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  • richardmitnick 3:01 pm on December 5, 2018 Permalink | Reply
    Tags: , COSINE-100 Dark Matter Experiment - Yale University, , Dark Matter search,   

    From Science Magazine: “Underground experiment casts doubt on controversial dark matter claim” 

    From Science Magazine

    Dec. 5, 2018
    Adrian Cho

    COSINE-100 at Yangyang underground laboratory in South Korea

    DAMA at Gran Sasso uses sodium iodide housed in copper to hunt for dark matter LNGS-INFN

    Gran Sasso LABORATORI NAZIONALI del GRAN SASSO, located in the Abruzzo region of central Italy

    Now, the first experiment designed to directly test DAMA’s controversial claim has released its first data. Physicists working with the COSINE-100 detector in South Korea say they see no sign of dark matter—but still need a couple more years to really put the screws to the DAMA claim.

    “They can’t rule out the DAMA signal yet,” says Katherine Freese, a theoretical physicist at the University of Michigan in Ann Arbor who is not involved in either experiment. “But the exciting thing is that they’ll be able to rule it out.” Or, as may be less likely, confirm it.

    Astrophysical observations show invisible dark matter makes up 85% of all matter. Our own galaxy is thought to reside within a vast cloud of the stuff. However, scientists still don’t know what dark matter is. For decades, experimenters have hunted for particles of it floating about, mostly to no avail. To search for dark matter, physicists deploy ultrasensitive detectors deep underground, where they are shielded from cosmic rays and other background radiation.

    However, since 1998, the DAMA collaboration has claimed to have seen just such a signal. The team’s detectors consist of sodium iodide crystals doped with thallium, which produce flashes of light when a particle of some sort—regular or dark matter—strikes a nucleus within the material and sends it flying. The DAMA team has seen a yearly variation in the collision rate that could be a strong sign of dark matter, as Freese and a colleague predicted in 1986.

    If our Milky Way galaxy is shrouded in dark matter, then as the sun wheels about the galactic center, it should regularly plow into a wind of dark matter particles. Moreover, as Earth circles the sun, it should alternately rush into and out of that wind, causing the rate of dark matter collisions to wax and wane over the course of the year. If dark matter consists of theorists’ favorite candidate particle, known as weakly interacting massive particles (WIMPs), the signal should peak in June and bottom out in December—just what DAMA sees.

    Several other detectors have failed to see the signal. However, those detectors use heavier elements such as xenon, silicon, and germanium for the target nuclei, DAMA researchers say, which could explain the discrepancy. “Even taking those results as they are, considering the large experimental and theoretical uncertainties there could be space for compatibility,” says Rita Bernabei, a physicist at the University of Rome Tor Vergata and leader of the DAMA team.

    To sort through the confusion, COSINE researchers built a detector that also uses thallium-doped sodium iodide crystals. “I got into this field to test the DAMA result, and I was surprised others hadn’t,” says Reina Maruyama, a physicist at Yale University and co-spokesperson for the 50-member COSINE team. Since 2016, the 106-kilogram detector has been collecting data 700 meters underground at Yangyang Underground Laboratory, on South Korea’s eastern coast. And its first 59.5 days of data show no sign of dark matter, COSINE researchers report today in Nature.

    So does the COSINE result nix the DAMA claim? Not quite. With only 2 months of data, COSINE researchers couldn’t look for the telltale annual variation, but simply looked for an excess of events above the backgrounds created by extraneous radiation. The lack of an excess rules out the possibility that DAMA is seeing WIMPs, Maruyama says. But Bernabei says the test is too weak to do that. “The modeling of a background is a quite uncertain procedure and at low energy is in general not reliable,” she says.

    However, Freese says WIMPs are already ruled out—by DAMA’s own data. The argument is tricky, but if dark matter particles are WIMPs, which are presumed to interact with the nucleus in a particularly simple way, then the peaks and valleys in DAMA’s annual cycle should shift by 6 months for lower-energy events, Freese explains. And low-energy data that DAMA presented earlier this year show an unshifted oscillation. DAMA could be seeing some other kind of dark matter particle, Freese says. Bernabei argues that DAMA could still be seeing WIMPs.

    All agree that to really put the DAMA claim to the test, COSINE researchers will have to look for the same annual variations that DAMA sees—which can help pull a weaker signal out of the background. COSINE already has 2 years of data in the can, Maruyama says, and it will need another 3 years to make that test. Two other experiments are also trying to directly challenge the DAMA result with sodium-iodide detectors.

    Ultimately, all physicists hope to detect dark matter. So Maruyama says she would “love to” reproduce the DAMA signal. If COSINE cannot do that, Freese says, “We may never know what created the DAMA signal.”

    See the full article here .


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  • richardmitnick 11:27 am on March 6, 2017 Permalink | Reply
    Tags: , COSINE-100 Dark Matter Experiment - Yale University, , , , Laboratori Nazionali del Gran Sasso in Italy, , Women in STEM - "Meet the South Pole’s Dark Matter Detective" Reina Maruyama,   

    From Nautilus: Women in STEM – “Meet the South Pole’s Dark Matter Detective” Reina Maruyama 



    Matthew Sedacca

    Reina Maruyama wasn’t expecting her particle detector to work buried deep in ice. She was wrong.

    In the late 1990s, a team of physicists at the Laboratori Nazionali del Gran Sasso in Italy began collecting data for DAMA/LIBRA, an experiment investigating the presence of dark matter particles.

    Gran Sasso LABORATORI NAZIONALI del GRAN SASSO, located in the Abruzzo region of central Italy

    DAMA/LIBRA at Gran Sasso
    DAMA/LIBRA at Gran Sasso

    The scientists used a scintillation detector to spot the weakly interactive massive particles, known as WIMPs, thought to constitute dark matter. They reported seeing an annual modulation in the number of “hits” that the detector receives. This was a potential sign that the Earth is moving through the galaxy’s supposed halo of dark matter—something that few, if any, researchers could claim.

    Reina Maruyama’s job, at a detector buried two-kilometers deep in the South Pole, is to determine whether or not these researchers’ findings are actually valid. Previously, Maruyama worked at the South Pole to detect neutrinos, the smallest known particle. But when it came to detecting dark matter, especially with using detectors buried under glacial ice, she was initially skeptical of the task. In those conditions, she “couldn’t imagine having it run and produce good physics data.”

    Contrary to Maruyama’s expectations, the detector’s first run went smoothly. Their most recent paper, published in Physical Review D earlier this year, affirmed the South Pole as a viable location for experiments detecting dark matter. The detector, despite the conditions, kept working. At the moment, however, “DM-Ice17,” as her operation is known, is on hiatus, with the team having relocated to Yangyang, South Korea, to focus on COSINE-100, another dark matter particle detector experiment, and continue the search for the modulation seen in DAMA/LIBRA.

    COSINE-100 Dark Matter Experiment – Yale University

    The shielding structure of COSINE-100 includes 3 cm of copper, 20 cm of lead, and 3 cm of 37 plastic scintillator panels for cosmic ray muon tagging. 18 5-inch PMTs are attached to the copper box to observe scintillation light from liquid scintillator, and each plastic scintillator has a 2-inch PMT attached on one side (top panels have a PMT on each side). http://cosine.yale.edu/about-us/cosine-100-experiment.

    Dark Matter?Data visuals from COSINE-100, a dark matter experiment in Yangyang, South Korea. Reina Maruyama

    Nautilus sat down with Maruyama at Yale this past January to talk about the potential nature of dark matter, the variety of ways scientists use to search for it, and what it’s like working in the South Pole.

    What do the scientists behind DAMA claim to have discovered?

    What this experiment with DAMA has seen is that in June, the velocity is odd. The sun and Earth are going in the same direction; in December, the velocities are in opposite directions, at about a 10 percent difference. That means in June we expect this signature to occur more frequently than in December. DAMA claims to have seen this annual modulation signature. People started to think about: “Well what is it that DAMA is seeing? Could it be some sort of environmental effect?” We don’t know. They’ve looked at their data, and they’ve argued against every possibility that people have come up with. One thing that the dark matter community has asked them to do is actually release their data, but so far they have refused to do that.

    The original idea of DM-Ice was to go to the southern hemisphere where the seasonal variation is opposite in phase, so if we continue to see the signal, then it would be really hard to attribute that signal to something seasonal. If we don’t see anything, then there is something in their data that they don’t understand.

    University of Wisconsin–Madison, DM-Ice collaborators

    So what is dark matter?

    We don’t know what it is. We know it exerts gravity. This is why we call it matter. We see evidence from it: in how stars move around in a galaxy, and galaxies around each other. When we look out at distant stars and galaxies, we can see light being bent around something that exerts gravity, even on photons, but we don’t see any light, x-rays, or clues of things existing.

    What we saw was that the speed of the rotating objects are much faster than what you would expect for something like that. So that seems to indicate there is more mass between these objects. You can do that by adding a clump of mass between. That’s what we see: not specific objects, but dark matter diffusely spread out all over, typically surrounding galaxies. There must be dark matter inside the orbit of our sun so that we can move at the speed that we are. That means we are going through this halo of dark matter, riding along with the sun and the earth.

    What can we do to prove that dark matter is causing these changes?

    Let’s just pick a volume, your coffee, right there. We are hypothesizing that if dark matter is WIMPs, then there’s a very small possibility that the WIMPs going at 300 kilometers per second could interact with the coffee nuclei. If that happens in our detectors, we can actually see a nucleus being kicked by a WIMP. That’s how a lot of particle detectors work: Either there are some energy transfers to the electrons, or there is some energy transfer into the nuclei, and then we detect the electrons or light emitted from that, or sound waves. If those occur at the right energy, with the right frequency, then we can say maybe we see dark matter in our detectors.

    When there is a knock into a nucleus you can actually collect two different kinds of signals: the charge and photon emissions. When nuclei get kicks, it transfers some of that energy into electrons, and then the electrons move around, and that process emits light, and in some of that, electrons can be collected, and that is a signal. You need some sort of mass, and you need to be able to tell if a nucleus got a kick. The most efficient way to do that is to have a detector that is also the target, where the nuclei is. You want some big volume to increase the likeliness this can occur. DAMA is using sodium iodide detectors. These are very sensitive experiments, and a lot of these can actually tell the difference between an initial electron kick versus an initial nuclear kick. The electron kicks actually occur much more often in these detectors, so you can reject those as background and just keep the nuclear kicks.

    Newer technologies are much more sensitive to nuclear kicks than sodium iodide. Every other experiment that has tried to look for a signature like this has not seen anything. They see nuclear kicks, but mostly attributable to neutrons. They cannot definitively say that this must be dark matter.

    Gamma Ray Shield, or Bath tub?Maruyama said, “We put detectors inside when we need to shield them from gamma rays that are present in a typical room. The box is made of lead bricks.” NO image credit.

    How did you come up with the design for your experiments?

    With DM-Ice, we wanted to be as similar to DAMA as possible: We want sodium iodide, and we want it to be low-background. So we need shielding around it to block the detector from gamma rays and cosmic rays. The only thing that’s changing should be the dark matter. It turns out the South Pole is actually a pretty good environment. You have an entire continent of ice, which is very stable. Once you go two and a half kilometers into the ice, nothing is changing. Ice at the South Pole, it’s super clean.

    Then you need to start worrying about practical things like: Can you get there, and do you have infrastructure to run the experiment? Is it affordable, do you have the right people to do this with? That starts to narrow down the site and the environment. You end up with the a few places in the world you could do this, and then maybe you want to partner with somebody else so that you can afford a bigger detector, and more, better infrastructure that’s more stable. That is the thinking process. Then you have to convince your colleagues in the field that this is a really good idea and need to share a pot of resources available to all U.S. funds. That’s the thought-process behind the experiment.

    What’s it like working in the South Pole?

    First you have to get approved to go, but that’s pretty competitive. A lot of people want to go and so if you have a good reason to go, you go. Before you go, you need to get medical clearance. You get checked out. It’s a remote location. They want to make sure you’re not gonna get sick while you’re there. So you spend one or two nights in Christchurch, New Zealand. You meet a lot of other people who might be going with you: engineers, geologists, biologists, other scientists, firemen, cooks, and bus drivers; a lot of really engaged and very passionate people.

    When you get to the South Pole, you have take it slow, even though you’re excited and working, it’s 10,000 feet, so they ask you to take it easy your first few days. You enter through what looks like a restaurant-refrigerator door. Keep the cold out kind of thing. Very comfortable, get your own room, dormitory-style living. Water is very precious. All of the energy is provided by jet fuel. So airplanes fly in and siphon off the fuel except for what’s needed for to get back. And there’s a power station where they generate electricity. They get water by melting the ice, and it’s a very expensive process. You get like two-minute showers twice a week. It’s on the honor system. That’s what it’s like living in the station.

    What are some problems that you faced when working down there?

    It’s 24/7 sunlight. So the sun circles above your head. Because you’re there to get things done, it’s hard to know when to stop working. But before you know it, it’s two in the morning, and the sun’s bright and shining. So you have to make sure you get enough sleep and ready to work the next day. That was a challenge for me.

    So when you’re not on site what are you doing in terms of research?

    We might have a small-scale detector here and do stress tests on it. Physicists love to tinker: How we can improve these detectors? What if we changed the temperature a lot? How can we make this detector even quieter so that we can look for even smaller signals, or a signal that exists that looks even bigger? People like to say things like we’re looking for a needle in a haystack, so can we reduce the haystack? Can we change the color of the haystack so that the needle looks even more visible?

    What’s the future for DM-Ice?

    Right now there is no drilling happening at the South Pole. We’ll keep pushing to do that experiment. In the meantime, the detector is buried and frozen into the ice, so we might as well just keep it running. We’re focusing on the Korean effort. What we can do there is look for the signal. If we continue to see the same signal, we can try to look for other correlations and cross them off on our own. If we cannot find other causes for it, I think the case for DAMA becomes stronger. Then DAMA’s signal is not specific to DAMA.

    See the full article here .

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    Welcome to Nautilus. We are delighted you joined us. We are here to tell you about science and its endless connections to our lives. Each month we choose a single topic. And each Thursday we publish a new chapter on that topic online. Each issue combines the sciences, culture and philosophy into a single story told by the world’s leading thinkers and writers. We follow the story wherever it leads us. Read our essays, investigative reports, and blogs. Fiction, too. Take in our games, videos, and graphic stories. Stop in for a minute, or an hour. Nautilus lets science spill over its usual borders. We are science, connected.

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