Probing a Light Scalar Boson with a few-MeV Proton Beam Deep Underground

TL;DR

The work proposes a novel, accelerator-based strategy to search for MeV-scale scalar bosons $\phi$ by producing them in low-energy proton–nucleus reactions at the underground LNGS Bellotti-IBF and detecting their flux with large-volume detectors like XENONnT and DarkSide-20k. It develops a nuclear-structure framework using multipole expansions to relate $\phi$ emission to nuclear transitions and internal $e^+e^-$ pair processes, and provides practical flux estimates for a representative setup. Three detection channels—$\phi\!+\!e^-\to e^-\gamma$, $\phi\to\gamma\gamma$, and $\phi\to e^+e^-$—are analyzed, with event-rate formulas in the far-field limit enabling projection of detector sensitivities. The projections indicate that this parasitic, underground approach can explore parameter space regions complementary to astrophysical and beam-dump bounds, and that XENONnT and DarkSide-20k could achieve meaningful sensitivity within a year of data-taking. Future work includes refined nuclear-production modeling and dedicated efficiency/background studies for the detectors, potentially extending the method to other light bosons beyond the scalar case.

Abstract

We propose to investigate the production of a light scalar boson $φ$ 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 $φ$ 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 $φ$ 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.

Figures (3)

• Figure 1: Projected sensitivity of XENONnT (green) and DarkSide-20k (blue), combined with scalar production at the Bellotti-IBF. The projections assume $g_e = m_e / f_\phi$ and correspond to 1 year of data taking with a 3 MeV, 1 mA proton beam on a 3 mm thick TaF$_3$ target. Constraints from LSND (grey), neutron scattering (dark grey), and astrophysics (black) are also shown, while those from the electron $g-2$, SNO and Borexino are subdominant (cf. App. \ref{['sec:constraints']}).
• Figure 2: Projected sensitivity of XENONnT (green) and DarkSide-20k (blue) combined with scalar production at the Bellotti-IBF, for fixed values of the scalar mass. The projections assume a data acquisition time of 1 year with the same beam and target as in Fig. \ref{['fig:prod']}. Constraints from LSND (grey), neutron scattering (dark grey), and the electron $g-2$ (red) are also shown, while SNO and Borexino remain subdominant.
• Figure 3: Projected sensitivity of XENONnT (green) and DarkSide-20k (blue) combined with scalar production at the Bellotti-IBF, for fixed values of the electron coupling. The projections assume a data acquisition time of 1 year with the same beam and target as in Fig. \ref{['fig:prod']}. Constraints from LSND (grey), neutron scattering (dark grey), and astrophysics (black) are also shown, while SNO and Borexino are subdominant.