Dark matter is a tough thing to study. There is no getting around it: any strategy we can come up with to look for these invisible mystery particles must hinge on the sneaky little creatures interacting in some way with ordinary Standard Model particles. Otherwise we haven’t got even the slightest chance of seeing them.
One of the most popular classes of dark matter candidates is the Weakly Interacting Massive Particles (WIMPs), so called because they do not interact electromagnetically, only weakly, with ordinary matter. In direct detection we look for WIMPs that interact by scattering off of a Standard Model particle. In contrast, indirect detection looks for interactions that consist of a dark matter particle (either a WIMP or a non-WIMP — it doesn’t matter) annihilating with another dark matter particle or decaying on its own into Standard Model particles. These Standard Model end products we have a good chance of detecting if we can just get our backgrounds low enough. In my last post, “Dark Skies: A Very Brief Guide to Indirect Detection,” I gave a more detailed look at the kinds of annihilation and decay products that we might expect from such a process and spoke briefly about some of the considerations that must go into a search for particles from these annihilation and decay processes. Today I will highlight three of the indirect detection experiments currently attacking the dark matter problem.
The Alpha Magnetic Spectrometer (AMS-02) is a large indirect detection experiment situated on the International Space Station. I am especially excited to talk to you about this experiment because just a couple of days ago AMS-02 released a very interesting result. Although I include a link to the press release and the relevant papers below, I intend to give away the punchline by summarizing here everything I know about the AMS-02 experiment and their result from this past week.
Fig. 1: (Left) A 3D rendering of the AMS-02 detector, from the AMS-02 group website located at www.ams02.org. (Right) A schematic of the AMS-02 detector showing all of its various subsystems .
First, let’s talk about the design of the experiment (Fig. 1). As the infamous Shrek once said, ogres are like onions. Well, most big particle physics experiments are like onions too. They consist of many layers of detectors interspersed with different kinds of shielding – the quintessential example being big collider experiments like ATLAS and CMS at the Large Hadron Collider. AMS-02 has just as many layers and almost as much complexity to it as something like ATLAS. In no particular order, these layers are:
- A donut-shaped superconducting magnet surrounding most of the AMS-02 detector. Any particles traversing through the hole in the donut will be deflected by the magnet, and particles of different charges are deflected in different directions. The magnet therefore is an effective way to separate positrons from electrons, positive ions from negative ones, and antimatter from ordinary matter.
- Veto or anticoincidence counters (ACCs) that lie just inside the superconducting magnet. The ACCs tag any particle that enters the detector through the side rather than the hole in the magnet thus allowing the AMS-02 to reject particle events that do not have well-understood trajectories.
- A Transition Radiation Detector (TRD) consisting of twenty layers of material (that provides discrimination between extremely high-energy leptons (positrons, electrons) and hadrons (protons, etc) that are traveling near the speed of light. Each time an electron or positron passes through the TRD, it produces a shower of x-rays as it crosses the interface between layers, but a proton or other hadron does not.
- The Time of Flight (ToF) system, which consists of 4 layers of scintillation counters, two above and two below the detector that measure the time it takes for a particle to traverse the detector and also serve as a trigger system, indicating to the other subsystems that a particle has entered the detector.
- The tracker, which consists of eight layers of double-sided silicon sensors that record the path of any particle that enters the the detector through the magnet. By the time a particle has reached the tracker, the magnet has already done half the work by separating positively-charged from negatively-charged particles. By looking at the trajectory of each particle inside the tracker, the AMS-02 can not only determine which particles are positive and negative but also the atomic number Z of any nucleus that enters the detector, hence the “Spectrometer” part of “Alpha Magnetic Spectrometer.”
- A Ring Imaging Cherenkov detector (RICH) which measures particle velocities by looking at the shape of the Cherenkov radiation that is produced by incident particles.
- An electromagnetic calorimeter (ECAL) consisting of a dense piece of material inside which incident particles produce secondary showers of particles. By looking at these particle showers, the ECAL helps to discriminated between leptons (electrons, positrons, gammas) and hadrons (e.g. protons) that, if they have the same trajectory through the magnet, might be otherwise impossible to tell apart.
Although it sounds complicated, the combined power of all these various subdetectors allows AMS-02 to do a spectacular job of characterizing many different types of particles. Here, the particles relevant to dark matter detection are antimatter particles such as positrons, antiprotons, and antideuterons. In the absence of exotic processes, we expect the spectra of these particles to be smoothly decreasing, isotropic, and generally well-behaved. Any bumps or features in, say, the positron or antiproton spectra would indicate some new process at work –like possibly WIMP annihilations .
Fig. 2: The first positron fraction measurement produced by the AMS-02 collaboration, released in April 2013 .
Back in April 2013, AMS-02 released its first measurement of the fraction of positrons as compared to electrons in cosmic rays (Fig 2). Clearly the curve in Fig. 2 is not decreasing – there is some other source of positrons at work here. There was some small speculation among the scientific community that the rise in positron fraction at higher energies could be attributed to dark matter annihilations, but the general consensus was that this shape is caused by a more ordinary astrophysical source such as pulsars. So in general, how do you tell what kind of positron source could cause this shape of curve? The answer is this: if the rise in positron fraction is due to dark matter annihilations, you can expect to see a very sharp dropoff at higher energies. A less exotic astrophysical source would result in a smooth flattening of this curve at higher energies .
Fig. 3: An updated positron fraction measurement from two years’ worth of data released by the AMS-02 collaboration on September 18, 2014 .
On September 18, 2014, AMS-02 released a followup to its 2013 result covering a larger range of energies in order to further investigate this positron excess (Fig. 3). The positron fraction curve does in fact begin to drop off at higher energies. Is it a smoking-gun signal of WIMP annihilations? Not yet – there are not enough statistics at high energies to differentiate between a smooth turnover or an abrupt drop in positron fraction. For now, the AMS-02 plans to investigate the positron flux at even higher energies to determine the nature of this turnover and to do a similar measurement with the antiproton fraction as compared to regular protons.
For a webcast of the official press release, you can go here. Or, to read about the AMS-02 results in more detail, check out the references  and  at the bottom of this article.
The Fermi Large Area Telescope (LAT) is another indirect detection experiment that has seen hints of something interesting. In this particular experiment, the signal of interest comes from gamma rays of energies ranging from tens of MeVs to more than 300 GeV. The science goals are to study and catalog localized gamma-ray sources, to investigate the diffuse gamma-ray background in our part of the universe, and to search for gamma rays resulting from new physics processes, such as dark matter annihilations.
Because the Earth’s atmosphere is not transparent to gamma rays, our best chance to study them lies out in space. The Fermi-LAT is a very large space-based observatory which detects gammas through a mechanism called pair conversion, where a high-energy photon rather than being reflected or refracted upon entering a medium converts into an electron-positron pair. Inside the LAT, this conversion takes place inside a tracker module in one of several thin-yet-very-dense layers of tungsten. There are sixteen of these conversion layers total, interleaved with eighteen layers of silicon strip detectors that record the x- and y- position of any tracks produced by the electron-positron pair. Beneath the tracker modules is a calorimeter, consisting of a set of CsI modules that absorb the full energy of the electrons and positrons and therefore give us a good measure of the energy of the original gamma ray. Finally, the entire detector is covered by an anticoincidence detector (ACD) consisting of plastic scintillator tiles that scintillate when charged particles pass through them but not when gamma rays pass through, thereby providing a way to discriminate the signal of interest from cosmic ray backgrounds (Fig. 5).
One of the nice things about the Fermi telescope is that it not only has a wide field of view and continually scans across the entire sky, but it can also be pointed at specific locations. Let’s consider for a moment some of the places we could point the Fermi-LAT if we are hoping to detect a dark matter signal .
- The probability of dark matter annihilations taking place is highest in regions of high density like our galactic center, but there is a huge gamma-ray background to contend with from many various astrophysical sources. If we look further out into our galactic halo, there will be less background, but also less statistics for our signal of interest. And there is still a diffuse gamma-ray background to contend with. However, a very narrow peak in the gamma spectrum that is present across the entire sky and not associated with any one particular localized astrophysical source would be very suggestive of a dark matter signal – exactly the kind of smoking gun we are looking for.
- We could also look at other galaxies. Certain kinds of galaxies called dwarf spheroidals are particularly promising for a number of reasons. First of all, the Milky Way has several close dwarf neighbors, so there are plenty to choose from. Second, dwarf galaxies are very dim. They have few stars, very little gas, and few gamma-ray sources like pulsars or supernova remnants. Should a gamma signal be seen across several of these dwarf galaxies, it would be very hard to explain by any means other than dark matter annihilation.
In spring 2012, two papers came out one after another suggesting that a sharp gamma peak had indeed been found near the galactic center, which you can see in Fig. 6 [7, 8]. What is the cause of this feature? Was it some kind of instrumental effect? A statistical fluctuation? Was it the dark matter smoking gun? The Fermi-LAT team officially commented on these papers later that November, reporting a feature that was much less statistically significant and much closer to 135 GeV and consistent with gamma rays produced by cosmic rays in the earth’s atmosphere .
Fig. 6: The gamma-ray spectrum reported by Fermi in 2012 . The black points show the number of counts observed in each energy bin. The green markers represent the best-fit background-only model, the red markers represent the best-fit background + WIMP annihilation model, and the blue dots represent the best-fit WIMP annihilation model with the background subtracted off.
This gamma line has been an active target of study since 2012. In 2013, the Fermi-LAT group released a further investigation of this feature using over 3.7 years of data. A bump in the spectrum at about 133 GeV was again observed, consistent with the 2012 papers, but with decreased statistical significance in part because this feature was narrower than the energy resolution of the LAT and because a similar (yet smaller) feature was seen in the earth’s “limb”, or outermost edges of the atmosphere . The hypothesis that this bump in the gamma-ray spectrum is a WIMP signal has all but fallen out of favor within the scientific community.
In the meantime, Fermi-LAT has also been looking for gamma rays from nearby dwarf galaxies. A survey of 25 dwarf galaxies near the Milky Way yielded no such statistically-significant signal . For the next few years, Fermi will continue its search for dark matter as well continuing to catalog and investigate other astrophysical gamma-ray sources.
Last but certainly not least, I wanted to discuss one of my particular favorite experiments. IceCube is really cool for many reasons, not the least of which is because it is situated at the South Pole! Like LUX (my home experiment), IceCube consists of a large array of light sensors (photomultiplier tubes) that look for flashes of light indicating particle interactions within a large passive target. Unlike LUX, however, the target medium in IceCube is the Antarctic ice itself, which sounds absolutely fantastical until you consider the following: if you go deep enough, the ice is extremely clear and uniform, because the pressure prevents bubble formation; and if you go deep enough it becomes very dark, so that any flashes of light inside the ice will stand out.
In IceCube, neutrinos are the main particles of interest. They are the ninjas of the particle physics world – neutrinos interact only very rarely with other particles and are actually rather difficult to detect. However, when your entire detector consists of a giant chunk of ice 2.5 kilometers deep, that’s a lot of material, resulting in a not-insignificant probability that a passing neutrino will interact with an atom inside your detector. A neutrino interacting with the ice will produce a shower of secondary charged particles, which in turn produce Cherenkov radiation that can be detected. Neutrinos themselves are pretty awesome on their own, and there is a wealth of interesting neutrino research currently taking place. They can also tell us a lot about a variety of astrophysical entities such as gamma-ray bursts and supernovae. And even more importantly for this article, neutrinos can occur as a result of dark matter annihilations.
Unfortunately for the dark matter search, muons and neutrinos produced by cosmic ray interactions in the atmosphere are a major source of background in the detector. Muons, because they are quite heavy, tend to travel long distances in most materials. Luckily, they don’t travel nearly as far as neutrinos – we’re talking on the order of only a few meters on average before they attenuate in a medium like ice. Neutrinos travel vastly further, so a good way to discriminate between cosmic-ray muons and neutrinos is to eliminate any downwards-traveling particles. Any upwards-traveling particle tracks must be from astrophysical neutrinos, because only they can traverse the entire diameter of the Earth without getting stopped somewhere in the ground. To put it more succinctly: IceCube makes use of the entire Earth as a shield against backgrounds! Atmospheric neutrinos are a little more difficult to distinguish from the astrophysical neutrinos relevant to dark matter searches, but are nevertheless an active target of study and are becoming increasingly better understood.
Now that we’ve talked about the rationale of building gigantic detectors out of ice and the kinds of signals and backgrounds to expect in such detectors, let’s talk the actual design of the experiment. IceCube consists of 86 strings of 60 digital optical modules, each consisting of a photomultiplier tube and a readout board, deployed between 1.5 and 2.5 kilometers deep in the Antarctic ice. How do you get the modules down there? Only with the help of very powerful hot-water drills. The drilling of these holes and the construction of IceCube is exciting enough that it probably warrants its own article.
Fig. 8: A schematic of the IceCube experiment. Note the Eiffel Tower included for scale. Image from .
Alongside the strings that make up the bulk of the detector, IceCube also contains a couple of other subdetectors. There is a detector called IceTop on the surface of the ice that is used to help veto events that are atmospheric in origin. There is another detector called DeepCore that consists of additional strings with optical modules packed much more tightly together than the regular strings for the purpose of looking of increasing the sensitivity to low-energy events. Other special extensions designed to look for very high and very low energy events are also planned for the near future.
With regards to the quest for dark matter, the IceCube strategy is to focus on two different WIMP annihilation models: χχ to W+W- (or τ+τ- for WIMPs that are lighter than W bosons) and χχ to b b-bar. In each case, the decay products produce secondary particles, including some neutrinos. By examining neutrinos both from the sun and from other galaxies and galaxy clusters, IceCube has produced a very competitive limit on the cross section for dark matter annihilation via these and other similar annihilation modes [12, 13].
Fig. 9: IceCube’s 2012 limit on dark matter – proton spin dependent interactions. All of the black curves correspond to different neutrino models. Image from .
For more information, there is a wonderful in-depth review of the IceCube detector design and status at the Annual Review of Nuclear and Particle Science.
So there you have it. These are some of the big projects keeping an eye out for WIMPs in the sky. At least some of these experiments have seen hints of something promising, so over the next couple years maybe we’ll finally get that five-sigma discovery that we want so badly to see.
 AMS Collaboration. “High statistics measurement of the positron fraction is primary cosmic rays of 0.5-500 GeV with the Alpha Magnetic Spectrometer on the International Space Station.” Phys. Rev. Lett. 113 (2014) 121101.
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 AMS Collaboration. “Electron and positron fluxes in primary cosmic rays measured with the Alpha Magnetic Spectrometer on the International Space Station.” Phys. Rev. Lett. 113 (2014) 121102.
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 Bloom, Elliott. “The search for dark matter with Fermi.” Conference presentation – Dark Matter 2014, Westwood, CA. http://www.pa.ucla.edu/sites/default/files/webform/ElliottBloom_UCLADMMeeting_022614_FinalkAsPlacedOnDM2014Website.pdf.
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