New proton emitter (188)At implies an interaction unprecedented in heavy nuclei.

We report the discovery of a new atomic nucleus (188)At, which is the heaviest proton-emitting isotope known to date. The new activity was observed through the (107)Ag((84)Sr, 3n)(188)At fusion-evaporation reaction using the focal-plane spectrometer of the gas-filled recoil separator in the Accelerator Laboratory of the University of Jyväskylä, Finland. To fully interpret the experimental data, we have expanded the non-adiabatic quasiparticle model to treat nuclei in the beyond-lead region. The description reproduced the measured decay rate and pointed towards emission from an extremely prolate-deformed state with a dominant s(1/2) proton component in the wave function. The Thomas-Ehrman shift can be enhanced in low angular momentum states, but such effects have not been observed in heavy nuclei. The single-proton separation energy of (188)At deviates from that extrapolated from the systematics, which can be interpreted as the first evidence of this effect in heavy nuclei.