  One important challenge in our field is to understand the initial condition
of the QGP and constrain it using sensitive experimental observables. Recent
studies show that the Pearson Correlation Coefficient between ${\bm V}_n$ and
event-wise mean transverse momentum $[p_{\mathrm{T}}]$, $\rho_{n}({\bm
V}_n,[p_{\mathrm{T}}])$ can probe number and size of sources, nuclear
deformation, volume fluctuation, and initial momentum anisotropy in initial
state of heavy-ion collisions. This proceeding presents new, precision
measurements of ${\bm V}_n-[p_{\mathrm{T}}]$ correlation in
$^{\mathrm{129}}\mathrm{Xe}+^{\mathrm{129}}\mathrm{Xe}$ and
$^{\mathrm{208}}\mathrm{Pb}+^{\mathrm{208}}\mathrm{Pb}$ collisions for
harmonics $n=2$, 3, and 4 using the ATLAS detector at the LHC. The values of
$\rho_{n}$ shows rich and non-monotonic dependence on centrality, $p_{T}$ and
$\eta$, reflecting that different ingredients of the initial state impact
different regions of the phase space. The ratio of $\rho_{2}$ between the two
systems in the ultra-central region suggests that ${^\mathrm{129}}\mathrm{Xe}$
has large quadrupole deformation and with a significant triaxiality. All
current models fail to describe many of the observed trends in the data.
