Ambiguity in running spectral index with an extra light field during
  inflation
  At the beginning of inflation there could be extra dynamical scalar fields
that will soon disappear (become static) before the end of inflation. In the
light of multi-field inflation, those extra degrees of freedom may alter the
time-dependence of the original spectrum of the curvature perturbation. It is
possible to remove such fields introducing extra number of e-foldings prior to
$N_e\sim 60$, however such extra e-foldings may make the trans-Planckian
problem worse due to the Lyth bound. We show that such extra scalar fields can
change the running of the spectral index to give correction of $\pm 0.01$
without adding significant contribution to the spectral index. The corrections
to the spectral index (and the amplitude) could be important in considering
global behavior of the corrected spectrum, although they can be neglected in
the estimation of the spectrum and its spectral index at the pivot scale. The
ambiguity in the running of the spectral index, which could be due to such
fields, can be used to nullify tension between BICEP2 and Planck experiments.
