15hydrogen

Hydrogen-15 (symbol 15H or H-15) is a theoretical isotope of hydrogen that would contain one proton and fourteen neutrons. With a mass number of 15, it lies far beyond the known light isotopes of hydrogen and is predicted to be unbound, existing only as a very short-lived resonance if it could form at all.

Most modern nuclear models place hydrogen-15 beyond the neutron drip line, meaning no bound nucleus would result from its formation and any such state would decay essentially immediately, likely by neutron emission. Some models also consider possible decay pathways such as beta decay to a more neutron-rich or proton-rich nucleus, but such routes would be highly suppressed due to energy considerations. To date, hydrogen-15 has not been observed experimentally.

If produced in high-energy collisions or fragmentation experiments, hydrogen-15 would be sought as a transient resonance in the breakup products rather than as a stable nucleus, necessitating rapid detection methods. Its existence is studied to test the limits of nuclear binding and the behavior of extremely neutron-rich systems, providing benchmarks for theoretical approaches such as ab initio methods and shell-model calculations with continuum coupling.