1. A computer-implemented method for determining if a user of a search engine is conducting a research mission, comprising: generating a plurality of features related to a series of queries submitted by the user to the search engine; based on a first subset of the plurality of features, detecting if the user is performing complex research using a first machine learned detector by measuring user's level of engagement in a search; and based on a second subset of the plurality of features, detecting if the user is performing a mission related to a particular information need using a second machine learned detector by measuring topical coherency between consecutive queries; determining if the user is conducting a research mission based on at least the complex research detection and the mission detection.