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

Chapter 1: Introduction and Theoretical Overview 22
at the LHC can help constrain them. We briefly describe the other improvements
that will be needed to reduce the overall theoretical error to the level of a percent.
1.5.1 Sources of theoretical errors
QCD corrections: QCD calculations to NLO accuracy have been available for
a long time. The event generator MC@NLO [84], for example, combines NLO QCD
matrix elements with the HERWIG [46] parton shower algorithm. The fixed-order
matrix element calculation gives a good description of processes where the partons are
energetic and widely separated, while parton showers provide an excellent description
of processes dominated by soft and collinear gluon emission. A tool that combines
both types of calculation is therefore very useful.
MC@NLO calculates W/Z boson production cross-sections including γ − Z in-
terference as well as spin correlations among leptonic decay products. It can be
interfaced with a particular detector simulation. For example, it has been interfaced
with the ATLAS simulation via the Athena computing framework.
Recently, NNLO QCD calculations have become available for some processes.
The program FEWZ [67] calculates W/Z production cross-sections through NNLO
including finite-width effects, γ − Z interference and spin correlations. The program
ResBos [19] resums soft and collinear initial-state gluon emission to all orders, giving
an accurate description of processes in the low-pT regime. It can also (optionally)
include initial-state NLO corrections. Both of these programs use VEGAS [72] for
evaluating phase-space integrals.
The authors of [75] recently studied the theoretical uncertainties in W/Z cross-