by Noah Kurinsky
For nearly 100 years, we have known that we can’t account for the vast majority of the matter we observe in the Universe. Observational evidence such as galaxy rotation curves and gravitational lensing have pointed to additional mass that we simply can’t see.
During the middle of the 20th century, as astrophysicists searched the cosmos, particle physicists at colliders like Fermi’s Tevatron found thousands of new particles, most of which are much heavier than normal atoms and decay extremely quickly. So when we began searching for what is now known as dark matter (DM), we looked for something that might fit into what was called the "particle zoo."
Without success.
Dark matter’s stubborn resistance to discovery has forced us to reevaluate what it may look like. If it is much lighter than we’ve assumed, there must be more of it around to make up the total mass required to hold galaxies together. Our challenge: the signals these lighter particles would leave in terrestrial detectors are smaller than any we’ve ever set out to measure. To answer that challenge, DM physicists are constructing the coldest, quietest, most sensitive particle detectors ever made.
Understanding low-mass dark matter interactions
A single DM particle of 10 GeV (10,000,000,000 electron volts; eVs are a common unit of mass/energy in particle physics), roughly the mass of a carbon atom, can deposit a total energy of about 1 keV in a detector. This is equivalent to a single soft X-ray photon, or about 500 photons of red light. Detecting keV-scale, or even eV-scale energies is common; experiments like LZ and SuperCDMS SNOLAB (both with KIPAC involvement, and discussed previously in this series in Aug 2017 and Nov 2014) have already laid much of the groundwork in this area.
Now consider a DM particle a million times lighter. What changes in a search for this type of particle? Well, for one thing, because we know the DM mass density of the solar system, we expect its number density to be dramatically increased. So, for a detector of a given size, if we expect one 10 GeV DM particle to pass through each year, we’d expect one 10 keV DM particle every 30 seconds.
The number we would actually observe also depends on how DM interacts with normal matter, but in most cases the number density increase works to our advantage. For our current benchmark DM model in the low-mass regime, we expect the event rate in the detector to be roughly one interaction per second for about 100 grams of material. For most solid detectors, 100g of detector material would be smaller than an iPhone (yes, even the mini).
The measurement problem: Tradeoffs between event rate and energy
You might say, well, Noah, if these events are happening so often, wouldn’t we see them constantly? Unfortunately, despite the large event rate, two issues work against us:
- The maximum energy we could extract from a 10 keV DM particle bound to our galaxy is 30 meV—that’s milli-, not mega-, or about 30 thousandths of an eV—roughly equivalent to long-wavelength infrared light, for which no single-event detectors currently exist. Worse yet, photons at these energies are emitted by all objects at room temperature. Without special handling, any such detector would be awash in light from trillions of interactions.
- Dark matter is so much lighter than any particles in our detector that, in a billiard ball-type collision, it will bounce off the nuclei or electrons without even depositing a substantial fraction of its energy, let alone the maximum! (See Figure 2, below.)
We can mitigate the first point by building detectors much more sensitive than typical infrared detectors and operating them at near absolute zero to eliminate infrared radiation from the environment. This is a challenging problem to solve, and a focus of the research performed in our lab.
Developing new technologies—quantum sensors for dark matter detection
With the second point we get a bit lucky because extremely light DM particles behave more like quantum mechanical matter-waves, without finite size or position. Their interactions with our detectors would produce collective excitations, called phonons, which are like sound waves and can be detected as heat energy. Going even further, for theorized ultra-light DM particles called axions (discussed previously here in Oct 2015 , Nov 2019 and Aug 2020), the wave-like nature of the interaction is even more pronounced; the detector acts like a tiny energy-collecting net, held fixed in a vast and steady “axion” wind.
Developing new technologies—quantum sensors for dark matter detection
Most quantum sensing labs, like ours, focus on detecting tiny energies for science and technology applications. In order to reduce thermal vibrations that would otherwise obscure the science under study, these experiments typically operate at 270 C below the freezing point of water. We like to operate at temperatures near 10 mK, which is as close to absolute zero as we can get right now.
Another reason for such frigid operating temperatures: Our sensors are made from low-temperature superconductors. The energy needed to flip them back to the non-superconducting (or normal) state is ~ 1000 times smaller than required by conventional detectors. By precisely measuring resistance changes in our sensors we can measure event energies far smaller than those detected by other means. A greater understanding of superconductors directly contributes to improving our ability to detect dark matter.
As a parting thought, note the term "quantum sensors." This is a precise term relating to the quantum nature of interactions at extremely low energy scales, which also applies to quantum computing. In fact, in principle a quantum computer can also be used to detect dark matter. This has led many people (including myself) to think about how qubits could be made more sensitive to dark matter to exploit their extreme sensitivity to small energies.
The author would like to thank his colleagues Kelly Stifter and Betty Young for their help with this post.
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