Abstract
Bringing an external radioactive source close to a large underground detector can significantly advance sensitivity not only to sterile neutrinos but also to “dark” gauge bosons and scalars. Here we address in detail the sensitivity reach of the Borexino-SOX configuration, which will see a powerful (a few PBq) 144Ce–144Pr source installed next to the Borexino detector, to light scalar particles coupled to the SM fermions. The mass reach of this configuration is limited by the energy release in the radioactive γ-cascade, which in this particular case is 2.2 MeV. Within that reach one year of operations will achieve an unprecedented sensitivity to coupling constants of such scalars, reaching down to g∼10−7 levels and probing significant parts of parameter space not excluded by either beam dump constraints or astrophysical bounds. Should the current proton charge radius discrepancy be caused by the exchange of a MeV-mass scalar, then the simplest models will be decisively probed in this setup. We also update the beam dump constraints on light scalars and vectors, and in particular rule out dark photons with masses below 1 MeV, and couplings ϵ≥10−5.
Highlights
Search for light weakly coupled states undergoes a revival in recent years [1]
We will not analyze constraints related to UV completion, concentrating instead only on the low-energy physics induced by (1)
The MeV-range masses suggested by the rp anomaly make nuclear physics tools preferable
Summary
Search for light weakly coupled states undergoes a revival in recent years [1]. There has been increased interest in models that operate with light sterile neutrinos, axion-like particles, dark photons, and dark scalars that can be searched for in a variety of particle physics experiments. We significantly expand the sensitivity reach by taking into account the decays of light particles inside the Borexino detector. Taking into account the decay and the Compton absorption of the scalars inside the detector we arrive at the expected counting rate, and derive the sensitivity to coupling constants within the mass reach of this setup
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