Is it possible to get a PhD position with only a Bachelor's degree?
It's certainly possible -- in fact, in the USA it's more common to start a PhD directly after a Bachelor's than after a Master's. In Europe, having a Master's first is a more typical requirement, but I have known some people to start PhDs straight after their Bachelor's (in physics, in the UK). Having significant research experience and published papers will certainly help your application a lot.

Why not apply for PhD and Master's positions at the same time? That way, you may get accepted to a PhD, in which case great, if not then you will likely have the Master's to fall back on.