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   매드박™ 
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   [re] Why do pure neutral nuclei NOT exist?

>
>>Hello!
>>
>>I wonder why pure small neutral nuclei (e.g. only 4 neutrons instead
>>of 2 protons and 2 neutrons like in He) seemed not to exist. I know
>>that giant stars can transform in so called "neutron stars", that
>>consist only of neutrons.
>>
>>These kind of single neutral nuclei normally should be more likely and
>>even more stable, because there is no electromagnetical repulsive
>>forces between each other like in normal nuclei. It is clear that
>>single neutrons are not stable and decay within about 10 Min., but
>>bounded neutrons should exist. In standardmodel of particle physics
>>all quarks are equal to the strength of the strong interaction. The
>>strong force do not distinguish between the flavor, so there is no
>>sensible reason why neutral nuclei should not exist.
>>
>>Maybe someone knows the answer...
>>
>>Kind Regards!
>>Dorian Credé
>>
>>
>They decay for the same reason that the single neutron does.
>With strong interaction it wouldn't, but with weak interaction, it does.
>
>
>Arnold Neumaier
>


That's wrong, even if there was no weak and electromagnetic
interactions, there couldn't be bound states of two equal nucleons. The
reason is that the tensor force of the NN-potential is the binding
force, and it's only attractive enough if you are in the right
spin-isospin state, the deuteron state.

The deuteron bound state is S=1, T=0, and it's the only bound state of
the two-nucleon system. Since the spin-isospin part is odd under
particle exchange, the orbital part is also even, because the total
angular momentum is J=1. To be precise: it's a mixing of the orbital
angular momentum l=0 (s-wave) and l=2 (d-wave) states due to the tensor
force, which is crucial for the existence of the deuteron bound state.
The mixing of the d-state is experimentally proven by the fact that the
deuteron has indeed an electrical quadrupol moment.

Two neutrons cannot be in this state, because both have T_z=-1/2 which
cannot build the isospin-singulett state T=0, which is  the
antisymmetric combination of the two-isospin state.

Singulett isospin state: |up down>-|down up>
Triplett state: |up up>, |down down>, |down up+up down>

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