  In the last few years we have seen an increase interest on gravitational
waves due to recent and striking experimental results confirming Einstein's
general relativity once more. From the field theory point of view, gravity
describes the propagation of self-interacting massless spin-2 particles. They
can be identified with metric perturbations about a given background metric.
Since the metric is a symmetric tensor, the massless spin-2 particles present
in the Einstein-Hilbert (massless Fierz-Pauli) theory are naturally described
by a symmetric rank-2 tensor. However, this is not the only possible consistent
massless spin-2 theory at linearized level. In particular, if we add a mass
term, a new one parameter $(a_1)$ family of models ${\cal L}(a_1)$ shows up.
They consistently describe massive spin-2 particles about Einstein spaces in
terms of a non-symmetric rank-2 tensor. Here we investigate the massless
version of ${\cal L}(a_1)$ in a curved background. In the case $a_1=-1/12$ we
show that the massless spin-2 particles consistently propagate, at linearized
level, in maximally symmetric spaces. A similar result is obtained otherwise
$(a_1 \ne -1/12)$ where we have a non-symmetric scalar-tensor massless model.
The case of partially massless non-symmetric models is also investigated.

High Energy Physics - Theory