Probing a light scalar boson with a few-MeV proton beam deep underground

We propose to investigate the production of a light scalar boson <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>ϕ</a:mi> </a:mrow> </a:math> in low-energy proton-nucleus interactions using the 3.5 MV accelerator of the Bellotti Ion Beam Facility, located in the underground Gran Sasso National Laboratory. Nuclear reactions induced by a few-MeV proton beam on suitable target materials can act as a controlled source of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>ϕ</c:mi> </c:math> particles. Owing to the deep-underground location, the facility benefits from substantial cosmic-ray shielding, enabling searches for rare processes with minimal background. The produced <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>ϕ</e:mi> </e:math> particles will be sought with large-volume, low-background detectors already operating or currently under construction at the Gran Sasso National Laboratory. This approach combines a tunable accelerator-based production mechanism with the exceptional sensitivity of underground rare-event searches, offering a novel avenue to probe light scalar bosons beyond the Standard Model.