  We calculate the long-distance matrix elements for the decays of the Upsilon
(eta_b) and chi_b (h_b) states in lattice QCD with two flavors of light
dynamical quarks. We relate the lattice matrix elements to their continuum
counterparts through one-loop order in perturbation theory. In the case of the
leading S-wave matrix element, we compare our result with a phenomenological
value that we extract from the experimental leptonic decay rate by using the
theoretical expression for the decay rate, accurate through relative order
alpha_s. Whereas estimates of the leading S-wave matrix element from quenched
QCD are 40--45% lower than the phenomenological value, the two-flavor estimate
of the same matrix element is close to the phenomenological value.
Extrapolating to the real world of 2+1 light flavors, we find that this matrix
element is approximately 6% higher than the phenomenological value, but that
the phenomenological value lies within our error bars. We also compute the
color-singlet and color-octet matrix elements for P-wave decays. We find the
value of the color-singlet matrix element for 2+1 flavors to be approximately
70% larger than the quenched value and the value of the color-octet matrix
element for 2+1 flavors to be approximately 40% larger than the quenched value.

High Energy Physics - Lattice;High Energy Physics - Phenomenology