is to remove the contribution below $Q_{3,4}=1.3$ GeV entirely. The result for MRST99 partons is shown by the dotted curve in Fig. 4. Even with this extreme choice, the $0^+$ cross section is not changed greatly; it is depleted by about 20%. On the other hand, as anticipated, we see for $0^-$ production, the infrared region is much more important and the cut reduces the cross section by a factor of 5.

Another uncertainty is the choice of factorization scale $\mu$. Note that in comparison with previous calculations [@INC], which were done in the limit of proton transverse momenta, $p_{3,4\perp}\ll Q_\perp$, now we include the explicit $p_\perp$-dependence in the $Q_\perp$-loop integral of (\[eq:rat3\]). Moreover, we resum the ‘soft’ gluon logarithms, $\ln\,1/(1-z)$, in the $T$-factor.[^5] So now the $T$-factor includes both the single soft logarithms and the single collinear logarithms. The only uncertainty is the non-logarithmic NLO contribution. This may be modelled by changing the factorization scale, $\mu$, which fixes the maximal $q_\perp$ of the gluon in the NLO loop correction. As the default we have used $\mu=m_h/2$; that is the largest $q_\perp$ allowed in the process with total energy $m_h$. Choosing a lower scale $\mu=m_h/4$ would enlarge the cross sections 