So much in science is based on constraints. If scientists don’t understand something, they try to constrain it as much as possible so that more precise experiments can finally detect whatever the theorized phenomenon is. Dark matter is notoriously difficult in this regard, as it has evaded detection for over a century at this point, despite even more precise instruments trying to capture a glimpse of it. One of those instruments is the Super Cryogenic Dark Matter Search (SuperCDMS), run by the SLAC National Laboratory and located in northern Minnesota. To help further the cause, researchers looked at the data from the experiment while considering a few new possibilities, and while they didn’t find any evidence of dark matter, they helped tighten the constraints even more.
SuperCDMS, like most dark matter detection experiments, relies on the theory of dark matter that believes it interacts with ordinary matter in some way, shape, or form. Most likely, this interaction will take the form of some sort of interaction with the nuclear of an atom it runs into.
When dark matter collides with an atom, there are two possibilities for what happens. First is an “elastic collision” – simply thought of as when billiard balls bounce into one another, as Noah Kurinsky, a staff member at SLAC and corresponding author on a new paper describing this research, points out. Data from SuperCDMS has already been thoroughly scouted for evidence of those types of interactions. However, there is another option – an “inelastic” collision.
In such a collision, the energy isn’t transferred to the atom’s nucleus but its electrons, or even photons, if they happen to be present. And there are two main ways they could potentially have such a collision.
One is called Bremsstrahlung radiation, meaning “braking radiation” in German. In this case, while the dark matter particle would still hit a nucleus, some energy would be transferred directly into a photon. Detectors at the SuperCDMS would then be able to pick up the increased energy of that photon and deduce that an inelastic dark matter collision could have caused it.
Bremsstrahlung radiation has been experimentally observed before, but the other potential inelastic collision method, known as the Migdal effect, has not yet. In this case, the dark matter particle again hits the nucleus, but the nucleus itself shifts ever so slightly, and the cloud of electrons surrounding it must shift as well. This shift should theoretically be detectable by experiments such as SuperCDMS, and indeed other theorists have calculated what that shift might look like in an experiment.
Unfortunately, after reanalyzing data previously collected by SuperCDMS, Dr. Kurinsky and his colleagues found no evidence of dark matter. However, they added some additional constraints by showing that a dark matter particle’s total “mass” (equivalent to the energy in high-end physics) is less than one-fifth the mass of a proton.
But they didn’t stop there. Another way the detector might not see the results they expect based on these new interaction theories is if the dark matter particles don’t make it all the way to the detector in the first place and get swallowed up by Earth’s ground or even the atmosphere beforehand. That would help to put an upper bound on the minimum level of mass dark matter to reach the detector.
To do this calculation, the physicists collaborated with another group of scientists not well-known for their interest in particle physics – geologists. That collaboration allows the physicists to calculate how much energy a dark matter particle would lose depending on which direction through the Earth it was passing. Calculations show a dramatic difference in lost energy as it passed the entire way through the Earth and its core to reach the mine in Minnesota where SuperCDMS is located finally, and another dark matter particles that might come from straight overhead and meet the path of least resistance into the detector.
Again, these calculations didn’t turn up any smoking gun of dark matter, so the search continued. However, they were able to say that dark matter must be below the energy limit that would allow the particles (if that is indeed what they are) to pass through to the detector, even via the most direct route. They can also confidently say that if it has enough energy to make it to the detector, it must have a mass of less than 1/5th of a proton.
Now the search continues, with more and more scientists joining it every year. And with more and more detectors coming online every year, maybe there is some hint of dark matter’s existence hiding in the data sets of other detectors just waiting to be analyzed using this new framework. But until then, details of this most mysterious of materials will continue to elude us.
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Graphic of the SuperCDMS facility layout
Credit – SLAC