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

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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.

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  • $\begingroup$ 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 '19 at 12:58
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    $\begingroup$ @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 '19 at 14:20
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    $\begingroup$ FWIW, in the solar core, the probability of a diproton transmuting to a deuteron (rather than just splitting into a pair of protons) is around $10^{-26}$. $\endgroup$ – PM 2Ring Mar 15 at 6:18
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Since this has been kicked up by community , I found this link:

The neutron–neutron fusion process, n n → d e ν , at very low neutron energies is studied in the framework of pionless effective field theory that incorporates dibaryon fields. The cross section and electron energy spectrum for this process are calculated up to next-to-leading order. We include the radiative corrections of O ( α ) calculated for the one-body transition amplitude. The precision of our theoretical estimates is found to be governed essentially by the accuracy with which the empirical values of the neutron–neutron scattering length and effective range are currently known. Also discussed is the precision of theoretical estimates of the transition rates of related electroweak processes in few-nucleon systems.

Here is a reference to an experimental program to study $nn$ scattering, which data would be necessary to use in the theoretical study above.

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  • $\begingroup$ Here's a free version of that paper: arxiv.org/abs/nucl-th/0507048 Note that "experimental observation of this reaction does not seem to belong to the near future". OTOH, the electrons emitted when nn decays to a deuteron have a higher peak energy than those emitted in neutron decay, so it's relatively easy to detect. $\endgroup$ – PM 2Ring Mar 15 at 6:11

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