Within transforming commands (eval, stats, timechart etc) and WHERE command, use the field within single quote
For other places (table, fields, search) put it within double quotes.
Have you tried the rename command?
... | rename "xyz (rpm)" as xyz_rpm | ...
I know you probably didn't create the field name this way, but FYI:
Field names that are made from the following character set will never require quotation marks and will be properly understood by all Splunk commands:
Upper case letters (A-Z)
Lower case letters (a-z)
Field names should begin with a letter.
Have you tried single quotes?
Another option is to use either a field alias (in props.conf or in Splunk Web on the Search Head via Settings -> Fields -> Field alias), or a
rename in your search before doing stats, eval, etc.