If I were ChongChong,

I would still go to HR. But instead of asking HR to get involved directly, I'll ask HR what is the company's policy on situation like this, if similar cases happened before and what were the outcome. And since there is no evidence, I don't want HR to take any action, but I want advice on what I could do to prevent it from happening again. I would also ask HR to keep it confidential, so neither my boss nor the co-worker would know about it. The first time is more a consulting session. The goal is to seek advice and building a personal relationship with the HR.

I will keep going to HR if it happens again. Sooner or later, with the repeated pattern, HR will take it more seriously ......