7 TeV ATLAS event
The single straight track in yellow on the right ends in a shower of energy in the electromagnetic calorimeter: this is a positron. To balance momentum, there must be an unseen partner particle--a ghostly neutrino. Its projected path is shown as a red dashed line. A positive W-boson can decay into a positron and a neutrino--and that is what the ATLAS people tell us this most likely is, a good candidate for a W decay.
Copyright 2010 The ATLAS Experiment at CERN