  A search is performed for delayed and nonpointing photons originating from
the displaced decay of a neutral long-lived particle (LLP). The analysis uses
the full Run 2 data set of proton-proton collisions delivered by the LHC at a
center-of-mass energy of $\sqrt{s}=13$ TeV between 2015 and 2018 and recorded
by the ATLAS detector, corresponding to an integrated luminosity of 139
fb$^{-1}$. The capabilities of the ATLAS electromagnetic calorimeter are
exploited to precisely measure the arrival times and trajectories of photons.
The results are interpreted in a scenario where the LLPs are pair-produced in
exotic decays of the 125 GeV Higgs boson, and each LLP subsequently decays into
a photon and a particle that escapes direct detection, giving rise to missing
transverse momentum. No significant excess is observed above the expectation
due to Standard Model background processes. The results are used to set upper
limits on the branching ratio of the exotic decay of the Higgs boson. A
model-independent limit is also set on the production of photons with large
values of displacement and time delay.
