The leading-twi
st parton di
stribution amplitude
s (PDA
s) of ground-
state <
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span> and <
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span>- and <
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span>-quarkonia are calculated u
sing a
symmetry-pre
serving continuum treatment of the me
son bound-
state problem which unifie
s the propertie
s of the
se heavy-quark
sy
stem
s with tho
se of light-quark bound-
state
s, including QCD'
s Gold
stone mode
s. Analy
sing the evolution of <
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span> and <
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span> PDA
s with current-quark ma
ss, <
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span>, increa
sing away from the chiral limit, it i
s found that in all ca
se
s there i
s a value of <
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span> for which the PDA matche
s the a
symptotic form appropriate to QCD'
s conformal limit and hence i
s in
sen
sitive to change
s in renormali
sation
scale,
ζ. Thi
s ma
ss lie
s ju
st above that a
ssociated with the
s-quark. At current-quark ma
sse
s a
ssociated with heavy-quarkonia, on the other hand, the PDA
s are piecewi
se convex&nda
sh;concave&nda
sh;convex. They are much narrower than the a
symptotic di
stribution on a large domain of
ζ ; but nonethele
ss deviate noticeably from <
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span>, which i
s the re
sult in the
static-quark limit. There are al
so material difference
s between <
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span> and <
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span> PDA
s, and between the PDA
s for different vector-me
son polari
sation
s, which vani
sh
slowly with increa
sing
ζ . An analy
si
s of moment
s of the root-mean-
square relative-velocity, <
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ssn=03702693&md5=111dcad601dc315fa21cc69cf6a0e481" title="Click to view the MathML
source">〈v<
sup>2m
sup>〉
span><
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span>, in <
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span> and <
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span>
sy
stem
s reveal
s that <
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ssn=03702693&md5=0ddd6ca29d3eae2185773a7ee561c4ef" title="Click to view the MathML
source">〈v<
sup>4
sup>〉
span><
span cla
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span>
span>
span>-contribution
s may be needed in order to obtain a reliable e
stimate of matrix element
s u
sing
such an expan
sion, e
specially for proce
sse
s involving heavy p
seudo
scalar quarkonia.