  The rejection of forward jets originating from additional proton--proton
interactions (pile-up) is crucial for a variety of physics analyses at the LHC,
including Standard Model measurements and searches for physics beyond the
Standard Model. The identification of such jets is challenging due to the lack
of track and vertex information in the pseudorapidity range $|\eta|>2.5$. This
paper presents a novel strategy for forward pile-up jet tagging that exploits
jet shapes and topological jet correlations in pile-up interactions.
Measurements of the per-jet tagging efficiency are presented using a data set
of 3.2 $fb^{-1}$ of proton--proton collisions at a centre-of-mass energy of 13
TeV collected with the ATLAS detector. The fraction of pile-up jets rejected in
the range $2.5<|\eta|<4.5$ is estimated in simulated events with an average of
22 interactions per bunch-crossing. It increases with jet transverse momentum
and, for jets with transverse momentum between 20 and 50 GeV, it ranges between
49% and 67% with an efficiency of 85% for selecting hard-scatter jets. A case
study is performed in Higgs boson production via the vector-boson fusion
process, showing that these techniques mitigate the background growth due to
additional proton--proton interactions, thus enhancing the reach for such
signatures.
