The picture shows the result of a collision between an electron and a positron in the Delphi detector. Two muons (red and green) moves to the upper left and upper right from the collision point in the centre. In such a collision the total momentum must be zero. Therefore we know that (at least) one neutrino have escaped the detector going downwards in this figure.

The picture shows the result of the collision of two protons in the Atlas detector. The created Z0 decays in one muon μ- and an anti-muon μ+.