MadSci Network: Physics
Query:

Re: antimatter neutrons

Area: Physics
Posted By: Jay H. Hartley, Post-doctoral physicist, Lawrence Livermore National Lab
Date: Sat Nov 15 17:41:29 1997
Area of science: Physics
ID: 878604608.Ph
Message:

Good question, Marcy. It's one of those things that always bugged me when I was learning about antimatter, and it's actually kind of hard to find a source for the answer without turning to graduate physics textbooks.

You are correct that the opposite-charge explanation of antimatter is the simplest. Like most simple explanations it is incomplete, as you have demonstrated with your question regarding neutrons. I read once, in a supposedly reputable source, that the neutron is its own antiparticle, because it has no charge. This is obviously incorrect, since one of the properties of a particle and antiparticle pair is that they will annihilate into energy if they collide with one another. Neutrons are a major component of all matter, and clearly those neutrons are not constantly annihilating one another.

The difference between a neutron and an anti-neutron has to do with the fact that neutrons are actually composite particles. Each neutron is made up of 3 fundamental particles called "quarks," specifically two d-quarks and one u-quark. (They are also referred to as "down" and "up" quarks.) The anti-neutron, on the other hand, is made up of two anti-down quarks and one anti-up quark.

If you think about it, this makes a great deal of sense. The up quark has a charge of 2/3, and the down has a charge of -1/3. (I'm using units where the electron has a charge of -1.) Add: 2/3+(-1/3)+(-1/3)=0. You get a zero-charge neutron. Now, the anti-up quark has charge -2/3, and the anti-down has charge +1/3. Add two anti-downs and an anti-up, and you still get zero charge, but in this case an anti-neutron.

So, it may help to think of a collision between a neutron and an anti-neutron instead as a collision of their component quarks and anti-quarks. You can learn more about the zoo of fundamental particles at The Particle Adventure.

As you will learn there, the picture I have described for you is part of the "Standard Model" of physics. We are not currently able to observe quarks directly, but only by including them in our equations are we able to explain all the phenomena we do see in particle experiments. The "Standard Model" is a theory that has been tested more extensively than any other, but that does not guarantee that it is a complete description.

One last note: in addition to charge, there are actually a few other properties of particles that get reversed in antiparticles. These aren't discussed much in popular science (I think they are ignored at The Particle Adventure as well) because these properties don't have intuitive meaning for people. Just to give you a taste, I'll break tradition and tell you that these other properties include "baryon number," "lepton number," and "color." The names don't really have much meaning outside of the mathematics of particle physics.

"Color," for example, has nothing to do with the colors of everyday life. It just serves as a convenient analogy because quarks come in three "colors," just like the three primary colors of light - Red, Green, and Blue. The anti-particle versions are given the names of the complementary colors - cyan (anti-Red), magenta (anti-Green), and yellow (anti-Blue). Neutrons and other similar particles are "white" - made up of one red, one green, and one blue quark. Cute, isn't it? Who says physicists don't have a sense of esthetics!


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