The Positron Excess

Searching for antimatter in the wild reveals a bit more than we expected. But only a bit. Are pulsars to blame? or is it Dark Matter?

Show Notes

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.

There are a few minor exceptions to these rules, and they have all been explained by various physical phenomena: like the distinction between lower energy cosmic rays from inside our galaxy to higher energy cosmic rays from outside our galaxy. Each of these minor bumps on the otherwise clean plots of counts of cosmic rays is a fascinating story in its own right. But today, we'll focus on one, massive, glaring irregularity:

Again, the number of positrons observed as cosmic rays at higher energies is much higher than we'd expect.

The Positron “Excess”

Check out this plot from a 2019 publication by the AMS-02 Collaboration, Towards Understanding the Origin of Cosmic-Ray Positrons:

Fig 4. from the above paper, https://doi.org/10.1103/PhysRevLett.122.041102

Per the most recent AMS-02 data analysis, the spectrum of positrons in cosmic rays can be cleanly represented with a two-component model. The first component, valid at lower energies, is the usual, expected effect of interstellar media collision debris with higher energy cosmic rays. It peaks at about 5 to 10 GeV with a slightly long tail towards higher energies. The second component, valid at higher energies, appears to be associated with a different and stronger source of positrons, whose peak is closer to a few hundred GeV. This model for an as-yet-unknown source of positrons, is skewed in the other direction, with a longer tail towards lower energies, and a sharp cut-off estimated at about 800 GeV.

Now, this two component model is just one interpretation of the data. An agnostic, best fit model. The essential point it captures, is the positrons in Cosmic Rays very likely come from multiple sources. The data associated to the second source term in the model is what we refer to as the “excess” of positrons.

As noted in the aforementioned publication, there are few possible explanations for that excess. Pulsars - fast spinning husks of recently desceased stars - may well lose some of their rotational energy to radiation and the production of particle/antiparticle pairs. They could be a source for these positrons (see also this).

Another, more intriguing explanation, is that the positrons are created as a byproduct of dark matter/antidark matter annihilation. [1] Dark Matter is a theoretical framework for explaining a vast array of astrophysical phenomena, which are all basically consistent with a new kind of stable or very long lived particle. Such a particle would not interact with light at all, hence the name, Dark Matter.

Of course, we don't know if Dark Matter really is made up of particles, and if so, we don't know what those particles would be. They would represent new particle physics, a further extension of the Standard Model. Because many such models of new particle physics include particles that could act like Dark Matter, the positron excess serves as a consistency check or constraint on such models.

If the annihilation of a new kind of Dark Matter particle were responsible for the excess of positrons [2], the AMS-02 data already highly constrains its properties. In particular, it has to be heavy, like around a TeV or more, and it has to decay through some intermediate state before producing any of those excess positrons. This scenario is at least qualitatively consistent with the fact that we haven't yet seen any evidence for Dark Matter at the LHC or in direct detectionexperiments [3]. 

In Summary

Antimatter is out there. It’s coming in from outer space. Like the antimuons and positrons that appear in our atmosphere from collisions with these high energy particles, antiprotons and positrons are occasionally formed by tiny collisions all over our galaxy.

The number of positrons we see are inconsistent with our understanding of how these secondary Cosmic Rays form. In certain energy ranges, we see far too many positrons. Something is definitely going on. Something we haven’t yet accounted for.

Something, perhaps... perhaps, like Dark Matter.

Footnotes

[1] Dr. Rebecca K. Leane, the author of that recent review on these kinds of Dark Matter Indirect Detection results, remarks that pulsars are currently favored to explain the excess. Of course, particle physicists remain excited until its ruled out! See also the following footnote.
[2] It’s worth pointing out that the as-yet statistically insignificant, slight overabundance of antiprotons could come from Dark Matter annihilation, too! Such antiprotons in cosmic rays also present constraints on Dark Matter annihilation models.
[3] The usual disclaimer, with a twist! The DAMA/LIBRA collaboration has been claiming the observation of Dark Matter for years now, although it remains unconfirmed by any other experiment. Convention wisdom remains that Dark Matter has yet to be identified. To bolster that conventional wisdom, a recent, second-party analysis of the DAMA/LIBRA data has suggested their signal may result from a kind of systematic, statistical error.

What is The Field Guide to Particle Physics?

This is your informal guide to the subatomic ecosystem we’re all immersed in. In this series, we explore the taxa of particle species and how they interact with one another. Our aim is give us all a better foundation for understanding our place in the universe.

The guide starts with a host of different particle species. We’ll talk about their masses, charges and interactions with other particles. We’ll talk about how they are created, how they decay, and what other particles they might be made of.