On The Heels of a Ghost: a Modern Glimpse into The Search for Dark Matter (Part 2)

The ancient gramophone played a tune.

It must have moved, or did I miss it do something so obvious just before my eyes? Perhaps the tune had been playing the whole time and I didn’t notice?

The ancient gramophone sat there. Yet the striking thing was not the sound it played at all, but the fact that it gleamed clean mahogany, fresh as it was the day it was first purchased.

Recall our Ghost Hunter that we imagined in Part 1. He can approach his hunt in one of two ways: directly and indirectly, akin to how our physicists must approach their experiments toward dark matter. Let’s explore a few current examples, and perhaps it could shed light on how our Ghost Hunter might approach his next hunt, as it seems this time he has missed his chance.

DEAP-3600 is an experiment at SNOLAB in Sudbury, Canada, and Coherent Captain Mills (CCM), is an experiment conducted at Los Alamos National Lab in New Mexico. They are experiments that each exploit scintillation to be able to measure and record events associated with dark matter. When a high energy particle is absorbed by a scintillating material it will be re-emitted as lower energy light, which defines the scintillation phenomenon. Liquid Argon is regarded as an excellent material for these scintillation techniques, not only because of its low cost and chemical inertness, but also because of the sheer amount of photons it produces in response to what it absorbs. Additionally, it is invisible to its own scintillation light. It is the ideal candidate, however it needs some help. Photomultiplier Tubes are used to aid in amplifying incident photons such that the detectors they are paired with can better detect events which began as the incident energy scintillating through the liquid argon.

In the DEAP-3600 experiment, they seek to directly detect Weakly Interacting Massive Particles (WIMPs), which have been the premiere candidate for dark matter a while. They are slow moving and heavy in comparison to other particles which is a means of justifying the formation of galaxies like we mentioned last time. You might ask yourself, “If there is so much excess mass in the form of dark matter, then perhaps dark matter is made up of massive particles.” The logic seems sound: Big Mass comes from Big Ghosts. Do you think the 3600-Litre DEAP detector is a big enough net to catch a WIMP?

The CCM experiment instead seeks to indirectly detect dark matter by confirming new physics regarding the neutrino. You remember the neutrino right? No charge, very small mass. There’s 3 kinds of them: tau neutrino , electron neutrino, and muon neutrino. In recent years, oscillations of neutrinos were confirmed: They switch forms in transit. What CCM hopes to confirm is if a supposed “sterile neutrino” exists during that form change as a middle step.

Global experiments have provided hints toward the possibility of sterile neutrinos due to anomalies in data, this experiment hopes to provide an irrefutable answer: yes or no. While active neutrinos (tau, electron, muon) interact with the weak nuclear force (their gravitational interaction is disregarded), sterile neutrinos would not interact with even the weak force. This could be a link to physics beyond the Standard Model. However, their Liquid Argon detector is set up to seek each type of neutrino. In doing so they offer the indirect detection of some dark matter phenomena too.

“Another exciting possibility that CCM will test is the production and detection of light sub-GeV dark matter, which will not require any experimental innovations – only a reanalysis of the neutrino data. In principle, dark matter could belong to a complex dark sector, which, like the Standard Model, could be comprised of several new particles and gauge interactions. Also belonging to this dark sector are “mediators”, i.e., particles that communicate interactions between the dark and visible sectors.”

– Richard Van De Water, Coherent Captain Mills Experiment project proposal, Los Alamos National Lab

A dark mediator sounds to me like a dark photon. That’s spooky.

So what can we draw from these experiments to recommend to our Ghost Hunter?

  1. If you have a big signature, you may have a Big Ghost, and therefore ought to use a Big Net. The detectors of our physicists are profound in size, and if they prove fruitless, well, maybe you just need a bigger one? Next gen detectors are on their way such as: Darkside 20-K, and LUX-Zeplin which up the scale even further.
  2. Use the location to your advantage; one haunted house is not equivalent to another. A dark matter facility below 2km of rock such as SNOLAB is quite different than the ones in the hills of New Mexico. They’ll see a range of particles with different incident energy, which constrains the search.
  3. Consider a more subtle way of searching. If you can outthink the ghost, and predict which corporeal things it might effect, maybe you can catch it in the act! Just so with dark matter experiments. While directly observing dark matter might be a glorious achievement, perhaps a more subtle indirect approach is better. Even if a dark matter confirmation can’t be found, any new scientific insights gained will help refine our search for the future.

At the end of the day it comes down to the ability to detect, and measure things. Without those means, you’ll just look like yore swiping around in the dark. The experiments mentioned are only a glimpse of what’s on going right now, but there are plenty more occurring and upcoming. It’s a game of persistence and creativity. Remember, these are potentially the first threads into the web of physics Beyond the Standard Model. So be patient in the face of failure. Whether physicists seek ground-breaking new science, or ghost hunters seek their next ghostly quarry, wouldn’t you agree that the thrill of the hunt is the best part?

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