The INT workshop in "Nuclear reactions from lattice QCD" will bring together researchers in the nuclear reaction, effective field theories, and lattice QCD communities to address a common goal: "How can one determine nuclear reaction cross sections directly from the underlying theory of QCD?" Until recently, this question was out of reach of our computational capabilities, but with ever-growing computational resources, it is not unrealistic to assume scattering processes involving light nuclei will soon be accessible through LQCD calculations. Despite extensive progress in LQCD calculations involving few-body bound and elastic scattering states, no calculations of nuclear reaction cross sections above the inelastic threshold have been performed. It remains a big challenge to extend previous methods and/or develop new methods to be able to study channels with energies above inelastic threshold where more particles are involved in the process.

The benefits of a finite-volume nuclear reactions formalism are directly applicable to diverse environments that range from the cosmos, such as BBN, stars, and supernovae, to terrestrial facilities such as nuclear reactors and high-energy density (HED) facilities like the national ignition facility (NIF) and international thermonuclear experimental reactor (ITER). In addition to being the main source of energy production in these environments, nuclear reactions are essential for producing key elements in astrophysical environments and providing diagnostics in terrestrial facilities. For example, in the former the reaction ⁴He + ⁴He → ⁸Be + γ is an important step in the life-enabling triple-alpha process, while in the latter the reaction that produces an excited state of ⁵He, ²H + ³H → ⁵He^*, serves as a direct probe of fusion within inertial confinement fusion experiments. Both reactions are poorly known empirically, but a finite-volume nuclear reactions formalism applied to LQCD calculations will allow us to eliminate gaps in our knowledge of these inelastic processes. Such a formalism is particularly attractive since such calculations, being truly ab initio, would include all NN and NNN (and higher) interactions in a consistent manner.

As the needed computational resources to perform lattice QCD calculations of multi-nuclei systems are under way, the cross-section of these reactions can be obtained once the required formalism is developed. This formalism has to build the bridge between the calculated lattice QCD observables and the elements of physical S-matrix for these reactions. Also it is necessary to identify how to best constrain the EFT low energy coefficients, most importantly the three-nucleon force parameter, from LQCD calculations of multi-nuclei systems. These coefficients can be used subsequently in ab-initio calculations of heavier nuclei reactions leading to improved uncertainties in these calculations.

This two-days workshop aims to gather theorists in this field to set the goals, investigate challenges and come up with a plan to be able to study nuclear processes beyond the inelastic threshold. Physicists from the nuclear reaction community will report on the important nuclear reactions and will set the long-term goals of the field. Physicists with expertise in EFT of few nucleon systems are able to exchange ideas with physicists from other areas and set short term goals in order to be able to start moving forward in this field. This is going to be a small workshop with a limited number of participants.