  The Standard Model (SM) of particle physics is both incredibly successful and
glaringly incomplete. Among the questions left open is the striking imbalance
of matter and antimatter in the observable universe which inspires experiments
to compare the fundamental properties of matter/antimatter conjugates with high
precision. Our experiments deal with direct investigations of the fundamental
properties of protons and antiprotons, performing spectroscopy in advanced
cryogenic Penning-trap systems. For instance, we compared the proton/antiproton
magnetic moments with 1.5 ppb fractional precision, which improved upon
previous best measurements by a factor of >3000. Here we report on a new
comparison of the proton/antiproton charge-to-mass ratios with a fractional
uncertainty of 16ppt. Our result is based on the combination of four
independent long term studies, recorded in a total time span of 1.5 years. We
use different measurement methods and experimental setups incorporating
different systematic effects. The final result,
$-(q/m)_{\mathrm{p}}/(q/m)_{\bar{\mathrm{p}}}$ = $1.000\,000\,000\,003 (16)$,
is consistent with the fundamental charge-parity-time (CPT) reversal
invariance, and improves the precision of our previous best measurement by a
factor of 4.3. The measurement tests the SM at an energy scale of
$1.96\cdot10^{-27}\,$GeV (C$.$L$.$ 0.68), and improves 10 coefficients of the
Standard Model Extension (SME). Our cyclotron-clock-study also constrains
hypothetical interactions mediating violations of the clock weak equivalence
principle (WEP$_\text{cc}$) for antimatter to a level of $|\alpha_{g}-1| < 1.8
\cdot 10^{-7}$, and enables the first differential test of the WEP$_\text{cc}$
using antiprotons \cite{hughes1991constraints}. From this interpretation we
constrain the differential WEP$_\text{cc}$-violating coefficient to
$|\alpha_{g,D}-1|<0.030$.
