Tetraneutrons
Franz Heymann
physicists in France have apparently acquired evidence in favour of the
idea that objects they call "tetraneutrons" have been discovered by
them.
The mind boggles at the idea that fout neutrons could form a bound state
which lives long enjough to travel macroscopic distances, like several
metres, in the lab.
Is there a nuclear physicist in either of these two groups who would
venture to comment?
Franz Heymann
John Baez
In article <apbpak$33i$2...@venus.btinternet.com>,
Franz Heymann <Franz....@btopenworld.com> wrote:
>I notice in the latest issue of New Scientist that a group of
>physicists in France have apparently acquired evidence in favour of the
>idea that objects they call "tetraneutrons" have been discovered by
>them.
>
>The mind boggles at the idea that four neutrons could form a bound state
>which lives long enjough to travel macroscopic distances, like several
>metres, in the lab.
Of course New Scientist specializes in wild and soon-forgotten
claims. It's getting to the point where the main reason to read
this magazine is to know what not to believe. That's sad, because
it wasn't always this way. They still have some good articles!
But "caveat emptor" is the order of the day here.
On this newsgroup we've already talked about the New Scientist
cover article on faster-than-light communication:
http://groups.google.com/groups?selm=8a3lbg%244a7%241%40pravda.ucr.edu
their claim that "anti-gravity research is on the rise":
http://groups.google.com/groups?selm=ld-usenet-sfm54-3A9F22.10234230072002%40news.west.cox.net
their report that geomagnetic fields may cause variations in the
gravitational constant:
http://groups.google.com/groups?selm=amu5gn%244d9%241%40inky.its.caltech.edu
and their discussion of a theory in which 3-dimensional space
arises from a "non-geometric order-disorder model driven by
self-referential noise", whatever that means:
http://groups.google.com/groups?selm=8a3lbg%244a7%241%40pravda.ucr.edu
.......................................................................
Anyway - about tetraneutrons.
I didn't say they don't exist... just that "New Scientist" is
not the best place to find out if they do! A better place is this:
F.M. Marques, M. Labiche, N.A. Orr, et al
The detection of neutron clusters
http://www.arXiv.org/abs/nucl-ex/0111001
The introduction has a nice review with references on attempts
to detect neutral nuclei - that is bound states consisting of
a bunch of neutrons. They say this subject has a "long and
checkered history" starting in the 1960s. I'll summarize a bit
of what they say about it, but I'm no expert on this so hopefully
a real expert will step in to help out.
The dineutron is not bound - if you take two neutrons, they
don't stick together. There's no clear evidence about the
trineutron. Most calculations suggest that the tetraneutron
is *not* bound. However, these calculations are tricky enough
that it might be a bound state with a very small binding energy -
less than 1 MeV.
So, they did some experiments. They had the most luck smashing
beryllium-14 nuclei against a carbon target. After a lot of
work they got 6 events with characteristics of a multineutron
cluster. The most plausible explanation was beryllium-14
splitting into beryllium-10 and a tetraneutron. It seemed
like these tetraneutrons decayed via beta decay, with a lifetime
of at least 100 nanoseconds - long enough to reach the detector,
which was 3.5 - 6.5 meters away.
So you see, tetraneutrons don't need to last *very* long to
get across the lab! I don't find it shocking that a bound
state of this sort could last 100 billionths of a second.
So, *this* New Scientist article could be just fine.
On the other hand, for a more skeptical theoretical perspective
on tetraneutrons, see:
N.K. Timofeyuk
On the existence of a bound tetraneutron
http://www.arXiv.org/abs/nucl-th/0203003
So, if I had to guess, I'd say that nobody is sure yet
whether tetraneutrons are bound.
Dirk Bruere
"John Baez" <ba...@galaxy.ucr.edu> wrote in message
news:apchvf$som$1...@glue.ucr.edu...
> In article <apbpak$33i$2...@venus.btinternet.com>,
> Franz Heymann <Franz....@btopenworld.com> wrote:
> >I notice in the latest issue of New Scientist that a group of
> >physicists in France have apparently acquired evidence in favour of the
> >idea that objects they call "tetraneutrons" have been discovered by
> >them.
> >
> >The mind boggles at the idea that four neutrons could form a bound state
> >which lives long enjough to travel macroscopic distances, like several
> >metres, in the lab.
>
> Of course New Scientist specializes in wild and soon-forgotten
> claims. It's getting to the point where the main reason to read
> this magazine is to know what not to believe. That's sad, because
> it wasn't always this way. They still have some good articles!
> But "caveat emptor" is the order of the day here.
It's as good as the people who write the articles.
If you think you could do better then give them a call.
Dirk
John Devers
> Is there a nuclear physicist in either of these two groups who would
> venture to comment?
Probably there is but it's not me:-) I can give you a link about this
story from six months ago though and a quote you could read anything
into.
http://physicsweb.org/article/news/6/5/8
"An analysis of the data led by Francisco-Miguel Marqués identified
six protons with energies that could be explained most easily by
collisions with newly formed tetra-neutrons."
(F Marques et al 2002 Phys. Rev. C 65 044006)
Franz Heymann
"John Devers" <johnd...@froggy.com.au> wrote in message
news:2978f9d5.02102...@posting.google.com...
Many thanks,
Franz Heymann
Grant Green
I've been wondering about that for a while, actually. The strong
nuclear force (color force?) manages to hold a number of protons
together against high electrostatic repulsion, and the neutrons usually
don't wander away. Does nuclear binding require protons? If not, then
aggregates of neutrons (of some size) ought to be stable as well... I
wonder if one could accumulate a macroscopic amount of neutronium...
[Moderator's note: aggregrates of neutrons are only
stable under very high pressure, e.g. that provided
by gravity in a neutron star. Under conditions
currently achievable in the laboratory, material
made solely from neutrons would be highly explosive. - jb]
Grant Green
I've been wondering about that for a while, actually. The strong
student
>Franz Heymann wrote:
>
>> I notice in thie latest issue of New Scientist that a group of
>> physicists in France have apparently acquired evidence in favour of the
>> idea that objects they call "tetraneutrons" have been discovered by
>> them.
>>
>> The mind boggles at the idea that fout neutrons could form a bound state
>> which lives long enjough to travel macroscopic distances, like several
>> metres, in the lab.
>>
>> Is there a nuclear physicist in either of these two groups who would
>> venture to comment?
>
>I've been wondering about that for a while, actually. The strong
>nuclear force (color force?) manages to hold a number of protons
>together against high electrostatic repulsion, and the neutrons usually
>don't wander away. Does nuclear binding require protons? If not, then
>aggregates of neutrons (of some size) ought to be stable as well... I
>wonder if one could accumulate a macroscopic amount of neutronium...
Since there has not been much comment outside of the excellent
contributions of Worley and Baez, I'll venture my own meager
contribution. The main problem with neutral nuclei can be
understood simply in terms of the exclusion principle, in a
very old-fashioned shell model context. With four neutrons
not all four can be in the lowest-energy orbitals without violating
the exclusion principle, so the tetraneutron would have a higher
energy than the alpha particle on the basis of that alone.
But there's a little more to it than that.
The tensor force would also be significantly suppressed in the case
of a tetraneutron or a dineutron. To see that, for either
case, note that the tensor force will couple a pairwise l=0 and l=2 state
as long as s=1, and this coupling is known to contribute significantly
to the binding energy in the case of a deuteron or alpha.
(So much so that the deuteron would not be bound without it).
But if both nucleons are in a ground state having zero angular momentum,
then the needed spin 1 state is forbidden by the exclusion principle
if they are identical particles. So the coupling to the lowest state,
and consequently its contribution to binding energy, is available for
the alpha and the deuteron but not the tetraneutron or dineutron.
This applies to pairwise forces -- and I would agree with Franz
that with pairwise NN forces alone a bound tetraneutron would be
highly improbable.
The idea of a 4-nucleon force, as in a url mentioned in Baez' post,
is a very interesting one, but I can't really comment on that.
Grant Green
<gdg...@NOSPAMcontrabass.com> wrote:
>I've been wondering about that for a while, actually. The strong
>nuclear force (color force?) manages to hold a number of protons
>together against high electrostatic repulsion, and the neutrons usually
>don't wander away. Does nuclear binding require protons? If not, then
>aggregates of neutrons (of some size) ought to be stable as well... I
>wonder if one could accumulate a macroscopic amount of neutronium...
>
>[Moderator's note: aggregrates of neutrons are only
>stable under very high pressure, e.g. that provided
>by gravity in a neutron star. Under conditions
>currently achievable in the laboratory, material
>made solely from neutrons would be highly explosive. - jb]
Thanks. I'd like to read more on the topic: if you have a moment, is
there a good undergrad or grad student level text that would make a
good introduction? Assume a reader with an MS in Chemistry (physical
organic), who's been away from the field for a couple decades ;-)
Many thanks,
Grant