Re: dineutron

[John Denker <jsd@MONMOUTH.COM>]

At 10:16 PM 10/29/00 -0600, Jack Uretsky wrote:
>    Yes, Daddy.   And how come we don't find some sort of a nucleus made
> out of two neutrons? After all neutrons have spin, so two neutrons with
> opposite spin could combine to make a nucleus.  No?

I agree with Jack that

1) Generally:  people went too far when they claimed in this thread that
nuclear structure is "pretty straightforward".

2) More specifically:  You cannot stick two neutrons together to make a
dineutron -- and this is not a "straightforward" consequence of the
hand-wavy nuclear structure models mentioned so far.

=============

A superficial look at deuterium makes the non-existence of dineutrons more
mysterious, but a deeper look is more comforting:  It turns out that all
deuterons have total spin = 1, i.e. the configuration that Jack mentioned
(constituent spins anti-aligned) is unbound for the n+p system as well as
the n+n system.  Here's a table:


                          n+n:                    n+p:

 spins anti-aligned:    unbound                 unbound

 spins aligned:         forbidden by            deuteron
                        exclusion               exists
                        principle


Also:  Although the dineutron is unbound in free space, in other situations
it makes a certain amount of sense to talk about such things.  You can
think of 6He as being 4He plus a dineutron;  in a collision it is easier to
strip the dineutron than to strip the neutrons individually.

=============

These experimental facts suggest that the strong force cannot be modelled
by a scalar potential alone (in analogy with electrostatics);  to explain
the spin dependence we also need some sort of vector potential (roughly
analogous to magnetism).

I'm sure Jack knows 100 times more about this than I do.