by about 30%.

Next there is some uncertainty in the gluon distribution itself. To illustrate this, we compare predictions obtained using CTEQ6M [@CTEQ], MRST99 [@MRST99] and MRST02 [@MRST02] partons. For $0^+$ production at the LHC, with $m_h=120$ GeV and $\mu=m_h/2$, we find that the effective gluon–gluon luminosity, before screening, is $$\left.\frac{d{\cal L}}{dyd\ln M^2}\right|_{y=0} = (2.2,\ 1.7,\ 1.45)\times10^{-2}$$ respectively. This spread of values arises because the CTEQ gluon is 7% higher, and the MRST02 gluon 4% lower, than the default MRST99 gluon, in the relevant kinematic region. The sensitivity to the gluon arises because the central exclusive diffractive cross section is proportional to the 4th power of the gluon. For $0^-$ production, the corresponding numbers are $(4.2,\ 2.7,\ 1.7)\times10^{-5}$. Up to now, we have discussed the effective gluon–gluon luminosity. However, NNLO corrections may occur in the $gg\to h$ fusion vertex. These give an extra uncertainty of $\pm20\%$. Note that we have already accounted for the NLO corrections for this vertex [@INC].

Finally, we need to consider the uncertainty in the evaluation of the soft rescattering correction factor $S^2$, which is the probability that the rapidity gaps survive the soft $pp$ interaction. The computation of $S^2$ is discussed in some detail in 