Measurement of the Z boson cross-section in - Harvard University ...

Chapter 1: Introduction and Theoretical Overview 32
Table 1.1 summarizes the error estimate from each source as well as the overall
errors expected in the cross-section and acceptance. As expected from the above
discussion, the dominant component of the PDF uncertainty comes from gluon dis-
tributions.
Source of uncertainty cross-section acceptance
uncertainty (%) uncertainty (%)
Total QCD uncertainty, including 1.57 2.55
missing NNLO corrections and scale
dependence
Uncertainty due to missing NLO EW 0.38 0.96
corrections
PDF uncertainty 3.52 1.85
Overall uncertainty 3.85 3.29
Table 1.1: Summary of theoretical uncertainties, derived in [75]. Note that the
authors of [75] also quote errors on the uncertainties from QCD and EW sources.
These derive mainly from the slow convergence of MC integration programs, which
limits the precision of the results. They term it ‘technical error’, which we do not
show in our table.
Hence, using current tools, it is possible to reach a precision of ≈4% on the Z
boson cross-section calculation and ≈3% on acceptance estimation. We will come
back to these numbers in Chapters 6 and 9 in the context of our analysis. To be
able to improve these errors to the level of 1%, developments on several fronts will be
needed:
• an event generator that combines NNLO QCD corrections with complete NLO
EW corrections
• improved PDF constraints, especially for the gluon PDF, from new data
• PDF fits that include NNLO QCD and NLO EW corrections