Instantons, Spin Crisis and High $Q^2$ Anomaly at HERA

Abstract

The contribution of the nonperturbative quark-gluon interaction induced by the instantons to $g_1^p(x,Q^2)$ and $F_2^p(x,Q^2)$ structure functions is estimated. It is shown that nontrivial $Q^2$ dependence of the instanton contribution to $g_1^p(x,Q^2)$ allows us to explain the observed decreasing of the part of the proton spin carried by quarks without involving a large positive gluon polarization. It is demonstrated that the anomalous enhancement of the instanton contribution to $F_2^p(x,Q^2)$ structure function, due to multiple emission of the gluons from instanton vertex, can be one of the reasons for the excess of the DIS events at HERA at high $Q^2 $ and $x$.