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Radio-loud Active Galactic Nuclei at z~2-4 are typically located in denseenvironments and their host galaxies are among the most massive systems atthose redshifts, providing key insights for galaxy evolution. Findingradio-loud quasars at the highest accessible redshifts (z~6) is important tostudy their properties and environments at even earlier cosmic time. They wouldalso serve as background sources for radio surveys intended to study theintergalactic 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 bycross-matching the optical Pan-STARRS1 and radio FIRST surveys. The radioinformation allows identification of quasars missed by typical color-basedselections. While we find no good 6.4 < z <7.2 quasar candidates at thesensitivities 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 notpreviously known to be radio-loud, nearly doubling the current z~6 sample. Weshow the importance of having infrared photometry for z>5.5 quasars to robustlyclassify them as radio-quiet or radio-loud. Based on this, we reclassify thequasar J0203+0012 (z=5.72), previously considered radio-loud, to beradio-quiet. Using the available data in the literature, we constrain theradio-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 evolutionof the radio-loud fraction with redshift, in contrast to what has beensuggested by some studies at lower redshifts.

Constraining the radio-loud fraction of quasars at z>5.5