Followup: Looking at this more, you are correct about the upper v. lower bound, and given the high energy levels in the paper, one expects it to be so close to the speed of light (whether the mass is real or imaginary) that the SN 1987A data isn’t relevant.
Followup: Looking at this more, you are correct about the upper v. lower bound, and given the high energy levels in the paper, one expects it to be so close to the speed of light (whether the mass is real or imaginary) that the SN 1987A data isn’t relevant.