Instantons in the Large $Q^2$, $x$ Region at HERA

A new mechanism for an increased event production in the large $Q^2$, $x$ region at HERA is suggested. This mechanism results from a new type of instanton-induced quark-quark interaction, which is related to non-zero quark modes in the instanton field. We estimate the contribution of this interaction to the valence quark structure function $F_2^v$ using the gas instanton model approximation for the QCD vacuum. It is shown that this interaction can give a large contribution to $F_2^v$, especially at large values of $Q^2$. The strong dependence of this contribution on the quark masses and $Q^2$ is discussed, and it is concluded that a large charm quark contribution to events at high $Q^2$ can result from charm pair creation in the instanton field. We also obtain a sizeable contribution to the valence quark longitudinal structure function $F_L^v$ at high $Q^2$. Experimental observables which can check this instanton mechanism in the high $Q^2$ region at HERA are suggested.