Neutrino oscillation measurements with KamLAND and JUNO in the presence of scalar NSI

Determination of neutrino mass ordering and precision measurement of neutrino oscillation parameters are the foremost goals of the JUNO experiment. Here, we explore the effects of scalar non-standard interactions (sNSI) on the electron anti-neutrino survival probability measured by JUNO. sNSI appear as corrections to the neutrino mass term in the Hamiltonian. We have considered the simplest scenario where there is only one NSI ($\eta_{ee}$) present in the theory. Our results show that sNSI can have a significant effect on neutrino oscillation probabilities at the medium- and long-baseline reactor experiments. We fit KamLAND data assuming non-zero sNSI in theory and find that {\it estimates of $\Delta m^2_{21}$ and $\theta_{12}$ from KamLAND deviate significantly from their standard best-fit values} if one assumes sNSI in the theory. $\eta_{ee} \in [-1.0, 1.0]$ is allowed by KamLAND. JUNO cannot constrain sNSI but it can robustly measure $\Delta m^2_{21}$ and $\theta_{12}$ even when they differ widely from their current best-fit values. {\it Our work highlights the necessity of global analysis of constraints on sNSI and standard two-flavour oscillation parameters before arduous three-flavour questions such as neutrino mass ordering or CP violation in their presence are attempted.