Constraint on the Mass Scale of a Left-Right-Symmetric Electroweak Theory from the $K_L-K_S$ Mass Difference

The $K_L-K_S$ mass difference provides a stringent constraint on the mass ($M_R$) of the charged right-handed gauge field occurring in a "manifest" left-right-symmetric electroweak theory, yielding $M_R\gtrsim 1.6$ TeV. Taken in the context of a grand-unifying gauge theory, e.g., O(10), such a large bound on $M_R$, along with the measured value of ${{sin}^2\theta }_W$, implies that $M_R\gtrsim {10}^9$ GeV.