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

  • $\begingroup$ 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. $\endgroup$ – Viktor Dec 19 '15 at 13:37
  • $\begingroup$ 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. $\endgroup$ – Stipe Galić Dec 19 '15 at 17:20
  • $\begingroup$ 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. $\endgroup$ – 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.

High-energy neutron-neutron collisions will excite the baryon-meson spectrum in the same way as high-energy proton-proton collisions, but it's hard to make high-energy free neutrons and there aren't pure neutron targets.

  • $\begingroup$ you are not stressing that the neutron is unstable with a weak decay lifetime of the order of 15 minutes, cannot be used as a target except statistically withing neutron rich nuclei $\endgroup$ – anna v Dec 29 '15 at 17:59
  • $\begingroup$ @annav Fair enough; but even if the neutron were stable I don't think it'd be possible to observe n-n fusion. $\endgroup$ – rob Dec 29 '15 at 18:06
  • $\begingroup$ The p+p reaction needs the high power density because it has to overcome the Coloumb-Potential. This isn't the case with n+n. $\endgroup$ – yippy_yay Mar 2 at 12:58
  • $\begingroup$ @yippy_yay You've misread the answer. The Sun's matter density is high, but its power density is low. For comparison, I metabolize food into heat at about 100 W, and I have a volume much less than a cubic meter. Removing Coulomb repulsion would speed up fusion in purely strongly-interacting systems, like d-t fusion or the triple-alpha process. But p-p fusion, and the unrealistic n-n fusion described here, have to wait for the weak interaction. $\endgroup$ – rob Mar 2 at 14:20

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