Knocking on Heaven's Door: How Physics and Scientific Thinking Illuminate the Universe and the Modern World - Lisa Randall (2011)
Part V. SCALING THE UNIVERSE
Chapter 21. VISITORS FROM THE DARK SIDE
When the LHC’s chief engineer, Lyn Evans, spoke at the California LHC/Dark Matter conference in January 2010, he closed by teasing the audience about how for the last couple of decades, “You theorists have been thrashing around in the dark (sector).” He added the caveat, “Now I understand why I spent the last fifteen years building the LHC.” Lyn’s comments referred to the paucity of high-energy data over the previous years. But they were also hints about the possibility that LHC discoveries might shed light on dark matter.
Many connections exist between particle physics and cosmology, but one of the most intriguing is that dark matter might actually be made at the energies explored by the LHC. The remarkable fact is that if a stable particle species with weak scale mass exists, the amount of energy carried by particles of this type that survived from the early universe to today would be about right to account for dark matter. The result of calculations about the amount of dark matter that is left over by an initially hot—but cooling—universe demonstrate that this might be the case. That means that not only is dark matter literally right under our noses, its identity might prove to be too. If dark matter is indeed composed of such a weak mass particle, the LHC might not only give us insight into particle physics questions, it might also provide clues to what is out there in the universe and how it all began—questions that are incorporated into the science of cosmology.
But LHC experiments are not the only way to search for dark matter. The fact is that physics has now entered a potentially exciting era of data, not just for particle physics, but also for astronomy and cosmology. This chapter explains how experiments in the upcoming decade will search for dark matter using a three-pronged approach. It first explores why weak-scale-mass dark matter particles are favored, and after that, how the LHC might produce and identify dark matter particles if this hypothesis is right. We’ll then consider how dedicated experiments that are specifically designed to search for dark matter particles look for their arrival to Earth and try to register their feeble but potentially detectable interactions. Finally, we’ll consider the ways in which telescopes and detectors on the ground and in space look for products of dark matter particles annihilating in the sky. These three different ways of searching for dark matter are illustrated in Figure 78.
[ FIGURE 78 ] Dark matter searches take a three-pronged approach. Underground detectors look for dark matter directly hitting target nuclei. The LHC might create dark matter that leaves evidence in its experimental apparatuses. And satellites or telescopes might find evidence of dark matter annihilating and producing visible matter out in space.
We know the density of dark matter, that it is cold (which is to say, it moves slowly relative to the speed of light), and that it interacts at most extremely weakly—certainly with no significant interaction with light. And that’s about it. Dark matter is transparent. We don’t know its mass, if it has any non-gravitational interactions, or how it was created in the early universe. We know its average density, but there could be one proton mass per cubic centimeter in our galaxy or there could be one thousand trillion times the proton mass stored in a compact object that is distributed throughout the universe every kilometer cubed. Either gives the same average dark matter density, and either could have seeded the formation of structure.
So although we know it’s out there, we don’t yet know the nature of dark matter. It could be small black holes or objects from other dimensions. Most likely, it is simply a new elementary particle that doesn’t have the usual Standard Model interactions—perhaps a stable neutral remnant of a soon-to-be-discovered physical theory that will appear at the weak mass scale. Even if that’s the case, we would want to know what the properties of the dark matter particle are—its mass and its interactions and if it is part of some such larger sector of new particles.
One reason the elementary particle interpretation is currently favored is the point alluded to above—the abundance of dark matter, the fraction of energy it carries—supports this hypothesis. The surprising fact is that a stable particle whose mass is roughly the weak energy scale that the LHC will explore (again via E = mc2) has a relic density today—the fraction of energy stored in the particles in the universe—in the right ballpark to be dark matter.
The logic goes as follows. As the universe evolved, the temperature decreased. Heavier particles that were abundant when the universe was hotter are much more dispersed in the later cooler universe since the energy at low temperature is insufficient to create them. Once the temperature dropped sufficiently, heavy particles efficiently annihilated with heavy antiparticles so that both of them disappeared, but the reverse process where they were created no longer occurred at any significant rate. Therefore, due to annihilation, the number density of heavy particles decreased very rapidly as the universe cooled down.
Of course, in order to annihilate, particles and antiparticles have to first find each other.69 But as their number decreased and they became more diffuse, this became less likely. As a consequence, particles annihilated less efficiently later in the universe’s evolution since it takes two of them in the same place to tango.
The result is that substantially more stable, weak-mass particles could remain today than a naive application of thermodynamics would suggest—at some point both particles and antiparticles became so dilute that they just couldn’t find and eliminate each other. How many particles are left today depends on the mass and the interactions of the putative dark matter candidate. Physicists know how to calculate the relic abundance if we know these quantities. The intriguing and remarkable fact is that stable weak-mass particles happen to be such that they are left with about the right abundance to be the dark matter.
Of course, since we know neither the exact particle mass nor the precise interactions (not to mention the model of which this stable particle might be a part), we don’t yet know if the numbers work out exactly. But the fortuitous, albeit rough, agreement between numbers associated with what on the surface appear to be two entirely different phenomena is intriguing, and might well be a signal that weak-scale physics accounts for the dark matter in the universe.
This type of dark matter candidate has become generically known as a WIMP, or a Weakly Interacting Massive Particle. The word “weak” here is a descriptive term and not a reference to the weak force—a WIMP would interact even more weakly than the Standard Model’s weakly interacting neutrinos. Without more direct evidence for dark matter and its properties of the sort the LHC might reveal, we won’t know whether dark matter indeed consists of WIMPs. This is why we need experimental searches such as those we now consider.
DARK MATTER AT THE LHC
The intriguing possibility of producing dark matter is one reason cosmologists are curious about the physics of the weak energy scale and what the LHC might find. The LHC has just the right energy to search for a WIMP. If dark matter is indeed composed of a particle associated with the weak energy scale as the above calculation suggests, it just might be created at the LHC.
Even if that’s the case, however, the dark matter particle won’t necessarily be discovered. After all, dark matter doesn’t interact a lot. Due to their limited interactions with Standard Model matter, dark matter particles certainly won’t be produced directly or found in a detector. Even if produced, they will just fly through. Nonetheless, all is not lost (even if the dark matter particle will be). Any solution to the hierarchy problem will contain other particles—most of which have stronger interactions. Some of these might be copiously produced and subsequently decay into dark matter that will then carry away undetected momentum and energy.
Supersymmetric models are the most well-studied weak scale models of this type that naturally contain a viable dark matter candidate. If supersymmetry applies in the world, the lightest supersymmetric particle (LSP) might constitute the dark matter. This lightest particle, which carries zero electric charge, interacts too weakly to be produced on its own sufficiently often to find. However, gluinos—supersymmetric partners of the strong-force-communicating gluons, and squarks—supersymmetric partners of quarks—would be created if they exist and are in the right mass range. And, as was discussed in Chapter 17, both of these supersymmetric particles would eventually decay into the LSP. So even though a dark matter particle wouldn’t be produced directly, decays of other more prolifically created particles could conceivably create LSPs at an observable rate.
Other weak-scale dark matter scenarios that have testable consequences would have to be produced and “detected” in much the same way. The mass of the dark matter particle should be around the weak scale energy that the LHC will study. Those particles won’t be produced directly because of their feeble interaction strength, but many models contain other new particles that could decay into them. We might then learn of the dark matter particle’s existence and possibly its mass through the missing momentum it carries away.
Finding dark matter at the LHC would certainly be a major accomplishment. If it is found there, experimenters could even study some of its properties in detail. However, to really establish that a particle found at the LHC indeed constitutes the dark matter would require supplementary evidence. That is what detectors on the ground and in space might provide.
DIRECT DETECTION DARK MATTER EXPERIMENTS
The LHC’s potential to create dark matter is certainly intriguing. But most cosmology experiments don’t take place at accelerators. Experiments on Earth and in space that are dedicated to astronomical and dark matter searches are primarily responsible for addressing and advancing our understanding of potential solutions to cosmological questions.
Of course, dark matter’s interactions with matter are very weak, so current searches rely on a leap of faith that dark matter, despite its near invisibility, nonetheless interacts feebly—but not impossibly so—with matter we know (and can build detectors out of). This isn’t merely a wishful guess. It’s based on the same relic density calculation mentioned above that shows that if dark matter is related to models proposed to explain the hierarchy problem, then the density of particles that remains is the correct amount to account for dark matter observations. Many of the WIMP dark matter candidates suggested by this calculation interact with Standard Model particles at rates that might well be detectable with the current generation of dark matter detectors.
Even so, because of dark matter’s feeble interactions, the search requires either enormous detectors on the ground or, alternatively, very sensitive detectors that look for the products of dark matter meeting, annihilating, and creating new particles and their antiparticles on Earth or in space. You probably wouldn’t win the lottery if you bought only a single ticket, but if you could buy more than half of what’s available, then your odds would be pretty good. Similarly, very large detectors have a reasonable chance of finding dark matter, even though dark matter’s interaction with any single nucleon in the detector is extremely small.
The challenging task for dark matter detectors is to detect the neutral—uncharged—dark matter particles, and afterward distinguish them from cosmic rays or other background radiation. Particles with no charges don’t interact with detectors in conventional ways. The only trace of a dark matter particle passing through a detector would be the consequences of hitting nuclei in the detector and changing its energy by a minuscule amount. Because this is the only observable consequence, dark matter detectors have no choice but to search for evidence of the tiny amounts of heat or recoil energy that get created when dark matter particles pass through. Detectors are therefore designed to be either very cold or very sensitive in order to record the small heat or energy deposits from dark matter particles subtly ricocheting off.
The very cold devices, known as cryogenic detectors, detect the small amount of heat emitted when a dark matter particle enters the apparatus. A small amount of heat added to an already hot detector would be too difficult to notice, but with specially designed cold detectors, the tiny heat deposit can be absorbed and recorded. Cryogenic detectors are made with a crystalline absorber such as germanium. Experiments of this sort include the Cryogenic Dark Matter Search (CDMS), CRESST, and EDELWEISS.
The other class of direct detection experiments involves noble liquid detectors. Even though dark matter doesn’t directly interact with light, the energy added to an atom of xenon or argon when a dark matter particle hits it can lead to a flash of characteristic scintillation. Experiments with xenon include XENONIOO and LUX, and the other noble liquid experiments, ZEPLIN and ArDM.
Everyone in the theoretical and experimental communities is eager to know what the new results from these experiments will be. I was fortunate to be present at a dark matter conference at the KITP in Santa Barbara organized in December 2009, by two leading dark matter experts, Doug Finkbeiner and Neal Weiner, when CDMS, one of the most sensitive dark matter detection experiments, was about to release new results. In addition to being young and tall contemporaries who had done their PhDs together at Berkeley, Doug and Neal both had a great understanding of dark matter experiments and what their implications might be. Neal had more of a particle physics background, and Doug had done more astrophysics research, but they converged on the topic of dark matter when it became clear that dark matter studies would involve both. At the conference, they had collected leading theoretical and experimental expertise on the subject.
The most riveting talk of the day occurred the morning I arrived. Harry Nelson, who is a professor at the University of California Santa Barbara, talked about year-old CDMS results. You might wonder why a talk about old results should receive so much attention. The reason was that everyone at the conference knew that only three days later the experiment would release new data. And rumors were flying that scientists at the CDMS experiment had actually seen compelling evidence of a discovery, so everyone wanted to understand the experiment better. For years theorists had listened to talks about dark matter detection but had listened primarily to their results and had paid only superficial attention to the details. But with imminent dark matter detection conceivable, theorists were eager to learn more. Later in the week, the results were released and disappointed the audience’s greatly exaggerated expectations. But at the time of the talk, everyone was absorbed. Harry steadfastly managed to give his talk despite the many probing questions about the soon-to-be-released results.
Because it was a two-hour informal presentation, those of us in attendance could interrupt whenever necessary to understand as much as possible. The talk nicely addressed questions that the audience, which consisted mostly of particle physicists, would find confusing. Harry, who was trained as a particle physicist—not as an astronomer—spoke the same language we did.
With these extraordinarily difficult dark matter experiments, the devil is in the details. Harry made that abundantly clear. The CDMS experiment is based on advanced low-energy physics technology—the kind more conventionally associated with so-called condensed matter or solid state physicists. Harry told us how before joining the collaboration he would never have believed such delicate detections could possibly work, joking that his experimental colleagues should be grateful he wasn’t a referee on the original proposal.
CDMS works very differently from scintillating xenon and sodium iodide detection experiments. It has hockey-puck-size pieces of germanium or silicon topped by a delicate recording device, which is a phonon sensor. The detector operates at very low temperature—low enough to be just at the border between superconducting and non-superconducting. If even a small amount of energy from phonons, the sound units that carry the energy through the germanium or silicon, much like photons are the units of light—hit the detector, it can be enough to make the device lose superconductivity and register a potential dark matter event through a device called a superconducting quantum interference device (SQUID). These devices are extraordinarily sensitive and measure the energy deposition extremely well.
But recording an event isn’t the end of the story. The experimenters need to establish that the detector is recording dark matter—not just background radiation. The problem is that everything radiates. We radiate. The computer I’m typing on radiates. The book (or electronic device) you’re reading radiates. The sweat from a single experimentalist’s finger is enough to swamp any dark matter signal. And that doesn’t even take into account all the primordial and man-made radioactive substances. The environment and the air as well as the detector itself carry radiation. Cosmic rays can hit the detector. Low-energy neutrons in the rock can mimic dark matter. Cosmic ray muons can hit rock and create a splash of material, including neutrons that can mimic dark matter too. There are about 1,000 times as many background electromagnetic events as predicted signal events, even with reasonably optimistic assumptions about the mass and interaction strength of the dark matter particles.
So the name of the game for dark matter experiments is shielding and discrimination. (This is the astrophysicists’ term. Particle physicists use the more PC term particle ID, though these days I’m not sure that’s so great either.) Experimenters need to shield their detector as much as possible to keep radiation out and discriminate potential dark matter events from uninteresting radiation scattering in the detectors. Shielding is ac-complished in part by performing the experiments deep in mines. The idea is that cosmic rays will hit the rock surrounding the detector before they hit the detector itself. Dark matter, which has far fewer interactions, will make it to the detector unimpeded.
Fortunately for dark matter detection, plenty of mines and tunnels exist. The DAMA experiment, along with experiments called XENONIO and the bigger version XENONIOO—as well as CRESST, a detector that uses tungsten—take place in the Gran Sasso laboratory, situated in a tunnel in Italy about 3,000 meters underground. A 1,500-meter-deep cavern in the Homestake mine in South Dakota, originally built for gold excavation, will be home to another xenon-based experiment known as LUX. This experiment will take place in the very same cavity where Ray Davis discovered neutrinos from nuclear reactions taking place in the Sun. The CDMS experiment is in the Soudan mine, about 750 meters underground.
Still, all that rock above the mines and tunnels is not enough to guarantee that the detectors are radiation-free. The experiments further shield the actual detectors in a variety of ways. CDMS has a layer of surrounding polyethylene that will light up if something too strongly interacting to be dark matter comes through from the outside. Even more memorable is the surrounding lead from an eighteenth-century sunken French galleon. Older lead that has been underwater for centuries has had time to shed its radioactivity. It is a dense absorbing material that is perfect for shielding the detector from incoming radiation.
Even with all these precautions, a lot of electromagnetic radiation still survives. Distinguishing radiation from potential dark matter candidates requires further discrimination. Dark matter interactions resemble nuclear reactions that occur when a neutron hits the target. So opposite the phonon readout system is a more conventional particle physics detector that measures the ionization created when the alleged dark matter particle passed through the germanium or silicon. Together, the two measurements, ionization and phonon energy, distinguish nuclear events—the good processes that might be the result of dark matter—from events due to electrons, which are just radioactivity induced.
Other beautiful features of the CDMS experiment include the excellent position and timing measurements that it can make. This is nice because although the position is only directly measured in two directions, the timing of the phonons gives the position in the third coordinate. So experimenters can locate exactly where the event happened and discard background surface events. Another nice feature is that the experiment is segmented into the stacked hockey-puck-size detectors. A true event will occur in only one of these detectors. Locally induced radiation, on the other hand, won’t necessarily be confined to a single detector. With all these features and an even better design to come, CDMS has a good chance of finding dark matter.
Nonetheless, impressive as it is, CDMS is not the only dark matter detector and cryogenic devices are not the only type. Later on in the week, Elena Aprile, one of the xenon experiment pioneers, gave comparable details about her experiments (XEN0N10 and XEN0N100), as well as other experiments performed with noble liquids. Since these would soon be the most sensitive detectors for dark matter, the audience paid rapt attention to her talk too.
Xenon experiments record dark matter events through their scintillation. Liquid xenon is dense and homogenous, has a large mass per atom (enhancing the dark matter interaction rate), scintillates well, ionizes fairly readily when energy is deposited so that the two types of signals described above can efficiently discriminate against electromagnetic events, and is relatively cheap compared to other potential materials—although the price had fluctuated by a factor of six in the course of the decade. Noble gas experiments of this type have become a lot better as they have gotten bigger, and they should continue to do so. With more material, not only is detection more likely, but the outer part of the detector can shield the inner part of the detector more efficiently, helping assure the significance of a result.
By measuring both ionization and the initial scintillation, experimenters distinguish signal from background radiation. The XEN0N100 experiment uses very special phototubes that were designed to work in the low-temperature, high-pressure environment of the detector to measure the scintillation. Argon detectors might provide even better scintillation information in the future through their use of the detailed shape of the scintillation pulse as a function of time, and that will also help separate the wheat from the chaff.
The strange state of affairs today (although this might soon change) is that one scintillation experiment—the DAMA experiment in the Gran Sasso Laboratory in Italy—has actually seen a signal. DAMA, unlike the experiments I just described, has no internal discrimination between signal and background. Instead it relies on identifying dark matter signal events solely by their time dependence, using the distinctive velocity dependence coming from the Earth’s orbit around the Sun.
The reason the velocity of incoming dark matter particles is relevant is that it determines how much energy is deposited in the detector. If the energy is too low, the experiment won’t be sensitive enough to know if anything was there. More energy means the experiment is more likely to record the event. Due to the Earth’s orbital velocity, the speed of dark matter relative to us (and hence the energy deposited) depends on the time of year—making it easier to see a signal at some times of year (summer) than at others (winter). The DAMA experiment looks for an annual modulation in the event rate that accords with this prediction. And their data indicates they have found such a signal. (See Figure 79 for the oscillating DAMA data.)
[ FIGURE 79 ] Data from the DAMA experiment showing the modulation of the signal over time.
No one yet knows for sure whether the DAMA signal represents dark matter or is due to some possible misunderstanding about the detector or its environment. People are skeptical because no other experiment has yet seen anything. This absence of other signals is inconsistent with the predictions of most dark matter models.
Although confusing for the time being, this is the sort of thing that makes science interesting. The result encourages us to think about what different types of dark matter might exist and whether dark matter might have properties that make it easier for DAMA to see it than other dark matter detection experiments. Such results also force us to better understand the detectors so that we can identify spurious signals and tell if the data mean what the experimenters claim.
Other experiments all over the globe are working to achieve greater sensitivity. They could either rule out or confirm the DAMA dark matter discovery. Or they might independently discover a different type of dark matter on their own. Everyone would agree that dark matter had been discovered if even one other experiment confirms what DAMA has seen, but this has not yet occurred. Nonetheless, answers should be available soon. Even if the results just presented are out of date when you read this, the nature of the experiments most likely won’t be.
INDIRECT DARK MATTER DETECTION
LHC experiments and ground-based cryogenic or noble liquid detectors are two ways to determine the nature of dark matter. The third and final way is through indirect detection of dark matter in the sky or on Earth.
Dark matter is dilute, but nonetheless occasionally annihilates with itself or with its antiparticle. This doesn’t happen enough to significantly affect the overall density, but it might be enough to produce a measurable signal. That’s because when dark matter particles annihilate, new particles get produced that carry away their energy. Depending on its nature, dark matter annihilation could sometimes yield detectable Standard Model particles and antiparticles, such as electrons and positrons, or pairs of photons. Astrophysical detectors that measure antiparticles or photons might then see signs of these annihilations.
The instruments that search for these Standard Model products of dark matter annihilation weren’t initially designed with this goal. They were conceived as telescopes or detectors out in space or on the ground to detect light or particles in order to better understand what is in the sky. By looking at what types of stuff gets emitted by stars and galaxies and exotic objects that lie within them, astronomers can learn about the chemical composition of astronomical objects and deduce the properties and nature of stars.
The philosopher Auguste Comte in 1835 mistakenly said about stars, “We can never by any means investigate their chemical composition,” which he thought beyond the boundary of attainable knowledge. Yet not too long after he said those words, the discovery and interpretation of the spectra of the Sun—the light that was emitted or absorbed—taught us about the composition of the Sun and proved him decidedly wrong.
Experiments today continue this mission when they try to deduce the composition of other celestial objects. Today’s telescopes are very sensitive, and every few months we learn more about what is out there.
Fortunately for dark matter searches, the observations of light and particles that these experiments are already engaged in might also illuminate the nature of dark matter. Since antiparticles are relatively rare in the universe and the distribution of photon energies could exhibit distinctive and identifiable properties, such detection could eventually be associated with dark matter. The spatial distribution of these particles might also help distinguish such annihilation products from more common astrophysical backgrounds
HESS, the High Energy Stereoscopic System located in Namibia, and VERITAS, the Very Energetic Radiation Imaging Telescopic Array System in Arizona, are large arrays of telescopes on Earth that look for high-energy photons from the center of the galaxy. And the next generation of very high-energy gamma-ray observatory, the Cherenkov Telescope Array (CTA), promises to be even more sensitive. The Fermi Gamma-ray Space Telescope, on the other hand, orbits the sky 550 kilometers above the Earth every 95 minutes on a satellite that was launched at the beginning of 2008. Photon detectors on Earth have the advantage of having enormous collecting areas, whereas the very precise instruments on the Fermi satellite have better energy resolution and directional information, are sensitive to photons with lower energies, and have about 200 times the field of view.
Either of these types of experiments could see photons from annihilating dark matter, or from radiation produced by electrons and positrons resulting from dark matter annihilation. If we see either, we stand to learn a lot about the identity and properties of dark matter.
Other detectors look primarily for positrons, the antiparticles of electrons. Physicists working on an Italian-led satellite experiment called PAMELA have already reported their findings, and they look nothing like what was predicted. (See Figure 80 for PAMELA results.) The acronym in this case stands for the mouthful “Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics,” which is somewhat mitigated by the nice way PAMELA sounds when spoken with an Italian accent. We don’t yet know if the PAMELA excess events are due to dark matter or to misestimations of astronomical objects such as pulsars. But either way, the results have absorbed the attention of astrophysicists and particle physicists alike.
[ FIGURE 80 ] Data from the PAMELA experiment, showing how badly experimental data (the crosses) agreed with theoretical predictions (the dotted curve).
Dark matter can also annihilate into protons and antiprotons. In fact, many models predict that this happens most frequently if dark matter particles do indeed find each other and annihilate. However, large numbers of antiprotons lurking in the galaxy due to known astronomical processes can mask the dark matter signal. Still, we might have a chance of seeing such dark matter through antideuterons, which are very weakly bound states of an antiproton and an antineutron, which might also be formed when dark matter annihilates. The Alpha Magnetic Spectrometer (AMS-02), now on the International Space Station, as well as dedicated satellite experiments, such as the General Antiparticle Spectrometer (GAPS), might ultimately find these antideuterons and thereby discover dark matter.
Finally, the uncharged particles called neutrinos that interact only via the weak force could be the key to the indirect detection of dark matter. Dark matter might get trapped in the center of the Sun or the Earth. The only signal that could get out in that case would be neutrinos, since unlike other particles, they won’t be stopped by their interactions as they escape. Detectors named AMANDA, IceCube, and ANTARES are looking for these high-energy neutrinos.
If any of the above signals is observed—or even if they are not—we will learn more about the nature of dark matter—its interactions and its mass. In the meantime, physicists have been thinking about what signal to expect according to predictions from various possible dark matter models. And of course we ask about what any existing measurements might imply. Dark matter is tricky, since it interacts so weakly. But the hope is that with the many different types of dark matter experiments currently in operation, dark matter detection may be within imminent reach, and along with results from the LHC and elsewhere will provide a better sense of what is out there in the universe and how it all fits together.