https://doi.org/10.1140/epja/s10050-026-01850-3
Regular Article –Theoretical Physics
Light Element Primary Process (LEPP): from discovery to modern understanding – A Tribute to Roberto Gallino
INAF, Astrophysical Observatory Turin, Strada Osservatorio 20, Pino Torinese, 10025, Turin, Italy
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Received:
5
February
2026
Accepted:
31
March
2026
Published online:
4
May
2026
Abstract
This contribution is dedicated to Roberto Gallino and to the longstanding collaboration I had with him, which began with my Diploma thesis. After a small section of due and well deserved tribute to his life and scientific legacy, I will devote this paper to one of the most important pieces of work we carried out together: the conception of the Lighter Element Primary Process (LEPP) LEPP was introduced to explain the persistent mismatch between observed and predicted abundances of light neutron capture elements – particularly Sr, Y, and Zr–in metal poor stars. Classical s and r process nucleosynthesis, as implemented in Galactic chemical evolution models, could not account for the relatively high and nearly constant [Sr,Y,Zr/Fe] ratios found in the early Galaxy. LEPP was therefore proposed as an additional, primary mechanism capable of operating in low metallicity environments and producing light trans-iron nuclei independently of pre-existing seed distributions. Over the past 2 decades, the concept has evolved from a single, hypothetical process into a broader framework encompassing multiple astrophysical channels. High resolution spectroscopic surveys have further revealed significant star to star scatter in light to heavy neutron capture ratios, reinforcing the need for diverse production pathways. Modern interpretations view LEPP not as a monolithic process but as a phenomenological label capturing all primary mechanisms responsible for light n capture enrichment in the early Universe. This reconceptualization aligns observational constraints, nucleosynthesis modeling, and Galactic chemical evolution, while preserving the historical role of LEPP as a diagnostic tool for identifying missing physics.
Communicated by M. Borge.
© The Author(s) 2026
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