  The quest to understand the fundamental building blocks of nature and their
interactions is one of the oldest and most ambitious of human scientific
endeavors. CERN's Large Hadron Collider (LHC) represents a huge step forward in
this quest. The discovery of the Higgs boson, the observation of exceedingly
rare decays of $B$ mesons, and stringent constraints on many viable theories of
physics beyond the Standard Model (SM) demonstrate the great scientific value
of the LHC physics program. The next phase of this global scientific project
will be the High-Luminosity LHC (HL-LHC) which will collect data starting circa
2029 and continue through the 2030s. The primary science goal is to search for
physics beyond the SM and, should it be discovered, to study its implications.
In the HL-LHC era, the ATLAS and CMS experiments will record around 100 times
as many collisions as were used to discover the Higgs boson (and at twice the
energy). Both NSF and DOE are making large detector upgrade investments so the
HL-LHC can operate in this high-rate environment. Similar investment in
software R&D for acquiring, managing, processing and analyzing HL-LHC data is
critical to maximize the return-on-investment in the upgraded accelerator and
detectors. This report presents a strategic plan for a possible second 5-year
funded phase (2023 through 2028) for the Institute for Research and Innovation
in Software for High Energy Physics (IRIS-HEP) which will close remaining
software and computing gaps to deliver HL-LHC science.
