  Although the $\Lambda$ Cold Dark Matter model is the most accredited
cosmological model, information at high redshifts ($z$) between type Ia
supernovae ($z=2.26$) and the Cosmic Microwave Background ($z=1100$) is crucial
to validate this model further. To this end, we have discovered a sample of
1132 quasars up to $z=7.54$ exhibiting a reduced intrinsic dispersion of the
relation between ultraviolet and X-ray fluxes, $\delta_\mathrm{F}=0.22$ vs.
$\delta_\mathrm{F}=0.29$ ($24\%$ less), than the original sample. This gold
sample, once we correct the luminosities for selection biases and redshift
evolution, enables us to determine the matter density parameter $\Omega_M$ with
a precision of 0.09. Unprecedentedly, this quasar sample is the only one that,
as a standalone cosmological probe, yields such tight constraints on $\Omega_M$
while being drawn from the same parent population of the initial sample.
