  We explore the plasma matter content in the innermost accretion disk/jet in
M87* as relevant for an enthusiastic search for the signatures of anti-matter
in the next generation of the Event Horizon Telescope (ngEHT). We model the
impact of non-zero positron-to-electron ratio using different emission models
including a constant electron to magnetic pressure (constant $\beta_e$ model)
with a population of non-thermal electrons as well as a R-beta model populated
with thermal electrons. In the former case, we pick a semi-analytic fit to the
force-free region of a general relativistic magnetohydrodynamic (GRMHD)
simulation, while in the latter case, we analyze the GRMHD simulations
directly. In both cases, positrons are being added at the post-processing
level. We generate polarized images and spectra for some of these models and
find out that at the radio frequencies, both of the linear and the circular
polarizations get enhanced per adding pairs. On the contrary, we show that at
higher frequencies a substantial positron fraction washes out the circular
polarization. We report strong degeneracies between different emission models
and the positron fraction, though our non-thermal models show more
sensitivities to the pair fraction than the thermal models. We conclude that a
large theoretical image library is indeed required to fully understand the
trends probed in this study, and to place them in the context of large set of
parameters which also affect polarimetric images, such as magnetic field
strength, black hole spin, and detailed aspects of the electron temperature and
the distribution function.
