I can't give you a specific number because any specific number will do, just as the standard equations for SR are valid for any chosen mass as long as you adjust energy and other values accordingly. You might as well ask for a specific mass for a regular particle.
Rest mass of a photon = 0. Not as hypothetical as the tachyon, doesn't it?
Ok. Let's be explicit: Your statement is wrong.
Really, so massless particles can go faster than the speed of light, while massive particles can accelerate past the speed of light?
Because that's only true for massive particles whose mass is a real number. If the mass is an imaginary number then the particle has to start out faster than c and can't slow down past c. See the issue?
I'm talking about my statement.
Here, if you can actually read.
Relative to all inertial observers, massless particles travel at c. Relative to all inertial observers, massive particles (or any particles with mass) may approach c but never get to or past c.