[Jemaclus]

I would not say anything at all. I would keep all interactions with Sarah as professional as possible, including not reciprocating physical contact or jokes. I would also keep a log of any times she engages in physical contact or inappropriate jokes, and document any interaction in which you felt uncomfortable or were accosted by her. Accusing you of sexual harassment is a non-starter, and that should immediately terminate any friendship you may have between each other. Friends don't accuse each other of creating hostile work environments. Sarah might let it slide this time, but next time you say something she deems inappropriate, you might be having a meeting with HR about "repeated patterns of sexism" or something.

Like others have posted, HR is not your friend. Do not voluntarily talk to them unless they have specific questions or concerns to ask you. If they do that, what you want is to have a strong enough paper trail that any complaints Sarah brings against you are properly rebutted.

One thing I would not do right now is apologize. An apology can be seen as a confession. You want to do what you said you did: assert your innocence in the matter, ask for a retraction, and walk away if none is given.

I have other thoughts on this (e.g., start looking for a new job), which I'd be happy to expand upon if you want, but I think the safest thing for you to do right now is just cease any non-professional contact and document the crap out of everything she says or does to use in case of an inquiry.

Good luck.