Accessing User class in Smart Search lucene syntax

I would like to weight my search results within the "Search condition" setting of a smart search web part. I have had success referencing page types using lucene syntax like this: +(classname:cms.MenuItem)^2

How do I do the same to refer to the User class within the membership module? I tried the following, but the class was not recognized: +(classname:cms.user)^2

Any ideas?