Why did finding W and Z bosons require 270 GeV beams when their mass is only 80–90 GeV?

[WarnK]

I read on http://cern-discoveries.web.cern.ch/cern-discoveries/Courier/HeavyLight/Heavylight.html that to find the W and Z bosons they used beams with 270 GeV of energy per beam. But the W and Z bosons have a mass of about 80/90 GeV, how come so high-energy beams was needed?

 

[dynamicsolo]

Are these bosons the only products of the collisions between particles in the beams?