How to change legend label in "theme" argument in ggplot2?

I am using ggplot2 ro draw error bar plot. For the legend, I want it like this:

1. inside the plot. I know function theme(legend.position) can do this.

2. use latex symbol in the legend labels with function expression.

I have read several references of constructing the legend, but still have not found what I want exactly. I can perform either 1 or 2, but can not do both in my plot.

Following is R code of the plot with the problem. Since my reputation is not enough to post an image, please copy the code into your R to see the plot. I only want to keep the label contains \beta_A and \beta_B, but not the label contains group 1 and group 2.

Any ideas? Thank you!

library(ggplot2); library(scales) #for alpha
varx <- rep(c("group1", "group2"), each = 3)
vary <- rep(c("A", "B", "C"), 2)
poi <- sample(20:30, 6)
upper <- sample(40:50, 6)
lower <- sample(1:10, 6)

dat <- data.frame(varx, vary, poi, upper, lower)
dat
  #      varx vary poi upper lower
  # 1 group1    A  29    42    10
  # 2 group1    B  21    48     9
  # 3 group1    C  26    47     8
  # 4 group2    A  30    44     4
  # 5 group2    B  27    49     6
  # 6 group2    C  24    43     7

pp <- ggplot(dat, aes(colour = varx, y = poi, x = vary)) 

limits <- aes(ymax = upper, ymin = lower)

pp  + geom_point(aes(shape=varx), position = position_dodge(0.3), size = 2) + 
  ## dodge make the lines not stack upon each other
  geom_errorbar(limits, size = 1, width = 0.15, position = position_dodge(0.3)) + 
  theme_bw() + ## Get rid of the grey background
  geom_hline(yintercept = 0, linetype = "dotdash") +
  coord_cartesian(ylim = c(1, 60)) + 
  scale_shape_manual(values = c(17, 19, 18)) + 
  scale_colour_hue(labels =  c(expression(beta[A]), expression(beta[N]))) + 
  theme(plot.margin = unit(rep(1.5, 4),"mm"),
        legend.justification= c(1, 0), 
        legend.position = c(1, 0.5),
        legend.key = element_blank(),## Get rid of the legend box 
        legend.title = element_blank(),
        legend.text = element_text(size = 10, face = "bold"),
        legend.background = element_rect(fill=alpha(0.0001))) +
  labs(x = NULL, y = NULL)