Normal distribution

What is the procedure for determining the mean z-score for all points
in a normal distribution that have z-scores larger than a given z-
score? For example, suppose I want to know the mean of all z-scores
that have positive values (i.e., the mean of all z-scores in the right
half of the normal distribution). How do I go about finding this mean?

At some point in the past, I saw a formula that could be used to
answer questions like this. However, I can't remember where I saw the
formula. If someone in the group has the formula at the fingertips,
I'd be most appreciative if you'd post it.

Thanks

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