Braking Protons at the Electron-Ion Collider: From Invisible Meson Decay to New Physics Searches

We investigate the sensitivity of the Electron-Ion Collider (EIC) to invisible final states in coherent exclusive electroproduction. The characteristic signal is a forward proton with reduced energy and little additional detector activity. Using the excellent particle detection capabilities and kinematics reconstruction at the EIC, we argue that backgrounds can be strongly suppressed. While our analysis applies to various states, we specifically focus on vector and pseudoscalar particles: (i) neutral mesons [P=π^{0},η^{(')},V=ρ^{0},ω,ϕ], whose invisible standard model decays are extremely suppressed, and (ii) gluon-coupled axionlike particles (ALPs) decaying invisibly to a dark sector. With the baseline detector and a conservative residual-background estimate, the EIC can improve the sensitivity to invisible η, η^{'}, ω, and ϕ decays by factors of about 4-40 and probe invisible ρ^{0} decays for the first time. With further background reduction and optimized detector performance, the projected sensitivity may reach branching fractions as small as 4×10^{-11}. In addition, under the same conditions, the EIC would directly probe invisibly decaying ALPs with the couplings up to f_{a}∼10^{5}  GeV and masses in the range 0.1-2 GeV.