Diquarks: A QCD sum rule perspective

We propose a phenomenological QCD sum rule with an explicit diquark field to investigate the essential ingredients inside the hadrons. By introducing the mass m_φ and the condensate φ2 for the diquark field as parameters in the model, we find that the sum rule works well for Λ, Λ_c, and Λ_b. This implies that these Λ baryons can be represented by a diquark and a quark configuration. We also find that there is a duality relation among the parameters (m_φ, φ2) for which the sum rule is equally good. In the limit when φ=0, we find m_φ=400 MeV, which can be thought of as the constituent diquark mass.