Intrinsic Charm at High - Q 2 and HERA Data

We compute the predictions of intrinsic charm for deep-inelastic scattering at high-Q 2 and compare to HERA data. With the inclusion of constraints from low-energy data, enhancements beyond the predictions of standard structure functions are very modest, but peaking in the leptoquark mass variable is present near 200 GeV. Ultimately, the ability of HERA to probe the intrinsic charm hypothesis could be very substantial. HERA deep-inelastic scattering data with significant statistics at high Q 2 and high x is now becoming available [1, 2]. The cross section exhibits an excess beyond the predictions based on standard QCD distribution functions. However, it is possible that unexpected contributions to the quark distribution functions might explain at least part of this excess [3]. In this Letter, we pursue the implications for the HERA data of an intrinsic charm (IC) component of the proton [4]. We find that intrinsic charm naturally predicts a peak in the leptoquark mass distribution in the vicinity of 200 GeV, but that existing constraints on intrinsic charm limit the size of the effect. As a function of leptoquark mass, the enhancement from intrinsic charm relative to standard model (SM) predictions is at most of order 15% in the e + p neutral current (NC) case but can be as large as 75% in e + p charged current (CC) scattering. Expectations for e − p NC and CC scattering are also given. The possibility of an intrinsic charm component in the proton bound state has substantial theoretical and phenomenological motivation. However, a conclusive observation or severe limit has remained elusive. In the intrinsic charm model, there is a |uudc¯ c component of the proton in which the c and ¯ c have become completely integrated members of the bound state, travelling coherently with the light quarks. In the minimal model, the |uudc¯ c wave function is taken to be proportional to the inverse of the light-cone energy difference m 2 p − (m 2 i /x i) ∝ x c x ¯ c /(x c + x ¯ c) where the x i are the light-cone momentum fractions and the light quark and proton masses are neglected compared to the charm quark mass. After squaring, the intrinsic charm Fock state probability distribution takes the form