Question: what is a hypothetical string theory weightless particle

Susan Cartwright answered on 16 Jun 2015:

Whatever you want it to be, since it’s hypothetical!

It may be that you are trying to ask about superstring theory, according to which all particles are made up of vibrating loops of string (in 10 dimensions, just to add a bit more surrealism). If this is the case, then all particles are vibrating strings, and the massless ones are the photon and the gluon.

String theory is very complicated and I can’t do it justice in a short answer. I suggest you take a look at Brian Greene’s “The Elegant Universe”, which talks a lot about string theory.