The Positron Excess

The Field Guide to Particle Physics : Season 3
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The Positron Excess

Space is not a safe place. Matter and energy take on a totally different form than is familiar from our planetary lifestyle. Radiation is everywhere, and with it we find high energy particles flying all over the place. One of the biggest challenges in a voyage to Mars is shielding the travelers from all that radiation. Our magnetosphere and atmosphere do an outstanding job of filtering out the most of the high energy particles flying at us from all directions.

Many energetic particles come from the sun. Fast moving protons and electrons that boil off our friendly plasma ball get trapped in the van Allen belts of our earth’s magnetic field. Way above the atmosphere, we can see them sometimes as the Aurora.

Other energetic particles come to us from inside the Milky Way galaxy. Exploding stars, neutron stars and other monsterous astrophysical objects can shed or accelerate their own high energy particles. Often these particles have more energy than those put off by the sun, but it’s the same story: A lot of protons, a few electrons, and also some heavier nuclei: like alpha particles. Much less often, we see cosmic rays made up of even bigger things, like the nuclei of Carbon, Silicon or even Iron!

Some particles come from outside our galaxy. These can sometimes have outrageously high velocities, and are observed as miles-wide particle showers by large, ground based detector arrays. They aren't common. One of the biggest of these was observed by the Fly’s Eye camera back in 1991. It had over 50 J of energy packed into a single particle - probably a proton. That’s about the same kinetic energy as baseball being thrown around… in a single particle.

Fast moving high energy particles - the ones flying in from outside our solar system -  are typically called Cosmic Rays. A tiny fraction of these Cosmic Rays are actually antimatter. Antiprotons and positrons, specifically.  Understanding where all these cosmic rays come from is an important scientific question in its own right, but understanding where the antimatter comes from - and how much of it there is - has been a truly fascinating question. Especially of late.

Where does the cosmic antimatter come from?

The ratio of matter to antimatter in Cosmic Rays is small, and varies with particle speed. Typical numbers are 1 or 2 antiprotons for every ten thousand protons. The ratio of positrons to electrons is higher, closer to a few parts in a hundred. One thing we haven't seen? Bigger antiparticles. No antideutrons or antialpha particles have been observed - at all - let alone bigger antinuclei. But of course, we see big nuclei in Cosmic Rays all the time.

Because Cosmic Rays come from other parts of the galaxy - or even outside of it - these ratios are basically consistent with our typical assumption that all observed antimatter is secondary. It is created - in other words - through collisions or decay of so-called “normal” matter.

Really fast Cosmic Rays occasionally interact with other particles in our galaxy: the tiny, sparse bits of gas and dust in the large voids between stars, sometimes called the interstellar medium. Those collisions often generate more particles, and just like in our own atmosphere, antiparticles are part of that collision debris.

Just like the proton and the electron, to the best of our knowledge, the antiproton and the positron are stable particles. So unless they annihilate, these particles of antimatter just hang around. The collective effect of all these Cosmic Rays bounding around our galaxy is a very small - but measurable - population of antiprotons and positrons flying at us as secondary cosmic rays.

If we were to assume that all antimatter is secondary - that is, if antiprotons and positrons are created only from collisions in the interstellar medium - we can use that assumption to calculate how much of it we expect to see. In these calculations, the number of antiprotons pretty much lines up expectations. While on the high side, the population of antiprotons in our galaxy essentially agrees with what you'd expect from collisions of other cosmic rays in the interstellar medium.

While it is possible that antideutrons and antialpha particles can be also created in these collisions, they are rare. The expected number of them is currently far below current experimental sensitivity.

Positrons are a different story. What’s fascinating astroparticle physicists these days is that the number of positrons observed in Cosmic Rays is noticeably higher than we expect from these calculations. In particular, the number of positrons at higher energies is much bigger than we’d expect if they were only created in collisions, upwards of 10 percent or more!

In short, we see too many positrons flying at us as Cosmic Rays and we don't know why!

What we do know about Cosmic Rays

Earth's atmosphere is much denser than interstellar space, so Cosmic Rays that make it to Earth typically collide dramatically with molecules in our upper atmosphere. With land-based detectors, we can see the resulting showers of particles down on Earth. We can calculate how much energy they had, but we can't exactly say what kind of particle they were.

To assess the species of particle that's slamming into the Earth, we need to capture, identify and count them before they strike the atmosphere. We need, in other words, particle detectors on satellites.

Older experiments like the Fermi Gamma Ray Telescope and the PAMELA detector were put in orbit around the earth on satellites. The current state of the art, the AMS-02 Cosmic Ray experiment is literally in a box attached to the side of the International Space Station.

All these experiments agree: Cosmic Rays follow a somewhat predictable pattern. Most particles come equally from every direction in space, so as a population of particles, they're very likely diffused around the entire galaxy. The number of particles we see depends on their energy. Roughly speaking, the more energy a particle has, less common it is to see. But this trend is also true by particle species. In aggregate, simpler particles are also more common than complex ones. And of course, antimatter is far, far less common than matter.

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