  Superstring/M-theory is the theory of quantum gravity that can provide the
UV-completion to viable inflation models. We modify the Starobinsky inflation
model by adding the Bel-Robinson tensor $T^{\mu\nu\lambda\rho}$ squared term
proposed as the leading quantum correction inspired by superstring theory. The
$(R+\frac{1}{6m^2}R^2 -\frac{\beta}{8m^6}T^2)$ model under consideration has
two parameters: the inflaton mass $m$ and the string-inspired positive
parameter $\beta$. We derive the equations of motion in the
Friedmann-Lemaitre-Robertson-Walker universe and investigate its solutions. We
find the physical bounds on the value of the parameter $\beta$ by demanding the
absence of ghosts and consistency of the derived inflationary observables with
the measurements of the cosmic microwave background radiation.
