For my inaugural post a few months ago I discussed dark matter direct detection and the search for WIMPs deep underground. As a graduate student on the Large Underground Xenon (LUX) experiment, this is the area that I am most familiar with, but it is by no means the only way to hunt for these elusive particles. The very idea of dark matter was first motivated by problems in astronomy (such as understanding the rotation curves of galaxies), so what better way to look for it than to turn our telescopes to the skies?
The best way to get an intuition for the physics behind dark matter detection is to look at the Feynman diagrams representing interactions between dark matter particles and standard model particles. For example, the relevant interactions in the direct detection of WIMPS have the general form:
Feynman diagrams are conventionally drawn with time as the horizontal axis, increasing as you go from left to right. In this particular diagram a WIMP χ and a standard model particle, which I somewhat un-creatively call sm, come in from the left, interact at the vertex of the diagram, and then a WIMP χ and a standard model particle sm, leave on the right. (Here I have deliberately obscured the vertex, since there are many possible interactions and combinations of interactions that yield Feynman diagrams with the same initial and final particle states.) More succinctly, we can think of this diagram as a WIMP χ and a standard model particle sm scattering off each other. Direct detection experiments like LUX or the Cryogenic Dark Matter Search (CDMS) look specifically for WIMPs scattering off protons or neutrons in an atomic nucleus, so the relevant Feynman diagrams are:
Feynman diagrams are kind of beautiful in that you can draw a diagram for most any particle interaction you can think of; you can flip it, rotate it, and smoosh it around; and because of certain symmetry considerations you will in general still end up with something representing a completely valid, physically-allowed particle interaction.
Let’s do this with our direct detection diagram. If we just rotate it a quarter-turn, we end up with the following:
We can interpret this as a two WIMPs colliding and annihilating to form standard model particles in a way analogous to how electron-positron annihilation produces photons. WIMPs might be Majorana particles, i.e. their own antiparticles, or they might be Dirac particles, that is, distinct from anti-WIMPs, but the bottom line is still the same: the detection of the annihilation products can be used to deduce the presence of the initial WIMPs. (It might also be that WIMPs are unstable and therefore decay into standard model particles, in which case we could also look for their decay products.)
“Indirect” detection is the rather apt name for the technique of searching for WIMPs by trying to detect the products of their annihilation to standard model particles.
This strategy presents an entirely different set of challenges than direct detection. For one thing, you can’t shield against backgrounds in the same way that you can with direct detection experiments. After all, your signal consists of ordinary standard model particles, albeit standard model particles from an exotic origin, so any attempt to shield your experiment will just block out your desired signal along with the background. So where LUX is a “zero-background” experiment, indirect detection experiments look for signals that manifest themselves as tiny excesses of events over and above a large background. Additionally, indirect detection requires that WIMPs in the universe be both abundant enough and close enough together that there is a non-negligible probability for annihilation to occur. If in fact WIMPs are the answer to the dark matter problem then this was most certainly true in the early universe, but today, cosmologists estimate the local density of WIMPs to be approximately 0.3 GeV/c2/cm3. This corresponds to only about three WIMPs per cubic meter! This is a challenge indeed, but luckily there are a few places in the universe where gravity helps us out.
First of all, we can look for WIMPs in the centers of galaxies, where gravity helps coalesce both standard model and exotic massive particles into higher-density clumps. Here there are a number of annihilation processes we can search for. For instance, we can look for WIMPs annihilating directly into gamma rays, in which case the signal would be a mono-energetic peak in the gamma ray spectrum:
Note that as in my direct detection diagrams I have deliberately obscured the vertex of this diagram. Because WIMPs by definition do not interact electromagnetically they cannot convert directly into photons. However, the interaction represented in this particular diagram could take place if it contains an intermediate step where WIMPs convert first into a non-photon standard model particle. Then this intermediate particle could produce a photon final state.
The galactic center is not the easiest place to search for rare events. Here, the hunt for gammas from WIMP annihilations is complicated by the existence of many bright, diffuse gamma backgrounds in the from astrophysical processes that are not necessarily well-understood. In addition, the density profile of our WIMP halo is not well-understood near the center of our galaxy. It might be that our dark matter halo has a very dense “cusp” near the center; on the other hand it might very well be that the dark matter density in our halo increases up to a point but then plateaus to a constant density toward the center of the galaxy. Regarding the latter, understanding this density profile is an active area of research in computational and observational cosmology today. After all, if we don’t know how much dark matter is in the center of our galaxy, then how can we predict what an annihilation signal in that location might look like?
In order to mitigate the first of these complications, we can look to galaxies other than our own. In the Milky Way’s Local Group there are a number of galaxies called “dwarf spheroidals” which have extremely low luminosities, little to no interstellar gas and dust, and as a result, much less overall background than in our own galaxy. This sort of environment might therefore be very conducive to the indirect detection of WIMPs.
We can also look for WIMPs annihilating into heavy standard model particles. Generally these decay rapidly, producing jets that in turn yield a whole continuous spectrum of gammas and other particles. Schematically, we can summarize this process as:
Perhaps the most interesting products of these annihilations are the antimatter particles produced in these jets. The matter/antimatter asymmetry in the universe is a whole other mystery to be solved, but it does provide for us a fairly conclusive smoking-gun WIMP signal. Antimatter in the universe is rare enough that a large flux of antimatter particles could suggest WIMP annihilation events are taking place. Some classes of indirect detection experiments look for positron excesses; others look for antiprotons or antideuterons. On the other hand, these experiments are also complicated by the existence of other cosmic-ray backgrounds and the diffusion of these annihilation products in the Earth’s atmosphere. Understanding and modeling the (non-WIMP-related) processes that produce cosmic rays is also a very active area of research.
Finally, we expect there to be high WIMP densities in the sun’s gravitational potential well. This means that we could conceivably hunt for WIMPs much closer to home and not have to worry about backgrounds from other sources in the galaxy. There is a catch, however. The sun is so incredibly dense that the mean free path of, say, a photon inside its center is only about a centimeter. Each particle that escapes to its surface can only do so after going through a random walk of many, many absorptions and re-emissions. On average, this can take as many as hundreds of thousands or even millions of years! Neutrinos are the sole exception: they interact so weakly with other standard model particles that for the most part they just zip straight through the sun with no problem. Searches for dark matter annihilations in the sun therefore focus on neutrino-producing processes.
Neutrinos themselves are difficult to detect, but fortunately we do have technologies that are capable of doing so.
Over the next decade or so, I predict that indirect detection will be a very hot topic in particle physics (and not just because I really like dark matter!) There are a number of clever experiments that have already produced some interesting results, and several more scheduled to be constructed over the next few years. Stay tuned, because there will be a Part II to this article that will look at some of these experiments in detail.