![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
DEAP thoughts...
Oct. 19th, 2017 09:07 pmI just got back from a talk on the DEAP-3600 experiment by DEAP Project Director, Dr. Mark Boulay. While I didn't learn much in the way of physics per se, it certainly gave me an appreciation of the scientific and engineering feat of building such an instrument 2km underground (spoiler: everything is radioactive in some way, but DEAP-3600 is one of the least radioactively contaminated instruments ever built, and certainly in a class of its own considering its size: the "3600" is for the number of kilograms of liquid argon that makes up the detector volume).
So, the background is that we have discovered that everything we have ever measured in human history (which is described by a stunningly successful framework called The Standard Model of particle physics) is about 5% of what the universe is made of. About 27% is made of something we call dark matter, and the remaining 69% or so is made of something we call dark energy. We know almost nothing about dark matter except that we have seen its gravitational effects when we look out into the universe (it's very convincing once you see all the evidence), but we have no idea what it might be and there have been no direct observations of such a thing and we're not even sure what to look for. We know absolutely nothing about dark energy except that it is making the universe expand faster and faster (again, the evidence is strong, but our clues are zero). So, we really don't have a lot of clues about what makes up 95% of our universe!
The DEAP-3600 experiment is looking for a dark matter candidate called a WIMP. Yup. A WIMP, which stands for Weakly Interacting Massive Particle. We know dark matter exists, but we've never observed anything but its gravitational effects. Based on experiments, we are pretty darned sure that dark matter doesn't interact electromagnetically (it is dark after all), or seemingly with the strong force (which holds atomic nuclei and certain subatomic particles like protons and neutrons together). That leaves two forces: gravity (which we know it interacts with), and the weak force (which is responsible for radioactive decay and other stuff). I asked this evening and confirmed that the "Weakly Interacting" in WIMP doesn't mean that it doesn't interact much (well, it doesn't, but...) but means that they are hoping that it interacts with regular matter (that 5% we know anything about) through the weak force. I also confirmed that if dark matter doesn't interact through the weak force, that DEAP-3600 and all the other detectors being built around the world to look for dark matter would not be able to detect it.
When I think of something tiny but "massive"... that it has weight in a gravitational field (inertia wherever it is)... running into an atomic nucleus or something, I envision a little marble hitting a billiard ball. In such a case, the billard ball will absord some energy from the marble (and if there's no friction, it would start to move in the opposite direction from the marble that hit it... remember, "equal and opposite reaction"). But a marble hitting a billiard ball is actually an electromagnetic interaction (when you try to push your hands together, it is light being emitted by the electrons in your atoms keeping your hands from passing through each other and forcing them apart when they get close... what we feel as "solid"). In these dark matter detectors, that are relying on the weak force, the recoil of the nuclei would be due to the exchange of the weak force particles (the Z and W bosons) and you still get an energy transfer kind of like the way the electromagnetic force works (in fact, the two forces have been combined mathematically into a single "electroweak" theory, but that's another story). But... if they don't interact weakly, they would only interact gravitationally and would likely pass right through the nuclei of atoms as if they weren't there (as if matter was empty space). So that was my revelation this evening: trying to imagine how particles (if that's what we want to call them) that only interact gravitationally would behave as they passed through matter. And if so, could we ever detect such particles (see the video above on The Standard Model to see how weak gravity is compared to the other forces)? Another thought that occurred to me (in my ignorance) is to consider the hypothesized "graviton", which is supposed to be a very, very small particle (again, if it exists at all), but instead ask a different question: if the Higgs particle is a resonance in the Higgs field that permeates the universe, could dark matter be a similar resonance in the gravitational field? But where we think of gravitons as tiny, maybe these resonances are huge (maybe there are different order resonances, but the higher order resonances of other quantum fields would be too massive to make any difference in our universe, but since gravity is so weak, higher order resonances could possibly exist and have an effect). Until I take quantum field theory, there is no way to know if what I'm saying is hogwash or not (I'm flying purely intuitively, which is a strength of mine, but quantum theory does not always lend itself to interpretation through intuition). I guess that would also make them GIMPs...
So, the background is that we have discovered that everything we have ever measured in human history (which is described by a stunningly successful framework called The Standard Model of particle physics) is about 5% of what the universe is made of. About 27% is made of something we call dark matter, and the remaining 69% or so is made of something we call dark energy. We know almost nothing about dark matter except that we have seen its gravitational effects when we look out into the universe (it's very convincing once you see all the evidence), but we have no idea what it might be and there have been no direct observations of such a thing and we're not even sure what to look for. We know absolutely nothing about dark energy except that it is making the universe expand faster and faster (again, the evidence is strong, but our clues are zero). So, we really don't have a lot of clues about what makes up 95% of our universe!
The DEAP-3600 experiment is looking for a dark matter candidate called a WIMP. Yup. A WIMP, which stands for Weakly Interacting Massive Particle. We know dark matter exists, but we've never observed anything but its gravitational effects. Based on experiments, we are pretty darned sure that dark matter doesn't interact electromagnetically (it is dark after all), or seemingly with the strong force (which holds atomic nuclei and certain subatomic particles like protons and neutrons together). That leaves two forces: gravity (which we know it interacts with), and the weak force (which is responsible for radioactive decay and other stuff). I asked this evening and confirmed that the "Weakly Interacting" in WIMP doesn't mean that it doesn't interact much (well, it doesn't, but...) but means that they are hoping that it interacts with regular matter (that 5% we know anything about) through the weak force. I also confirmed that if dark matter doesn't interact through the weak force, that DEAP-3600 and all the other detectors being built around the world to look for dark matter would not be able to detect it.
When I think of something tiny but "massive"... that it has weight in a gravitational field (inertia wherever it is)... running into an atomic nucleus or something, I envision a little marble hitting a billiard ball. In such a case, the billard ball will absord some energy from the marble (and if there's no friction, it would start to move in the opposite direction from the marble that hit it... remember, "equal and opposite reaction"). But a marble hitting a billiard ball is actually an electromagnetic interaction (when you try to push your hands together, it is light being emitted by the electrons in your atoms keeping your hands from passing through each other and forcing them apart when they get close... what we feel as "solid"). In these dark matter detectors, that are relying on the weak force, the recoil of the nuclei would be due to the exchange of the weak force particles (the Z and W bosons) and you still get an energy transfer kind of like the way the electromagnetic force works (in fact, the two forces have been combined mathematically into a single "electroweak" theory, but that's another story). But... if they don't interact weakly, they would only interact gravitationally and would likely pass right through the nuclei of atoms as if they weren't there (as if matter was empty space). So that was my revelation this evening: trying to imagine how particles (if that's what we want to call them) that only interact gravitationally would behave as they passed through matter. And if so, could we ever detect such particles (see the video above on The Standard Model to see how weak gravity is compared to the other forces)? Another thought that occurred to me (in my ignorance) is to consider the hypothesized "graviton", which is supposed to be a very, very small particle (again, if it exists at all), but instead ask a different question: if the Higgs particle is a resonance in the Higgs field that permeates the universe, could dark matter be a similar resonance in the gravitational field? But where we think of gravitons as tiny, maybe these resonances are huge (maybe there are different order resonances, but the higher order resonances of other quantum fields would be too massive to make any difference in our universe, but since gravity is so weak, higher order resonances could possibly exist and have an effect). Until I take quantum field theory, there is no way to know if what I'm saying is hogwash or not (I'm flying purely intuitively, which is a strength of mine, but quantum theory does not always lend itself to interpretation through intuition). I guess that would also make them GIMPs...
