On a set of equivalent theories to special relativity

Following the guidelines drawn from Selleri, we propose an extended approach which does not use the assumption of slowing down of clocks in motion with respect to those in the preferred frame but solely the independence of the two-way speed of light over a closed path in any inertial frame. It results a set of equivalent theories to special theory of relativity depending on two free parameters, namely a synchronization parameter and a scaling factor. Then, in this context, we discuss the general problem on superluminal velocities and causal loop paradoxes connection for different values of the synchronization parameter