  The transport of energetic charged particles (e.g., cosmic rays) in turbulent
magnetic fields is usually characterized in terms of the diffusion parallel and
perpendicular to a large-scale (or mean) magnetic field. The nonlinear guiding
center theory (NLGC) has been a prominent perpendicular diffusion theory. A
recent version of this theory, based on random ballistic spreading of magnetic
field lines and a backtracking correction (RBD/BC), has shown good agreement
with test particle simulations for a two-component magnetic turbulence model.
The aim of the present study is to test the generality of the improved theory
by applying it to the noisy reduced magnetohydrodynamic (NRMHD) turbulence
model, determining perpendicular diffusion coefficients that are compared with
those from the field line random walk (FLRW) and unified nonlinear (UNLT)
theories and our test particle simulations. The synthetic NRMHD turbulence
model creates special conditions for energetic particle transport, with no
magnetic fluctuations at higher parallel wavenumbers so there is no resonant
parallel scattering if the particle Larmor radius $R_{\rm L}$ is even slightly
smaller than the minimum resonant scale. This leads to non-monotonic variation
in the parallel mean free path $\lambda_\parallel$ with $R_{\rm L}$. Among the
theories considered, only RBD/BC matches simulations within a factor of two
over the range of parameters considered. This accuracy is obtained even though
the theory depends on $\lambda_\parallel$ and has no explicit dependence on
$R_{\rm L}$. In addition, the UNLT theory often provides accurate results and
even the FLRW limit provides a very simple and reasonable approximation in many
cases.
