Leading-twist parton distribution amplitudes of S-wave heavy-quarkonia

The leading-twist parton distribution amplitudes (PDAs) of ground-state S01 and S13 cc-- and bb--quarkonia are calculated using a symmetry-preserving continuum treatment of the meson bound-state problem which unifies the properties of these heavy-quark systems with those of light-quark bound-states, including QCD's Goldstone modes. Analysing the evolution of S01 and S13 PDAs with current-quark mass, m_q, increasing away from the chiral limit, it is found that in all cases there is a value of m_q for which the PDA matches the asymptotic form appropriate to QCD's conformal limit and hence is insensitive to changes in renormalisation scale, ζ. This mass lies just above that associated with the s-quark. At current-quark masses associated with heavy-quarkonia, on the other hand, the PDAs are piecewise convex-concave-convex. They are much narrower than the asymptotic distribution on a large domain of ζ but nonetheless deviate noticeably from φQQ-(x)=δ(x-1/2), which is the result in the static-quark limit. There are also material differences between S01 and ³S₁ PDAs, and between the PDAs for different vector-meson polarisations, which vanish slowly with increasing ζ. An analysis of moments of the root-mean-square relative-velocity, 〈v2m〉, in ¹S₀ and ³S₁ systems reveals that 〈v⁴〉-contributions may be needed in order to obtain a reliable estimate of matrix elements using such an expansion, especially for processes involving heavy pseudoscalar quarkonia.