A stringent constraint on the fractional change of proton g-factor

We report a constraint on the cosmological variation of the proton g-factor, $g_p$. By comparing the measured redshifts between \mbox{H\,{\sc i}} 21 cm and OH 18 cm lines observed with the newly commissioned Five-hundred-meter Aperture Spherical radio Telescope (FAST) toward PKS 1413+135 at $z$ = 0.24671, we obtain $\Delta g_{p}/g_{p} = (-4.3\pm2.5)\times10^{-5}$, which is more than two orders of magnitude more sensitive than previous constraints. In addition, we obtain sensitive constraints of $\Delta (\mu\alpha^{2})/(\mu\alpha^{2}) = (2.0\pm1.2)\times10^{-5}$ and $\Delta (\mu\alpha^{2}g_{p}^{0.64})/(\mu\alpha^{2}g_{p}^{0.64}) = (-4.7\pm1.9)\times10^{-6}$.