Recent years have seen dramatic progress in the determination of multi-hadron observables using lattice QCD (LQCD). Examples include the determination of scattering amplitudes involving multiple two-particle channels (e.g. the scalar-isocalar sector involving two pion and kaon-antikaon channels), and the first study of a nuclear fusion reaction. As the field moves towards more complicated observables it is essential to advance numerical and formal LQCD techniques and to collaborate with experts in effective field theories, dispersive and amplitude analysis, Dyson-Schwinger equations and other few-body methods.

With the aim of reviewing the status of multi-hadron LQCD and strengthening the ties to related subfields, this international workshop brought together 45 physicists, working at institutions in over 10 countries, and including 24 faculty members, 17 postdoctoral fellows and 4 graduate students. The group consisted of 26 lattice practitioners and 19 experts in related non-lattice subfields, with the latter group largely very knowledgeable about the possibilities and aims in multi-hadron LQCD.

Highlights of the conference included:
A structured distinction between more introductory or overview talks, both on theoretical issues and experimental motivation, and more technical, detailed presentations in specific topics.

A program organized into clearly divided sub-themes including two-particle systems in LQCD, dispersive approach to three-body physics, progress of three-body physics in LQCD, multi-baryon systems in LQCD, alternative methods for multi-baryon systems, and electroweak calculations of multi-hadron systems.

A session dedicated to 5-slide talks each designed to highlight a specific result or theoretical idea.

A total of 9 moderated discussion sessions, each about one hour long, led by a moderator who stimulated discussion often with the aim of establishing future goals and bridging the gap between various sub-fields.

A diversity lunch, lead by Raúl Briceño, in which everyone was encouraged to participate and share experiences concerning diversity in physics as it arises in the various countries represented at the workshop.

One of the main benefits of the workshop was the overlap between physicists working within and outside of LQCD. For example there was extensive discussion between those using unitarity and analyticity to constrain three-particle amplitudes and those focusing on methods to extract these amplitudes from finite-volume energies. Other productive discussions concerned the most relevant experimental observables that one should aim to calculate in LQCD, the nature of systematic uncertainties in lattice calculations and techniques for reducing these, and the connection between EFT methods and lattice calculations. Finally there was a productive, albeit technical discussion of the current theoretical approaches for dealing with three-body systems in LQCD.

Some of the main results presented at the workshop included:

• The status and derivation of the two-particle finite-volume formalism used to extract scattering, transition and decay amplitudes from LQCD.
• A review of numerical results for two-body systems including a discussion of future prospects and the role of systematic uncertainties.
• The status of three-body methods both within and outside LQCD, including a serious attempt to compare the existing finite-volume formalisms in the three-particle sector.
• A collection of numerical LQCD results for two-baryon systems including results for
  • meson-baryon scattering and the Delta(1232) and Lambda(1405) resonances
  • baryon-baryon scattering at the SU(3)-flavor-symmetric point
  • light nuclear matrix elements
  • the H dibaryon
  • NN scattering at heavy pion masses
• Updates in progress for electroweak multi-hadron physics including K -> pi pi, neutral kaon mixing, the time-like pion form factor, the B -> K^* transition amplitude and resonance form factors.