Neutron

[Little Bang]

Have anti-matter neutrons been produced?

[Erasmus00]

Have anti-matter neutrons been produced?

 

Yes, in the late 1950s at the Bevatron, for the first time.

[Little Bang]

Since it can't be observed directly the products of it's annihilation is proof of it's existence? Those products are exactly what?

[Erasmus00]

Generally a pair of 1.8 GeV photons. They can also first go to pions, and then photons.

[CraigD]

Those products [of antineutron-baryon annihilation] are exactly what?

Pseudoclassically, an antineutron and a neutron or proton annihilate producing many photons and more or less 3 (more likely more) mesons and anti-mesons, mostly pi mesons (pions) and antipions.

 

The essential particle physics accounting here is conserving baryon number, the count of quarks – antiquarks. A neutron has 3 quarks, [imath]u+d+d[/imath], an antineutron has the antiparticles of these three, [imath]\hat{u}+\hat{d}+\hat{d}[/imath]. A proton has [imath]u+u+d[/imath]. A charged pion has [imath]u+\hat{d}[/imath], a neutral one a superposition of [imath]u+\hat{d}[/imath] and [imath]\hat{u}+d[/imath]. Pions mass much less than protons or neutrons, so there’s lots of extra mass-energy to allow for many possibly annihilation products, following statistical laws.

 

Mesons are short lived, none with a mean lifetime greater than [imath]10^{-8}[/imath] s, but long enough that they can be detected in cloud chambers and on photographic films. Ultimately, all their short-lived decay products end up long-lived photons and neutrinos.

 

Generally a pair of 1.8 GeV photons. They can also first go to pions, and then photons.

Although electrons-positrons annihilation generally produce pairs of photons (much lower energy than 1.8 GeV), in my inexpert reading of various sources, everything I’ve encountered describes the much higher energy baryon-antibaryon annihilation as producing “star-bursts” of pions. I am, though ... inexpert. :shrug:

[Erasmus00]

I had a long thing typed up about when the quark picture would give an understanding of a collision, and when you are better off thinking of mesons and baryons as degrees of freedom, but sadly lost it due to browser hiccup . I'll maybe add it later.

 

Although electrons-positrons annihilation generally produce pairs of photons (much lower energy than 1.8 GeV), in my inexpert reading of various sources, everything I’ve encountered describes the much higher energy baryon-antibaryon annihilation as producing “star-bursts” of pions. I am, though ... inexpert. :)

 

In general, you'll have tons of pions, and a whole mess of their decay products, but it's really hard to untangle this and say "thats was an anti-proton/anti-neutron."

 

However, if you are reasonably confident that your anti-neutrino is being produced right at threshhold, then the "golden mode" is probably the two photon signal. This depends on your backgrounds, of course, but I imagine that any collider producing lots of anti-neutrons is also producing lots of pions.

[Little Bang]

Then we have no guarantee of it's existence. That means there is a possibility that the mainstream does not understand not only the neutron but also the proton.

[maddog]

Anti-Neutrons can exist simply because that anti-quarks of the appropriate mix can be used to form

them. Thus an annihilation pair of Neutron - Anti-Neutron can be formed.

 

The rub comes since Neutrons are electrically neutral, thus are impervious to electric and magnetic

fields. So you are not able to direct them by fields nor able to sense them to know where they are

located. The best you can hope for is knowing there trajectory from when they were created

(if that were possible). So any such annihilation pair would be mostly a random occurrence.

 

maddog

[Erasmus00]

Then we have no guarantee of it's existence. That means there is a possibility that the mainstream does not understand not only the neutron but also the proton.

 

All our current theories say is "if this particle exists, we should see some features of the data that look like this." Some of these features more resemble "seeing" the particle than others. But the thing is, the same theories that predict anti-neutrons also predict many other, more mundane scenarios (protons scattering off protons, electrons scattering off electrons, etc), so each verification strengthens the core of the theory.

 

Any theory that seeks to replace the standard model has its job cut out- the standard model successfully predicts decades of particle accelerator data.

[Qfwfq]

Anti-Neutrons can exist simply because that anti-quarks of the appropriate mix can be used to form them.

This isn't how to prove their existence, it would be begging the question because our only evidence of quarks is the behaviour of hadrons.

 

Then we have no guarantee of it's existence. That means there is a possibility that the mainstream does not understand not only the neutron but also the proton.

There's a plethora of phenomenology, starting from around Rutherford, showing that atomic nuclei, alpha particles, protons and neutrons exist.

 

When an acid corrodes something, turns out it's protons that are doing the dirty work. Loads of scattering experiments on hydrogen rich targets have enabled piecing together info about the proton. Neutrons are tricky to deal with; they can't be accelerated but ducts with appropriate reflective inner surfaces act as crude waveguides for them. A wealth of low energy nuclear reaserch says plenty about both nucleons.

[CraigD]

Researching for this thread, I found an interesting bit of history about the ca. 1956 work of Bruce Cork on antineutrons.

 

While Dirac described antiparticles ca. 1930, so physicists of Corks generation were long acquainted and comfortable with the idea and formalism, Gell-Mann and Zweig wouldn’t think up quarks ‘til ca. 1961, so physicists weren’t especially sure what a particular particle-antiparticle interaction would actually produce. According to this archived Waterloo U webpage, Cork hypothesized that a proton – antiproton collision wouldn’t result in a shower of photons and pions, but rather a neutron + antineutron pair.

 

If I may be so presumptuous, I think LB may be thinking along similar, “what if quarks don’t exist” lines (1950s physicists didn’t so much as doubt their existence, rather they hadn’t yet imagined them in much detail, but the result’s much the same). If you drop back to a 1950s position where hadrons (protons, neutrons, and their more exotic cousins) are considered fundamental (not made up of quarks and gluons), it’s difficult to imagine how there could be an antineutron, as unlike the proton, it has not charge to reverse in the case of its antiparticle.

 

Quarks pretty nearly resolve this problem (allow theory to match observation) as antineutrons are just [imath]\hat{u}+\hat{d}+\hat{d}[/imath] quark triples, with [imath]-\frac13 -\frac13 +\frac23 = 0[/imath] charge. Like protons, the antiparticle all the quarks have nonzero charge, so their antiparticles simple have reversed sign charge (and a couple of other, less obvious properties).

 

What are you think, LB? :QuestionM

[Rade]

If you drop back to a 1950s position where hadrons (protons, neutrons, and their more exotic cousins) are considered fundamental (not made up of quarks and gluons), it’s difficult to imagine how there could be an antineutron, as unlike the proton, it has not charge to reverse in the case of its antiparticle.

 

Here is a paper published in 1950 that claims to have an understanding of the antineutron and how it helps form a meson, well before any concept of quarks or partons, or before the antineutron was experimentally observed.

 

Phys. Rev. 80, 177 (1950): The Meson as a Composite Particle

[Little Bang]

Suppose we had two magnets made of an exotic material that would allow them to pass through each other never touching except via the field of each. Each would be made in the shape of a washer or doughnut and ones diameter is several hundred times smaller than the other. If they are separated out in space by say a half a meter when released what would you suppose their final position with respect to each other would be? I'll try to connect this to the thread shortly.

Edited August 10, 2010 by Little Bang

[Erasmus00]

Assuming there is some way they can lose energy to the surroundings, and they start close enough (the initial attraction will go like r^4), they will try to find the minimal energy solution. This is the smaller of the dipoles sitting in the middle of the large dipole, aligned with its field.

[Qfwfq]

They will certainly lose energy, at the least by the changing field radiating it. If there is no constraint at all, they will bang into each other no matter how carefully coaxial they are initially positioned, unless they initially repell and end up going further and further apart. If there is some kind of constraint, the final position depends also on how it is contrived and especially on whether it allows either magnet to flip at otherwise unstable configurations.

 

(the initial attraction will go like r^4)

The inverse of. :confused:

[Little Bang]

Now lets take one of the torus magnets and lay it on a table. The north pole is above the torus and the south pole is down through the table. If we now twist the torus into the shape of a figure eight what would the field do?

[Erasmus00]

Now lets take one of the torus magnets and lay it on a table. The north pole is above the torus and the south pole is down through the table. If we now twist the torus into the shape of a figure eight what would the field do?

 

The resulting field at large distances would be a quadrupole, most likely. Though some details of the twist would be relevant at intermediate distances.