W Pair Production

At LEP2, the three basic W-pair production diagrams contain Z $\rightarrow$ WW, $\gamma$ $\rightarrow$ WW and t-channel neutrino exchange (which dominates near the W-pair threshold). The issue of directly reconstructing the mass of a heavy boson naturally focusses on these events, since W-pairs have already been produced in their thousands in each of the four LEP detectors. Typical statistics of W pair events are shown in Table 1.

Table 1: Year, e⁺e^- centre-of-mass energy, integrated luminosity per LEP experiment and approximate number of W-pair events per LEP experiment.

Year	$\sqrt{s}$ (GeV)	$\mathcal {L}$(pb-1)/expt	NWW/expt
1996	161	$\sim$ 10	$\sim$ 35
	172	$\sim$ 10	$\sim$ 120
1997	183	$\sim$ 57	$\sim$ 850
1998	189	$\sim$ 175	$\sim$ 2700

Given that BR(W $\rightarrow$ q$\overline{q}$) $\simeq$ 68% and that the remaining decays are leptonic, one sees that the W pair sample subdivides as follows: hadronic channel (WW $\rightarrow$ q$\overline{q}$q$\overline{q}$) $\simeq$ 46%, semileptonic channels (WW $\rightarrow$ l$\nu$q$\overline{q}$) $\simeq$ 44% and the fully leptonic channels (WW $\rightarrow$ l$\nu$l$\nu$) are $\simeq$ 10%, where l denotes e,$\mu$ or $\tau$.