BUST: 34-Year Dark Matter Project Finds Nothing

By Tom Hartsfield


A grad student involved in starting the Baksan Underground Scintillator Telescope (BUST) project would now be almost 60 years old. And his experiment, which took more than half of his life to complete, would be just that: a bust. (How's that for irony?) Running since 1978, BUST collected 24 years of live data over 34 years of operation; the final null result was just submitted last week.

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But all is not lost. What were they looking for, and what does this "failure" teach us?

BUST was designed to look for evidence of dark matter. When we look out through telescopes at galaxies many light-years away, the amount of mass those galaxies need to stay together through gravitational attraction is far greater than the amount that appears to generate the light that we see from them. Dark matter is the fudge factor for the fact that we see roughly 80% less mass than we expect to see. It may also be a real thing.

Gravity as we know it doesn't seem to describe these galaxies correctly; it would however, if they had five times more mass in them than we observe. Given how well gravitational theories built by Newton and Einstein work, it's reasonable to conjecture that there really is a lot of stuff out there that we just can't see.

The essential characteristic of dark matter would have to be that it does not interact with any other matter except through gravity. If there were other interactions, those interactions would shoot off photons, which we could see with telescopes. That's why when we look for dark matter, we can't see it directly. Instead, we look for the signature byproducts of a dark matter particle hitting other particles. Muons would be the final byproducts in this particular case. 

To look for these muons, a giant telescope was built. But the telescope doesn't use lenses: instead it is a huge stack of tanks full of liquid. When a muon flies through a tank, there is a small chance that it may hit (interact with) a molecule of the special liquid inside. This causes the liquid to scintillate -- i.e., to emit light because its molecules absorb energy from the incoming muon.

Each tank is a bit more than two feet by two feet by a foot tall in size (70 cm x 70 cm x 30 cm), and there are 3150(!) of them. The tanks are arrayed as a stack of planar slabs 36 feet tall (11 m), and the whole stack is surrounded by light detectors. By detecting the light from a particle passing through each planar slab, the trajectory of a muon passing through the detector can be measured.

The goal is to see whether these trajectories point towards the sun. The sun is by far the most massive thing for several light-years around, so there would likely be more dark matter in its core than anywhere else nearby. Within the sun, some of the dark matter would probably self-annihilate. Two dark matter particles could crash into each other and be destroyed, sparking a shower of particles, such as muon neutrinos. These muon neutrinos almost never interact with matter, but when they occasionally do, another particle called a muon is created. This is the particle BUST was looking for.

The final twist in this story is that the detector looks not up at the sky, but down through the core of the earth. Many interactions occurring in the atmosphere cause muons to rain down on us constantly. Using the earth itself to shield the detector from many of these muons, the detector can more clearly see the (very) rare muon which was created by a neutrino from space hitting the nucleus of an atom deep inside the earth. Over 24 years, BUST found only 1255 of these muons, and each one was hand-checked. That's roughly only one per week!

The result of the experiment is a plot of muons detected from every direction in space. If far more muons were seen coming from the sun's direction, this would mean it is likely that dark matter annihilation is taking place, supporting our speculative theories of dark matter.

However, the data shows that there is absolutely no meaningful excess of muons measured coming from the sun's direction nor from any other direction. Thus, there is no concentration of the hypothetical dark matter particles of this type near the sun. (Since dark matter particles are only influenced by gravity, they would almost certainly be concentrated in the sun.)

This result places strong limitations on the existence of theorized dark matter. Further, the BUST finding corroborates other evidence collected from newer technology, including machines built 20 or 30 years later. This is truly an amazing experiment, despite its discovery of nothing!

Tom Hartsfield is a Ph.D. candidate in physics at the University of Texas and a regular contributor to the Newton Blog.

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