The search for dark matter is intriguing. It is, quite literally, a bullet in the dark. Although scientists are sure of this dark matter It exists – since all the normal matter in the universe simply can’t explain the way galaxies hold together – they don’t know what it is. They also don’t really know where he is (although they do have some ideas). They certainly don’t know what it looks like.
However, the physics community is keen to investigate these elusive particles because the dark side of our universe represents an alarming 95% of our universe when taken into account. dark energythe invisible force that accelerates the expansion of space.
But how does one analyze something without really knowing what to analyze? Well, there is one way. Although we don’t know what dark matter is yet, scientists can slowly discover what it isn’t.
This is exactly what several researchers dedicated to the research have recently done by sifting through data captured by a detector buried deep inside a mine in Minnesota. Although they haven’t found evidence of dark matter, they say they’ve set one of the narrowest limits ever to detect this phenomenon one day. had a full outline of their results published In June in Physical Review D.
“It’s all about mindset in science, where a null result can be just as impactful as a positive one,” Daniel Jardine, study co-author and postdoctoral researcher at Northwestern University, told Space.com. “Obviously it was cool to find dark matter, but we were able to cut out a new slice of dark matter parameter space.”
Related: “Hidden” photons can shed light on the mysterious dark matter
These latest discoveries are related to the Super Cryogenic Dark Matter Search (SuperCDMS) collaboration, of which Jardin is a member.
To put it briefly, the team concluded that the experimental SuperCDMS detector can now exclude dark matter particles down to about About one-fifth the mass of a proton – and possibly fewer masses.
“I’ve always loved chasing the unknown, and this is as big as it gets,” Jardine said. “I am very happy that my career has led me here, and however short it may be, I can always say that this result was the best in the world until other trials inevitably catch up.”
Wait, what is SuperCDMS?
To capture evidence of dark matter particles, the SuperCDMS collaboration is working with an experiment also referred to as SuperCDMS.
This experiment essentially harnesses the power of detectors that can determine if and when a dark matter particle (whatever it is) collides with the atomic nuclei of the materials embedded in the detectors themselves, specifically either germanium or silicon.
“I’ve been interested in space since I was little because it makes everything on Earth seem so small and insignificant,” Jardine said. “Then I learned about dark matter and couldn’t believe that all the stars and galaxies and things we see in the night sky make up less than 5% of the universe.”
Getting a little more technical, SuperCDMS can determine whether these dark matter particles are involved in what are known as “elastic collisions.” If they did, what would happen is that any energy lost by the dark matter particle when it broke up would be transferred to the motion of the affected atomic nuclei. In turn, the two bits will bounce back.
It would be like two billiard balls hitting each other only to bounce slightly backwards on the table, says study co-author Noah Korinsky, a scientist at the SLAC National Accelerator Laboratory.
But here’s the thing.
SuperCDMS apparently hasn’t found any elastic collisions yet – according to Jardin, we’d have heard about it by now because such a discovery would likely earn a Nobel Prize. However, this team of researchers, including Rob Calkins, a research assistant professor at Southern Methodist University, came up with an intriguing question.
What if SuperCDMS catches another type of collision no one has been looking for all this time? In particular, inelastic collisions.
Given these new findings, they were definitely on to something.
“Looking for elastic collisions remains a major driver of SuperCDMS, but looking at inelastic collisions has opened up an avenue for a dark matter parameter space to which the experiment was previously insensitive,” Jardine explained.
There are two ways a possible collision of inelastic dark matter might work. The first, according to the team, has to do with something called Bremsstrahlung radiation. At the detector, if this type of inelastic collision occurs, the dark matter particle will transfer some of its energy to a light particle, or photon, instead of just bouncing off as in the billiard ball example.
Although, on the other hand, an inelastic collision may occur through something called Midgall effect. If this release occurred, a dark matter particle hitting the nucleus would cause the nucleus itself to be thrown out of position, messing up the electron cloud’s distribution. Upon returning to their original place, some of those flowing electrons will be expelled.
At the risk of oversimplification, this means that the team was looking for SuperCDMS signals of either a flying photon or a lone electronic hardcore.
“It wasn’t as easy as counting,” Jardine noted. “This analysis used spectral shapes to model the power profile of the signal as well as several known background sources.”
And after all that, the search came up empty – but the story didn’t end here.
Jardin continued, “Then we used statistics to answer the question,” What is the probability that we will see the signal against the known background? This question is repeated hundreds of thousands of times and we leave out the parameter space where we should have been able to see the signal and we couldn’t.
There is always a silver lining
“There are about a billion particles of dark matter passing through you every second, but they interact so rarely that you can’t tell,” Jardine said. “We’re looking for a 1 in a billion billion billion billion interaction opportunity.”
Although this golden ticket was not found, other forms of treasure came to light.
Above all, all those statistical studies on the SuperCDMS signals ended up providing the team with their conclusions about potential low-mass limits for dark matter particles.
“Another dark matter experiment that was not as sensitive to dark matter mass as low as SuperCDMS for elastic collisions published a similar analysis that expanded its reach and flattened the playing field. Reading that, we wondered how far lower we could go if we used the same method,” Jardine said.
In addition, he explained, the team also “added more to the analysis such as more complex statistics and the inclusion of interactions with Earth.”
Yes, the Earth
Perhaps even more impressive is the way the team took into account that all LandIts position in space can influence these dark matter signals.
As they show, if dark matter interacts strongly enough with things, it’s likely interacting with literally everything in its path on its way to our tiny underground Earth detectors. One of those things that are ripe for interaction is our planet’s atmosphere.
The team reasoned that if a dark matter particle interacted with our atmosphere, this planetary shield would have taken some of the particle’s energy by the time we pick up its signal.
“Dark matter is thought to be more or less ubiquitous in a large ball around the galaxy,” Jardine said. Our solar system is in a spiral arm milky way That is, the earth revolves around the sun and the earth rotates on its own axis. This astronomical motion means that Earth is passing through a sea of dark matter particles, but from our perspective, dark matter particles seem to be constantly bombarding Earth and our detectors.”
Thus, the researchers realized that there is likely to be an upper energy limit that these interactive dark matter particles can punch through — if these are reactive, that is.
By modeling things like the density of Earth’s atmosphere, working with geologists to figure out what types of rocks are above the Minnesota mine where SuperCDMS is buried and tons of other variables, they’ve already discovered the upper dark matter energy boundary.
“When you fit a line to some data, there are two parameters: regression and intercept,” Jardine said. “In this analysis, we had more than 50 fit criteria simultaneously.”
As for what’s next, well, Jardine says this Sherlock Holmes-style deduction will continue. And if any of that occurs to your mind, emphasize a visual way of looking at team scores that puts everything into perspective.
“This result — the black lines — leaves out some new parameter space that others haven’t accessed before, but there is a lot more open space to the left, less blocks up and down, which represents less opportunity for interaction,” he said. These are getting increasingly difficult to investigate, but dark matter physicists are smart.
These dark matter hunters have certainly reached for the stars and managed to land softly on the moon.
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