Impact of extra particles on indirect Z' limits
  We study the possibility of relaxing the indirect limits on extra neutral vector bosons by their interplay with additional new particles. They can be systematically weakened, even below present direct bounds at colliders, by the addition of more vector bosons and/or scalars designed for this purpose. Otherwise, they appear to be robust. 
F. del Aguila, J. de Blas, P. Langacker, M. Perez-Victoria
hep-ph hep-ex