For the 2026 edition of the EJC, the school will develop its lecture program around the theme “Theory to Shape Experiments.” The development of scientific theories lies at the heart of our understanding of physical phenomena. It relies on the construction of models capable of interpreting existing observations, guiding future experiments, and providing a robust predictive framework. In this sense, the aim is to capture physical reality through abstract theoretical concepts, whose degree of sophistication depends on the level of accuracy required. This effort relates to theoretical formulations based on the notion of effectiveness and emergence: rather than attempting to describe reality in all its complexity, one selects the relevant degrees of freedom that are expected to govern the phenomenon at the scale of interest. The choice of these degrees of freedom is therefore central to theory building, as it determines both the structure of the model and the limits of its applicability.
At the same time, the development of physical theories requires a clear statement of their underlying assumptions and of their domain of validity. In practice, the many available models are constantly tested against experiment. This ongoing process calls for close interactions between theory, numerical simulation, and experimental data. In recent years, nuclear physics has entered an era of precision, which will undoubtedly challenge some of the foundations of existing theories and models in the field.
Regardless of the participants’ background—experimentalists or theorists, early-career researchers or senior scientists—the need to understand the mathematical models used to predict, or more often postdict, experimental results is becoming increasingly pressing, especially in a context of high experimental precision and growing attention to the quantification of theoretical uncertainties.
Moreover, it is no longer uncommon for different models or theoretical frameworks to be compared directly. Such comparisons, however, can be misleading or even hazardous: one approach or another may be underdetermined by the available data; conceptual breaks between methods may generate problems of incommensurability; and the intrinsic limitations of each framework may give rise to various forms of inconsistency. These issues make it all the more important to clarify the assumptions, scope, and limits of the theories used in modern nuclear physics.
These questions resonate with broader epistemological discussions, such as those addressed in the ESNT program “Sous-détermination, incomplétude, incommensurabilité : la pensée des limites,” which highlights how underdetermination, conceptual discontinuities between theories, and internal inconsistencies shape the way scientific models must be assessed and compared.
On the first day of the event, we will host a colloquium in a related, though distinct, area of physics. While not explicitly intended as an epistemological reflection, the talk should, through the example of the scientific method applied in another field, encourage participants to reflect on the role and purpose of a physical theory. In this way, the opening lecture will invite the audience to consider some of the broader questions that motivate the school and that are shared across many areas of the natural sciences.
The program will then offer lectures on several major theoretical approaches closely connected to experimental investigations:
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Few-body reactions, with applications to nuclear astrophysics and to our understanding of the Universe;
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Many-body methods, aimed at predicting nuclear spectroscopy, deformation, pairing, and correlations;
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Nuclear Density Functional Theory, for nuclear structure and dynamics, with applications ranging from fission to superheavy nuclei.
Participants will have the opportunity to learn about the mathematical foundations of these theoretical methods, as well as the assumptions on which they rely and the scope of their validity, before examining how they confront and interpret state-of-the-art experimental data.
In the afternoon, we plan to organize hands-on sessions based on small codes and simplified formalisms, allowing participants to explore in practice how physical models are constructed and used in nuclear physics. These sessions are intended to provide a concrete understanding of the modeling strategies that have been developed in the field over many decades.