Remarks on diquarks, strong binding, and a large hidden QCD scale

We present arguments regarding the possible role of diquarks in low-energy hadron phenomenology that has so far escaped theorists' attention. Good diquarks, i.e. the 0⁺ states of two quarks, are argued to have a two-component structure with one of the components peaking at distances several times shorter than a typical hadron size (a short-range core). This can play a role in solving two old puzzles of the 't Hooft 1/N expansion: strong quark-mass dependence of vacuum energy density and strong violations of the Okubo-Zweig-Iizuka (OZI) rule in quark-antiquark 0^± channels. In both cases empiric data defy 't Hooft's 1/N suppression. If good diquarks play a role at an intermediate energy scale they ruin 't Hooft's planarity because of their mixed-flavor composition. This new scale associated with good diquarks may be related to a numerically large scale discovered in [V. A. Novikov, M. A. Shifman, A. I. Vainshtein, and V. I. Zakharov, Nucl. Phys. B 191, 301 (1981)] in a number of phenomena mostly related to vacuum quantum numbers and 0^± glueball channels. If SU(3)_color of bona fide QCD is replaced by SU(2)_color, diquarks become well-defined gauge-invariant objects. Moreover, there is an exact symmetry relating them to pions. In this limit predictions regarding good diquarks are ironclad. If passage from SU(2)_color to SU(3)_color does not lead to dramatic disturbances, these predictions remain qualitatively valid in bona fide QCD.