New Constraints on Quasar Evolution: Broad Line Velocity Shifts over
  $1.5\lesssim z\lesssim 7.5 $
  We present the results of a model-independent investigation of the rest-frame
UV spectra from a comprehensive sample of $394$ quasars in the redshift range
$1.5\leq z \leq 7.5$. We fit the main Broad Emission Lines (BELs) in the
rest-frame range $1280 \text{\AA} \leq \lambda \leq 3000 \text{\AA}$ (O I, C
II, Si IV, C III, C IV and Mg II) with a lightly-supervised spline fitting
technique. Redshifts are derived from the peaks of each fitted BEL and used to
compute relative velocity shifts. We show that our method gives unbiased
velocity shifts and is insensitive to spectral resolution and instrumental
parameters. It is found that the average blueshift of the \cfour\, line with
respect to several low-ionisation lines in luminosity-matched samples does not
significantly evolve over $1.5\gtrsim z\gtrsim6$. However, the average
blueshift increases significantly by a factor $\sim 2.5$ at $z\gtrsim 6$. We
propose that this redshift evolution can be explained by \cfour\, winds
launched perpendicularly to an accretion disk with increased torus opacity at
high-redshift, coupled with a potential orientation-driven selection bias. Our
results open new exciting avenues of investigation into young quasars in the
reionisation epoch.
