The claim that neutrons and protons experience identical gravitational acceleration follows directly from the universality of free fall and the weak equivalence principle, which are foundational principles of modern physics that do not require specific citation.
The claim addresses the fundamental equivalence principle of gravity acting equally on all matter (neutrons and protons), which is a bedrock principle of general relativity and standard physics education. None of the retrieved papers specifically test the differential gravitational acceleration of neutrons versus protons directly; instead, they cover unrelated topics like particle detectors, cosmology, rotating stars, and general equivalence tests for neutrons or fundamental constants. Because this is a well-established foundational tenet of physics rather than a disputed empirical hypothesis, it is classified as common knowledge.