Neutral Current Four-Fermion Production

In addition to the charged current four-fermion processes already discussed, there is another class of four-fermion final state events called the neutral current (NC) processes. Unlike the charged current processes they do not contain a W boson, so rather than a final state of ${\rm q}\bar{\rm q}\ell\bar{\nu}_{\ell}$, these events contain two quarks and two charged leptons, ${\rm q}\bar{\rm q}\ell^{+}\ell^{-}$. With a final state such as this, if one of the charged leptons is not detected by the OPAL detector, the event will appear to have a final state of ${\rm q}\bar{\rm q}\ell\bar{\nu}_{\ell}$.

There are 24 neutral current processes which can produce a final state with either electrons, muons or taus in it. These are called the NC24 diagrams. The diagrams for these processes are shown in figure 5.1.

Figure 5.1: NC24 diagrams.

There are a further 24 neutral current processes that can only produce a final state with the charged leptons being an electron and a positron. The diagrams representing these events are shown in figure 5.2. When combined with the NC24 events, this group is called the NC48 set.

Figure 5.2: Along with the NC24 diagrams, these make up the NC48 diagrams.