A representation of the forward Compton amplitude in which the $A_2$ meson breaks scale invariance is shown to be consistent with existing data for the difference between the proton and neutron structure functions $\nu W_2^p-\nu W_2^n$, while ensuring a finite proton-neutron mass difference $\Delta M$. The conjecture that $W_L\equiv W_1+\left(\frac{{\nu }^2}{q^2}\right)W_2\to 0$ as $\nu \to \infty$ for fixed $q^2$ leads to an expression for $\Delta M$ in terms of measurable quantities.

• Received 10 September 1971

DOI:https://doi.org/10.1103/PhysRevD.5.75