Hunting for Ghosts: The Race to Find Dark Matter
“The most terrifying fact about the universe is not that it is hostile but that it is indifferent, but if we can come to terms with this indifference, then our existence as a species can have genuine meaning. However vast the darkness, we must supply our own light” — Stanley Kubrick
Humans have achieved some remarkable feats of technological advancement and scientific discovery over the course of the past 200 years. We have scoured the night sky for celestial events with powerful Earth and space based telescopes since Galileo to peer back into time and space. We have unlocked the nature of genetics and can now manipulate the biology of Homo Sapiens and all other cell-based life on Earth. We have constructed devices that connect us digitally through the internet, defining the dawn of the Information Age. We have bounced around near weightlessly on the Lunar surface, glancing back at the cosmic oasis we call home. It may then come as a shock to the less scientifically-inclined person that the brightest minds in physics today have been unable to account for or detect 85% of the universe’s mass. You read that right, we have yet to discover the true nature of this sneaky and elusive particle or whatever it really is (could be a gravitational effect, possibly from extra dimensions). We have some good guesses, but technically speaking, we really don’t know yet.
Dark matter, as physicists like to call it, is an ever pervasive yet incredibly elusive substance that plays a crucial role in the evolution and structure of the universe. It was first suggested by Swiss Astronomer Fritz Zwicky in the early 1930’s to explain the shocking observation that galaxies occupying the massive Coma Cluster seemed to behave as if gravitationally bound by a colossal distribution of non-luminous matter in the cluster to which Zwicky coined the term “dunkle Materie” or “Dark Matter”. This was pretty big news but it didn’t spark real attention until in 1968, Astronomer Vera Rubin noticed the same phenomenon Zwicky described by studying galactic rotation curves and the motion of stars within the galaxies to come to the startling yet reassuring conclusion that there was something deeply flawed about our understanding of matter in the universe. Rubin measured the amount of shifting in frequency from stars in a distant galactic disk as they whirl around the center of the galaxy. The light from the stars coming towards us in the rotating galaxy is shifted towards blue wavelengths since they are moving faster towards us while the stars on the other side are moving away, shifting to more red wavelengths. Rubin observed that contrary to their expectations of a falling off of the velocities as you move away from the center in accordance with conventional Newtonian mechanics, the velocity did not in fact fall off, it remained relatively constant near the edge of the disc. This was baffling. It verified Zwicky’s and others suspicions of an unseen substance gluing everything in the universe together.
Rubin’s claims of an invisible substance that dominated the universe’s mass distribution and was likely responsible for holding galaxies together was initially met with skepticism, not surprisingly, Rubin being a woman in physics at a time when women had very little scientific credence in academia and society as a whole. The scientific community quickly realized Rubin’s claims were valid and there was indeed an omnipresent, non-luminous, weakly interacting form of matter that has evaded science since Newton’s Principia and the dawn of our understanding of matter and gravity.
Physicists and Astronomers from around the world swiftly began to construct instruments and telescopes capable of surveying and researching the phenomenon of Dark Matter from all different fronts and fields, ushering in an explosion of interest in the topic. There are essentially three methods for detecting an invisible substance like Dark Matter:
- Direct Detection: Observing the direct interaction between a Dark Matter particle and a detector medium through light and charge.
- Indirect Detection: Observing the effects of the annihilation of Dark Matter particles into visible Standard Model constituents.
- Make it: Creating Dark Matter particles with particle accelerators and observing missing energy in the detector.
Now, there are some genuinely decent ideas about the nature and origin of Dark Matter, but again we are really just spit-balling here, that’s science. The most popular is the concept of Weakly Interacting Massive Particles or WIMPs. They are just what they sound like, weakly interacting massive particles that do not couple to the electromagnetic force (hence the invisibility), are practically transparent to normal matter, and are extremely abundant, apparently. We are almost certain it is some type of particle since our current understanding of matter and energy boils down to the idea of particles just being excitations of energy in the quantum fields that permeate the universe. This theory has worked out pretty well so far (actually, like really well, it’s super accurate) and it’s the dominant theory for explaining any type of matter we observe (or don’t). Other popular theoretical culprits are Axions, teeny-tiny little particles that zip around undetected throughout the universe. Ideas range from senile black holes, neutrinos, extra-dimensions, and dark photons all the way to frustrated physicists babbling about which theory is better (just kidding). Seriously though, it could literally be anything within reason. The parameter space for Dark Matter is notoriously vast and varies in mass range from particles that are dwarfed by the proton to bodies that are ~30 times the mass of the Sun. There’s a lot of work to do for physicists. Luckily they’re persistent and curious creatures, devising ingenious traps and experiments to catch Dark Matter red-handed.
While there are many experiments and research groups investigating the origin of Dark Matter, there are just a couple that could be on the verge of detecting the ever elusive particle by designing the perfect environment for Dark Matter to show its true colors. The goal is to create a region of space on Earth where we can rigorously scrutinize each interaction as either signal or background events. Background events are the noise, the junk that flies off of radioactive elements such as Radon and different isotopes of elements that are present in everything around us, even the material the detector is made from. All of these backgrounds must be accounted for in order to detect Dark Matter. Creating such an environment is anything but trivial. Backgrounds are so incredibly dominant that current WIMP detectors must be shielded from cosmic radiation by placing them deep within the Earth in mine shafts and mountain tunnels. Even underground, physicists must be extremely meticulous and careful about radioactive contamination and possible sources of background.
The principle of operation for these popular types of liquid Xenon particle detectors is pretty simple and elegant:
- A dark matter particle passes through Earth’s surface and into the detector medium
- The dark matter particle collides with the Xenon nucleus within the detector, exciting and ionizing electrons in its orbital which emits light at a specific frequency, read as the first light signal (S1) by Photomultiplier Tubes at the bottom and top of the detector.
- The ionized electrons released by the collision drift upwards in a strong, uniform electric field oriented vertically.
- The electrons reach the liquid-gas interface near the top of the detector which can then accelerate through the xenon gas causing an even larger light signal read by the PMTs at the top, the second (S2) signal.
These two signals, S1 and S2, tell physicists nearly everything they need to know about the location and type of interaction that caused it. The trick is discriminating which interactions come from background events and which are from Dark Matter particles. It’s a bit like trying to find a specific type of needle in a stack of needles; very difficult. It boils down to eliminating everything that could be a background and whatever is left unexplained could be a Dark Matter interaction.
America’s frontier into Dark Matter is spearheaded by the LUX-ZEPLIN collaboration, the fusion of two previous Dark Matter experiments, LUX, and ZEPLIN III, both Xenon Time Projection Chamber detectors that searched for the particles and pushed further into the sensitivity parameter space than ever before. While this experiment is funded by the U.S. Department of Energy, physicists from around the world work together to search for this one particle. The LUX-ZEPLIN experiment is a giant 7 ton vat of liquefied xenon housed nearly a mile underground down a decommissioned mine shaft at the Sanford Underground Research Facility (SURF) in South Dakota. The detector is currently scheduled to begin taking data in 2020. Concurrent rival experiments are racing alongside LUX-ZEPLIN in the search, competing to achieve the highest sensitivity and push deeper into the unexplored mass regions of interest.
The race for finding Dark Matter is a race for the best sensitivity. The plot shown above is a sensitivity plot showing WIMP-nucleon cross section on the y-axis and WIMP mass on the x-axis in GeV. It essentially shows the expected sensitivity, or how well our detectors can search certain parameter spaces. It also shows a big orange region at the bottom. This is the neutrino floor, an irreducible background of solar neutrinos that will begin to show up as backgrounds in the detector if it reaches such sensitivities. The rival experiment, XENONnT, aims to achieve similar sensitivity as L-Z. XENONnT is buried deep within a mountain at the Gran Sasso National Laboratory in Central Italy.
These experiments represent Homo Sapiens deepest curiosities at play as we search for the true nature of reality and the origin of matter and energy. It is no different than voyaging into uncharted lands to discover something that was there all along but was never attainable until now. Humans had to create the most radioactively quiet environment in the known universe in order to be sensitive enough to discover Dark Matter, setting up a clever trap for the most elusive substance known to man. If Dark Matter exists as we think it does, it must obey what we believe to be the laws of physics. If it obeys the laws of physics, it should have a chink in its highly elusive armor, giving physicists a shot at catching it in the act.
Dark Matter particles are likely whizzing through your face and body as we speak, yet you won’t feel a thing. Billions of particles of different types beam through our bodies each second, mostly neutrinos from the sun. Neutrinos are essentially transparent to ordinary matter, able to pass through a light year of lead without interacting with any atoms. It is a miracle of physics and human ingenuity that we have been able to see them and study their wildly bizarre characteristics such as oscillations and mysterious interstellar origin as high energy neutrinos bombard the planet constantly. Dark Matter is similar in this nature, yet has proven to be much more elusive. If the current experiments underway searching for Dark Matter are successful in their pursuits, it would constitute one of the most significant discoveries in the history of our species. There is a physics beyond our understanding and it is waiting to be uncovered by conscious beings that inhabit and appreciate the beauty of the universe.