This past month in Geneva a conference took place bringing together the world’s foremost experiments in cosmic ray physics and indirect dark matter detection: “AMS Days at CERN”. I took a break from thesis-writing, grabbed a bag of popcorn, and sat down to watch a couple of the lectures via webcast. There was a stellar lineup, including but not limited to talks from IceCube, the Pierre Auger Observatory, H.E.S.S. and CTA, Fermi-LAT, and CREAM. The Alpha Magnetic Spectrometer (AMS) experiment was, of course, the star of the show. It is the AMS and its latest results that I’d like to focus on now.
But first, I’d like to give a brief introduction to cosmic rays, since that’s what AMS studies.
It turns out that space is not as empty as one might think. The Earth is constantly being bombarded by extremely-high-energy particles from all directions. These cosmic rays were discovered in the early twentieth century by the Austrian physicist Victor Hess. Hess made several balloon-borne measurements of the Earth’s natural radiation at various altitudes and observed that the incidence of ionizing radiation actually increased with ascent, the exact opposite of what you would expect if all radioactivity came from the earth.
The word “ray” is actually something of a misnomer – Cosmic rays are primarily charged matter particles rather than electromagnetic radiation. Their makeup goes as follows: approximately 98% are nuclei, of which 90% of are protons, 9% are alpha particles (helium nuclei), and only a small proportion heavier nuclei; and approximately 2% electrons and positrons. Only very small trace amounts (less than one ten-thousandth the number of protons) of antimatter are present, and of this, it is all positrons and antiprotons – not a single antihelium or heavier anti-nucleus has been discovered. There are two types of cosmic rays: primary rays, which come directly from extrasolar sources, and secondary rays, which come from primary rays crashing into the interstellar medium and forming new particles through processes such as nuclear spallation. Particles resulting from cosmic ray collisions with the Earth’s atmosphere are also considered secondary cosmic rays – these include particles like pions, kaons, and muons, and their decay products.
Despite being discovered over a hundred years ago, cosmic rays remain in a lot of ways a big mystery. For one thing, we don’t know exactly where they come from. Because cosmic rays are generally electrically charged, they don’t travel to us straight from the source. Rather, they are accelerated this way and that by magnetic fields in space so that when they finally reach us they could be coming from any direction at all. Indeed, the cosmic ray flux that we see is completely isotropic, or the same in all directions.
Not only do they not come straight from the source, but we don’t even know what that source is. These particles move orders of magnitude faster than particles in our most powerful accelerators on Earth. Astronomers’ best guess is that cosmic rays are accelerated by magnetic shocks from supernovae. But even supernovae aren’t enough to accelerate the highest-energy cosmic rays. Moreover, there are features in the cosmic ray energy spectrum that we just don’t understand (see Fig. 2). Two kinks, a “knee” at about 1016 eV and an “ankle” at about 1018 eV could indicate the turning on or off of some astrophysical process. Experiments like the Pierre Auger Observatory were designed to study these ultra-high-energy particles and hopefully will tell up a little bit more about them in the next few years.
The AMS is primarily interested in lower-energy cosmic rays. For four years, ever since its launch up to the International Space Station, it’s been cruising the skies and collecting cosmic rays by the tens of billions. I will not address the experimental design and software here. Instead I refer the reader to one of my previous articles, “Dark Skies II- Indirect Detection and the Quest for the Smoking Gun”.
In addition to precision studies of the composition and flux of cosmic rays, the AMS has three main science goals: (1) Investigating the matter-antimatter asymmetry by searching for primordial antimatter. (2) Searching for dark matter annihilation products amidst the cosmic rays. And (3), looking for strangelets and other exotic forms of matter.
The very small fraction of cosmic rays made up of antimatter is relevant not just for the first goal but for the second as well. Not many processes that we know about can produce positrons and antiprotons, but as I mention in “Dark Skies II”, dark matter annihilations into Standard Model particles could be one of those processes. Any blips or features in the cosmic ray antimatter spectrum could indicate dark matter annihilations at work.
Fig. 3. The positron fraction measured by AMS. Image from L. Accardo et al. (AMS Collaboration), September 2014.
On April 14 at “AMS Days at CERN”, Professor Andrei Kounine of MIT presented the latest results from AMS.
The first part of Kounine’s talk focused on a precise characterization of the positron fraction presented by the AMS collaboration in September 2014 and a discussion of the relevant systematics. In the absence of new physics processes, we expect the positron fraction to be smooth and decreasing with energy. As you can see in Fig. 3, however, the positron fraction starts rising at approximately 8 GeV and increases steadily up to about 250 GeV. The curve hits a maximum at about 275 GeV and then appears to begin to turn over, although at these energies the measurements are limited by statistics and more data is needed to determine exactly what happens beyond this point. Models of dark matter annihilation predict a much steeper drop-off than do models where the positron excess is produced by, say, pulsars. Five possible sources of systematic error were identified, all of which have been heavily investigated. These included a small asymmetry in positron and electron acceptance due to slight differences in some of the bits of the tracker; variations in efficiency with respect to energy of the incoming particle; binning errors, which are mitigated due to high experimental resolution; low statistics at the tails of the electron and positron distributions; and “charge confusion”, or the misidentification of electrons as positrons, which happens only in a very small number of cases.
Kounine also presented a never-before-seen, not-yet-published measurement of the antiproton-proton ratio as measured by AMS, which you can see in Fig. 4. This curve represents a total of 290,000 antiprotons selected out of total of 54 billion events collected by AMS over the past 4 years. Many of the same systematics (acceptance asymmetry, charge confusion, and so on) as in the positron measurement are relevant here. Work on the antiproton analysis is ongoing, however, and according to Kounine it’s too soon to try to match models to the data.
Fig. 4. AMS’s latest antiproton-proton ratio measurement, from Prof. Andrei Kounine’s presentation at “AMS Days at CERN”.
As a dark matter physicist, the question in my mind is, do these measurements represent dark matter annihilations? Professor Subir Sarkar of Oxford and the Niels Bohr Institute in Copenhagen thinks not. In his talk at “AMS Days”, Sarkar argues that the dark matter annihilation cross-section necessary to match the positron flux seen by AMS and other experiments such as Fermi-LAT and PAMELA needs to be so large that by all rights the dark matter in the universe should have all annihilated away already. This is inconsistent with the observed dark matter density in our galaxy. You can get around this with theoretical models that incorporate new kinds of long-range forces. However, the observed antiproton flux, according to Sarkar, is consistent with background. Therefore dark matter would have to be able to annihilate into leptons (electrons and positrons, muons, neutrinos, and so on) but not quarks. Such models exist, but now we’re starting to severely restrict our model space. Moreover, dark matter annihilating in the early universe near the time of recombination should leave visible imprints in the Cosmic Microwave Background (CMB), which have not yet been seen. CMB experiments such as Planck therefore disfavor a dark matter explanation for the observed peak in positron fraction.
Sarkar then goes on to present an alternate model where secondary cosmic ray particles such as positrons are accelerated by the same mechanisms (magnetic shocks from supernovae, pulsars, and other cosmic accelerators) that accelerate primary cosmic rays. Then, if there are invisible accelerators in our nearby galactic neighborhood, as seems likely because electrons and positrons can’t propagate very far without losing energy due to interactions with starlight and the CMB, it could be possible to get very large fluctuations in the cosmic ray flux due purely to the randomness of how these accelerators are distributed around us.
Regardless of whether or not the AMS has actually seen a dark matter signal, the data are finally beginning to be precise enough that we can start really pinning down how cosmic rays backgrounds are created and propagated. I encourage you to check out some of the webcasts at “AMS Days at CERN” for yourself. Although the event is over the webcasts are still available in the CERN document archive here.