“On what can we now place our hopes of solving the many riddles which still exist as to the origin and composition of cosmic rays?” –Victor Francis Hess
We’ve come a tremendously long way in our understanding of the Universe, but there are plenty of mysteries still to be revealed. Way back in the late 1800s, we noticed that there was an unaccounted-for excess in the amount of ionization in the upper atmosphere, something that our Sun and Earth, by themselves, couldn’t explain.
Victor Francis Hess — the originator of the quote atop — decided to go see for himself, and conducted a series of balloon-borne experiments to get to the bottom of the puzzle.
100 years ago, Hess became the discoverer of cosmic rays, or very high energy particles whose origin comes from beyond our own Solar System. Since that time, many advances in understanding the origin and production of these particles has occurred, as well as the measurement of just what’s flying around through interstellar space.
Most of the high-energy particles flying around are protons, the most common atomic nucleus in the Universe. These cosmic rays have a huge energy range, ranging from non-relativistic speeds all the way up to around a whopping 1020 electron-Volts, or about a factor of 10,000,000 greater than the LHC — even at maximum energy after the upgrade-in-progress — will ever achieve. There are also much smaller numbers of cosmic-ray electrons, followed by trace amounts of cosmic-ray antimatter, including positrons and anti-protons.
Where do they come from? That’s part of the fun!
There are tons of astrophysical sources for cosmic rays, both in our galaxy and beyond. These range from young stars surrounded by dust and debris, white dwarfs and neutron stars (including pulsars), as well as black holes, including stellar-mass-size ones, supermassive ones, and everywhere in between.
Some cosmic rays show a clear origin from point sources, such as the pulsar at the center of the crab nebula, below.
That’s the cosmic ray side of things.
At the same time, there’s another puzzle in the Universe: the dark matter puzzle.
Since the latest results from PLANCK came in, we’ve had our picture of the Universe confirmed once again: there is, in fact, some type of dark, non-standard-model mass out there, in about five times the abundance of all the normal matter in the Universe combined.
Now, dark matter — in most models — happens to be its own antiparticle, which means if you have a collision of a dark matter particle with itself, you’re going to get a bunch of energy out. That can come in any form that the mass of dark matter allows: photons, electron/positron pairs, proton/anti-proton pairs, etc. Although experiments have placed constraints on what the masses and cross-section of dark matter is allowed to be, we have yet to have a definitive positive particle detection of dark matter.
It’s important to point out, at this point, that dark matter could have a mass anywhere from a few micro-electron-Volts all the way up to, in principle, 1024 electron-Volts. We’re only focused on the GeV-to-TeV mass range because that’s what we’re most capable of detecting!
Well, enter the latest attempt at indirect detection of dark matter, the Alpha Magnetic Spectrometer.
Launched aboard the Space Shuttle Endeavour’s final mission, the Alpha Magnetic Spectrometer (AMS) was attached to the International Space Station, where it’s quickly become the most advanced, sophisticated and prolific cosmic ray detector of all-time.
With over 31 billion cosmic ray particle detections (and counting), AMS is giving us an unprecedented amount of data to study cosmic rays.
This includes a huge amount of antimatter data as well, including positrons and antiprotons in huge numbers. They’ve just released their first science results, and rather than read a terrible press release that overhypes dark matter and SUSY, let me give you the actual science instead.
As with any particle, you can apply a magnetic field and determine, based on the direction it curves (or if it doesn’t curve), what its electric charge is. This is what AMS does, and over their first 25 billion cosmic ray detections, they’ve detected about 400,000 positrons of varying energies.
It’s a remarkable technical achievement, and by measuring the energy, momentum and curvature of these particles, they can tell, for example, what’s a positron from what’s a proton.
Now, here’s the fun stuff. Based on what we know about astrophysics, there are many things that can cause an excess of positrons: stars, pulsars, black holes, annihilating dark matter, extragalactic sources, etc. The key to telling them apart is to look at the spectrum of positrons, along with the corresponding spectrum of electrons and the locations where these cosmic rays originate.
First off, the AMS positron results, as compared with earlier FERMI and PAMELA results.
Nothing funny at all here. As you can see, this is consistent with previous results, but also shows no evidence of a sharp drop-off, which is what we’d need for dark matter evidence.
Also, not presented in their paper is the corresponding electron-fraction spectrum. A dark matter signal would show the same “bump-and-drop-off” in the electron spectrum as in the positron spectrum, something that Fermi has reported to not exist all the way up to energies of 1 TeV.
Finally, a dark matter signature would also be anisotropically concentrated towards the direction of the galactic center, and would very likely show a preferred directionality towards other large galaxies in the sky, such as M87. No evidence for directionality exists in the AMS data. The AMS data shows unprecedented precision in the measurement of the antimatter cosmic ray energy spectrum, and there’s plenty to be learned from this.
Just not about dark matter, at least, based on what it’s seen so far.
To reiterate, based on what AMS has presented, there is nothing to suggest that they have detected any evidence whatsoever for particle dark matter. The press release (and the earlier press conference) — reported upon by many that I will not link to — suggest otherwise, and that is misleading. In fact, calling it misleading is generous, because I personally believe it is deceitful, and it’s a deceit that I even anticipated a few weeks ago!
Matt Francis has a good, brief writeup as well, and it’s worth keeping in mind that not every scientific mission that we run will find the next great breakthrough that provides the solution to our deepest mysteries. That’s not how science works; I loathe overhyping results to say something that the data doesn’t. There might be particle dark matter out there, and it might even have the right parameters to be detectable by AMS. But if it is, we haven’t seen it yet. And unless subsequent data comes in with better statistics at higher energies to suggest otherwise, there will be no reason to think it ever saw even a hint of dark matter.