The R plotmath expression system can be somewhat cumbersome to use. If you wish, you can directly include Latex in plots using the latex2exp package. For example:

library(latex2exp)
library(tidyverse)
ggplot(mtcars, aes(mpg, hp)) +
geom_point() +
annotate(geom='text', label=TeX("$\\frac{\\sigma}{\\mu}exp(-\\pi \\phi)$"),
y=300, x=22, size=5) +
theme_bw()
#> Warning in is.na(x): is.na() applied to non-(list or vector) of type
#> 'expression'

^{Created on 2021-05-07 by the reprex package (v1.0.0)}