potentially deadly, potentially not. A guy did get his head in a particle acelerator beam when he thought it was turned off. Saw a really bright flash and all. It kinda’ messed up that narrow channel of flesh and he has some cignitive issues as a result.