LAUSR

We demonstrate that the currently running short baseline reactor experiments, especially Daya Bay, can put a significant upper bound on $\Delta m^2_{21}$. This novel approach to determining $\Delta m^2_{21}$ can be performed with the current data of both Daya Bay \& RENO and provides additional information on $\Delta m^2_{21}$ in a different $L/E$ range ($\sim$ 0.5 km/MeV) for an important consistency check on the 3 flavor massive neutrino paradigm. Upper limits by Daya Bay and RENO and a possible lower limit from Daya Bay, before the end of 2020, will be the only new information on this important quantity until the medium baseline reactor experiment, JUNO, gives a very precise measurement in the middle of the next decade. In this study $\theta_{12}$ value is fixed since its impact on the $\Delta m^2_{21}$ measurement is relatively small as discussed in the Appendix.