Dark Antimatter

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Every particle of matter had a corresponding particle of antimatter. When a particle of matter and a particle of antimatter come together they annihilate one another, creating two gamma rays traveling in opposite directions. So, when an electron and a positron (an anti-electron) meet, they produce two gamma rays having 511keV of energy. The amount of energy in the gamma rays can be computed using Einstein’s famous equation: E=mc2. So, What is dark antimatter like?

What is antimatter like?

In basically all appearances, antiparticles look and act like particles, with the exception that they have an opposite electrical charge. An electron is negative, so a positron is the same mass, same spin, same other properties, but with a positive charge. A proton is positive, so an antiproton is negatively charged. I find it rather interesting that these oppositely charged particles, having electric fields, annihilate to form electromagnetic radiation.

I am not a theoretical physicist, but it makes me wonder whether or not if antimatter had negative mass if it would produce gravitational radiation upon annihilation. The problem, though, comes from neutral particles. For example, a neutron has no electric charge. So, what distinguishes it from an antineutron? To answer that, you have to look deeper.

Neutrons and protons are examples of a group of particles called hadrons. In fact, they are part of a subgroup of hadrons called baryons. Hadrons are particles composed of smaller particles called quarks. Baryons have three quarks, and eventually decay into protons. Talking about the subatomic particle zoo is a bit more than I want to do in this post.

But, I wanted to mention this because neutrons are composed of an up quark and two types of down quarks. An up quark has a charge of 2/3 e, and down quarks have a charge of -1/3 e. Only quarks have fractional charges of the fundamental charge ‘e’. Antineutrons are composed of an anti-up quark and two anti-down quarks.   (We won’t really worry about how these baryons are really composed of lots of virtual quarks, too). 

Also Read: Finding Dark Matter

So, what happens when a neutron and an antineutron come together is that the up and anti-up quarks annihilate, and the anti-down quarks annihilate withe the down quarks.  But, things get a bit more complicated with neutrinos.  You see, neutrinos are leptons, which are fundamental particles, themselves.  They aren’t made of more basic things.  So, neutral neutrinos are pretty much indistinguishable from antineutrinos.  Indistinguishable means just that in particle physics.  So, the neutrinos can act as their own antiparticles.

They were talking about the gamma ray glow of the galaxy.  It turns out that wherever you point a gamma ray telescope, you get a slight glow.  And that glow seems to be brightest from the direction of the center of the galaxy.  Other galaxies may also have this gamma ray halo.  It is actually pretty much a mystery as to what it may be.  Well, some people are now suggesting that perhaps this may be a sign of dark matter.  In particular, they propose that perhaps dark matter has a dark antimatter counterpart. 

A particle of dark matter and dark antimatter would annihilate each other when running into one another, perhaps producing these gamma rays.  Well, that might actually be a problem, because the gamma rays have the wrong energy from what theorists would suggest.  Remember, the amount of energy is related to the energy of the gamma rays by Einstein’s mass-energy equivalence relationship.  But, that works both ways. 

You can also create more particles and antiparticles from energy.  So, very high energy gamma rays can also produce particle antiparticle pairs.  And, some of those particles might be unstable, decaying into other particles that then produce the gamma rays that we see.  Yeah, it seems like it may be a little of a stretch, but it is a very interesting idea, and it definitely deserves more work. Interestingly, these gamma rays do seem to be concentrated in regions where you’d expect dark matter to concentrate.

What differentiates dark matter from dark antimatter?

But, as I said, there is still a need for a lot of work.  After all, it isn’t even clear what dark matter is, much less if it even has a dark antimatter type of symmetry.  And, since dark matter does not interact via the electromagnetic forces, that means that it has no charge.  What differentiates dark matter from dark antimatter?  Is it composed of more fundamental dark things that do have charge, but only occur in pair or triplets like quarks?  Hmm.  This raises lots of questions.  And, as I said, high energy physics isn’t really my thing.  But, I do find it interesting.


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