We study the primordial scalar and tensor perturbations in inflation scenario involving a spectator dilaton field .
In our setup , the rolling spectator dilaton causes a tachyonic instability of gauge fields , leading to a copious production of gauge fields in the superhorizon regime , which generates additional scalar and tensor perturbations through gravitational interactions .
Our prime concern is the possibility to enhance the tensor-to-scalar ratio r relative to the standard result , while satisfying the observational constraints .
To this end , we allow the dilaton field to be stabilized before the end of inflation , but after the CMB scales exit the horizon .
We show that for the inflaton slow roll parameter \epsilon \gtrsim 10 ^ { -3 } , the tensor-to-scalar ratio in our setup can be enhanced only by a factor of { \cal O } ( 1 ) compared to the standard result .
On the other hand , for smaller \epsilon corresponding to a lower inflation energy scale , a much bigger enhancement can be achieved , so that our setup can give rise to an observably large r \gtrsim 10 ^ { -2 } even when \epsilon \ll 10 ^ { -3 } .
The tensor perturbation sourced by the spectator dilaton can have a strong scale dependence , and is generically red-tilted .
We also discuss a specific model to realize our scenario , and identify the parameter region giving an observably large r for relatively low inflation energy scales .