To get the full cross section the gluon cross section has to be integrated with the structure functions of the gluons
|(pp H ) = dx1dx2x1g(x1,mH2)x2g(x2,mH2)(gg H ).||(22)|
The value of the cross section including the K-factor has two main
uncertainties. The first is from the gluon structure function which
still has a large uncertainty in the low x region. The cross section
using a large set of todays best available structure functions was
calculated in  and their relative differences shown in
fig. 2.3. It can be seen that they differ by
around 20% which can be taken as the theoretical uncertainty from the
gluon structure function. At the time of data taking for the LHC it can be
expected to have much better structure functions available with data
from HERA, the Tevatron and even the LHC itself.
The second uncertainty in the gluon fusion cross section is from corrections above the next-to-leading order. The cross section changes with the renormalisation scale as an effect of un-calculated higher order effects. By changing between mH/2 and 2mH it is in  concluded that the remaining uncertainties from higher order effects is below 15%.
The uncertainty in the cross section arising from uncertainties in the top quark mass are small and will be insignificant with an improved measurement of the top mass at the starting time of the LHC.
The production of the Higgs through gluon fusion is sensitive to a
fourth generation of quarks. Because the Higgs couples in
proportion to the fermion mass, a heavier generation of quarks is not
suppressed in the process fig. 2.1a as would be
expected for a loop process with a heavier particle in the loop.
Including a fourth generation of very heavy quarks will more than
double the cross section as shown in fig. 2.4. This has
the consequence that the Higgs cross section is sensitive to a fourth
generation of quarks even if the quarks are too heavy for a direct
discovery at the LHC. The mass range is limited by the scale of new
physics where the standard model breaks down.