Constraining the radio-loud fraction of quasars at z>5.5
  Radio-loud Active Galactic Nuclei at z~2-4 are typically located in dense
environments and their host galaxies are among the most massive systems at
those redshifts, providing key insights for galaxy evolution. Finding
radio-loud quasars at the highest accessible redshifts (z~6) is important to
study their properties and environments at even earlier cosmic time. They would
also serve as background sources for radio surveys intended to study the
intergalactic medium beyond the epoch of reionization in HI 21 cm absorption.
Currently, only five radio-loud ($R=f_{\nu,5{\rm GHz}}/f_{\nu,4400\AA}>10$)
quasars are known at z~6. In this paper we search for 5.5 < z < 7.2 quasars by
cross-matching the optical Pan-STARRS1 and radio FIRST surveys. The radio
information allows identification of quasars missed by typical color-based
selections. While we find no good 6.4 < z <7.2 quasar candidates at the
sensitivities of these surveys, we discover two new radio-loud quasars at z~6.
Furthermore, we identify two additional z~6 radio-loud quasars which were not
previously known to be radio-loud, nearly doubling the current z~6 sample. We
show the importance of having infrared photometry for z>5.5 quasars to robustly
classify them as radio-quiet or radio-loud. Based on this, we reclassify the
quasar J0203+0012 (z=5.72), previously considered radio-loud, to be
radio-quiet. Using the available data in the literature, we constrain the
radio-loud fraction of quasars at z~6, using the Kaplan--Meier estimator, to be
$8.1^{+5.0}_{-3.2}\%$. This result is consistent with there being no evolution
of the radio-loud fraction with redshift, in contrast to what has been
suggested by some studies at lower redshifts.
