The Collaborative International Dictionary of English v.0.48:

Researcher \Re*search"er\ (-[~e]r), n.
   One who conducts research. In the field of scientific
   research, also called an investigator or scientist.
   [1913 Webster]

WordNet (r) 3.0 (2006):

researcher
    n 1: a scientist who devotes himself to doing research [syn:
         research worker, researcher, investigator]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "researcher":
   analyst, analyzer, assayer, delver, digger, essayer, experimenter,
   hunter, perquisitor, ransacker, research worker, researchist,
   rummager, searcher, seeker, test driver, test pilot, tester,
   tryer-out, zetetic