  During the 2015-2018 data-taking period, the Large Hadron Collider delivered
proton-proton bunch crossings at a centre-of-mass energy of 13 TeV to the ATLAS
experiment at a rate of roughly 30 MHz, where each bunch crossing contained an
average of 34 independent inelastic proton-proton collisions. The ATLAS trigger
system selected roughly 1 kHz of these bunch crossings to be recorded to disk.
Offline algorithms then identify one of the recorded collisions as the
collision of interest for subsequent data analysis, and the remaining
collisions are referred to as pile-up. Pile-up collisions represent a
trigger-unbiased dataset, which is evaluated to have an integrated luminosity
of 1.33 pb$^{-1}$ in 2015-2018. This is small compared with the normal
trigger-based ATLAS dataset, but when combined with vertex-by-vertex jet
reconstruction it provides up to 50 times more dijet events than the
conventional single-jet-trigger-based approach, and does so without adding any
additional cost or requirements on the trigger system, readout, or storage. The
pile-up dataset is validated through comparisons with a special
trigger-unbiased dataset recorded by ATLAS, and its utility is demonstrated
through a measurement of the jet energy resolution in dijet events, where the
statistical uncertainty is significantly reduced for jet transverse momenta
below 65 GeV.
