# Is neutron-neutron fusion viable?

Do I get unusual baryon maybe like pentaquark or just a pair of conjoined twin that is very unstable?

• Well you would need to produce the neutrons for fusion first... secondly you would need to somehow contain and move the neutrons, which is difficult since they do not have a charge so the electric force is out of the question. I would say it is not viable. – Viktor Dec 19 '15 at 13:37
• Two neutrons don't posses bound state, so one of those need to undergo "isospin flip" - that is n + n -> d + e + v. It is not forbidden from energy conservation point of view (you get an ~1/2 MeV from n -> p + e + v conversion and extra 2Mev from p+n->d) but I really didn't ever work this out, so I don't know is it possible. – Stipe Galić Dec 19 '15 at 17:20
• But in any case, I expect much lower rate than, let say, n+p->d because n+n reaction needs to go through weak channel. – Stipe Galić Dec 19 '15 at 17:29

Under isospin symmetry, the dineutron should be a "mirror nucleus" with the diproton and the spin-zero deuteron. Neither of those are bound (the deuteron has spin $\hbar$, and no stable excited states), and so there's no stable dineutron to fuse into.
Stipe Galic points out the possiblity of the weak interaction process $$\rm n + n \to d + e^- + \bar\nu$$ as the isospin analogue to the proton-proton reaction in the core of the Sun, $$\rm p + p \to d + e^+ + \nu$$ The core of the Sun is dense hydrogen under enormous pressure with a power density of about $100\rm\,W/m^3$; I'll let you work out for yourself the (in)feasibility of observing neutron-neutron fusion under terrestrial conditions.