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

Chapter 1: Introduction and Theoretical Overview 16
conserve energy or flavor. Momentum rescaling is therefore necessary at the end of
the hadronization process.
The Lund string model: The color field lines between a quark-antiquark pair can
be thought of as a tube, or a string in one dimension, with a constant string tension
k ≈ 1 GeV/fm. As the partons move apart, the tension in the string increases until
there is enough energy to produce a q¯q pair. Consequently, two color-singlet q¯q pairs
form that can fragment again, and so on until only on-shell hadrons remain. In this
model, baryons are generated by diquark-antiquark production.
The Lund model is intuitively appealing and physically motivated. The Pythia [92]
Monte Carlo program uses it for hadronization.
The cluster model: This model works by following the color structure of a par-
ton shower such that, at the end of the showering process, color-singlet clusters are
formed. The model splits all gluons into quark-antiquark and diquark-antidiquark
pairs, forming clusters with predominantly small masses. The light clusters decay to
hadrons, while the heavier clusters are split into lighter ones.
The advantage of the cluster model is that the properties of the hadrons are
determined by the properties of the parton shower. However, it runs into problems
with baryon production. The HERWIG [46] Monte Carlo program makes use of this
model.
1.4 Z boson production at the LHC
The main Z boson production mechanism at the LHC is the Drell-Yan process
q¯q → Z → µ + µ − , as illustrated in Figure 1.4. This process may or may not involve