ar X iv : h ep - p h / 96 11 38 8 v 2 7 M ar 1 99 7 Next - to - leading nonperturbative calculation in heavy quarkonium

The next-to-leading nonperturbative contributions to heavy quarkonium systems are calculated. The applicability of the Voloshin-Leutwyler approach to heavy quarko-nia systems for the physical cases of Bottomonium and Charmonium is investigated. We study whether the background gluon field correlation time can be considered to be infinity or not, by calculating the leading correction to this assumption and checking whether the expansion is under control. A phenomenological analysis of our results is also performed. The results make us feel optimistic about the Υ(1S) and to a lesser extent about the J/ψ but do not about higher levels. We also briefly discuss the connection with different models where a finite gluon correlation time is introduced.