What is meant by the term gauge singlet ?

[arroy_0205]

What is meant by the term "gauge singlet"?

Can anybody please explain what is meant by the term "gauge singlet"?
To be more specific, I got the term in a discussion on wess-zumino lagrangian where the superpotential contains the term:$$a_i \phi_i$$. The author claims that in order the theory to be gauge invariant and invariant under susy, the fields present in this term must be gauge singlets. I do not understand this at all. (The fields mentioned are left handed chiral superfields.)

 

[ansgar]

Singlet means that something is inaffacted by a certain symmetry transformation.

Examples:

The neutrino is a U(1) - electric charge singlet.

The electron is a SU(3) - color singlet

The Higgs boson is a Lorentz: SO(3,1) - singlet (scalar particle)

 

[blechman]

In other worlds, "neutral"; zero charge.