Title: Massive Fields as Systematics for Single Field Inflation,
Abstract: During inflation, massive fields can contribute to the power spectrum of
curvature perturbation via a dimension-5 operator. This contribution can be
considered as a bias for the program of using $n_s$ and $r$ to select inflation
models. Even the dimension-5 operator is suppressed by $\Lambda = M_p$, there
is still a significant shift on the $n_s$-$r$ diagram if the massive fields
have $m\sim H$. On the other hand, if the heavy degree of freedom appears only
at the same energy scale as the suppression scale of the dimension-5 operator,
then significant shift on the $n_s$-$r$ diagram takes place at $m=\Lambda \sim
70H$, which is around the inflationary time-translation symmetry breaking
scale. Hence, the systematics from massive fields pose a greater challenge for
future high precision experiments for inflationary model selection. This result
can be thought of as the impact of UV sensitivity to inflationary observables.