  We discuss a mechanism of particle production during inflation that can
result in a blue gravitational wave (GW) spectrum, compatible with the BICEP2
result and with the r < 0.11 limit on the tensor-to-scalar ratio at the Planck
pivot scale. The mechanism is based on the production of vector quanta from a
rolling pseudo-scalar field. Both the vector and the pseudo-scalar are only
gravitationally coupled to the inflaton, to keep the production of inflaton
quanta at an unobservable level (the overproduction of non-gaussian scalar
perturbations is a generic difficulty for mechanisms that aim to generate a
visible GW signal from particle production during inflation). This mechanism
can produce a detectable amount of GWs for any inflationary energy scale. The
produced GWs are chiral and non-gaussian; both these aspects can be tested with
large-scale polarization data (starting from Planck). We study how to
reconstruct the pseudo-scalar potential from the GW spectrum.
