  One of the main limitations in precision cluster cosmology arises from
systematic errors and uncertainties in estimating cluster masses. Using the
Mock-X pipeline, we produce synthetic X-ray images and derive cluster and
galaxy group X-ray properties for a sample of over 30,000 simulated galaxy
groups and clusters with $M_{\rm 500crit}$ between $10^{12}$ and $2\times
10^{15}$ M$_{\odot}$ in IllustrisTNG. We explore the similarities and
differences between IllustrisTNG predictions of the Sunyaev-Zel'dovich and
X-ray scaling relations with mass. We find a median hydrostatic mass bias $b =
0.125 \pm 0.003$ for $M_{\rm 500crit}$ $>10^{13}$ M$_{\odot}$. The bias
increases to $b = 0.17 \pm 0.004$ when masses are derived from synthetic X-ray
observations. We model how different underlying assumptions about the
dependence of $Y_{\rm X}$ on halo mass can generate biases in the observed
$Y_{\rm SZ} - M_{Y_{\rm X}}$ scaling relation. In particular, the simplifying
assumption that $Y_{\rm X} - M_{\rm tot}$ is self-similar at all mass scales
largely hides the break in $Y_{\rm SZ} - M_{\rm tot}$ and overestimates $Y_{\rm
SZ}$ at galaxy and groups scales. We show that calibrating the $Y_{\rm X}-$mass
proxy using a new model for a smoothly broken power law reproduces the true
underlying $Y_{\rm SZ} - M_{\rm tot}$ scaling relation with high accuracy.
Moreover, $M_{Y_{\rm X}}$ estimates calibrated with this method lead to $Y_{\rm
SZ} - M_{Y_{\rm X}}$ predictions that are not biased by the presence of lower
mass clusters or galaxy groups in the sample. Finally, we show that our
smoothly broken power law model provides a robust way to derive the $Y_{\rm
X}-$mass proxy, significantly reducing the level of mass bias for clusters,
groups, and galaxies.

Cosmology and Nongalactic Astrophysics;Astrophysics of Galaxies