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  • richardmitnick 1:57 pm on August 15, 2014 Permalink | Reply
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    From Fermilab: “Physics in a Nutshell What is a WIMP?” 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Friday, Aug. 15, 2014
    Fermilab Don Lincoln
    Don Lincoln

    If you want to understand dark matter, you need to understand terms such as MACHO and WIMP. It’s enough to recall one of those 1970s comic book advertisements for Charles Atlas’ body building program (well, for those of us of a certain age anyway).

    To understand the term WIMP, we need to go back to the idea of dark matter and why we think it exists. The easiest-to-understand evidence for the existence of dark matter involves spinning galaxies. As early as the 1930s, scientists combined measurements of the rotational speed of galaxies with [Isaac] Newton’s theory of gravity and determined that something was awry. The galaxies were spinning so fast that they could not be held together by the gravitational force of the observed matter and should have torn themselves apart. After decades of studies, scientists have determined that the most probable explanation is that there exists another form of matter that we now call dark matter. It is generally imagined that dark matter is essentially a diffuse gas of massive subatomic particles.

    wimp
    Possibility

    Astronomical evidence has allowed us to determine a fairly specific list of properties for dark matter, if it exists. Because this matter neither emits nor absorbs light, it neither is charged nor contains charge within it. This is why we call it dark. It is also stable. We know this because galaxies persist for billions of years. It does not interact via the strong force, as we see no evidence of cosmic rays (made of protons) interacting with it. And because this matter causes galaxies to rotate quickly, we know it both contains mass and participates in the gravitational force.

    That last point is crucial. There are four known forces: the strong and weak nuclear forces, electromagnetism and gravity. We know that dark matter does not experience the strong or electromagnetic forces. We know it does experience gravity. We don’t know about the weak force.

    So let’s think about that for a bit. While the weak force is … well, weak … gravity is incredibly weak, about a trillion trillion trillion times weaker than the weak force. We have never measured the force due to gravity between two subatomic particles (and we probably never will). So if gravity is the only force that dark matter feels, we will likely never detect it, nor will we ever make it any conceivable particle accelerator.

    So how is it that Fermilab (and others) have a vibrant research program looking for dark matter? Is it all wishful thinking?

    The answer is, “Of course not.” However, it does bring forward an assumption buried inside most dark matter searches. This assumption is that dark matter also experiences the weak nuclear force. Like weakly interacting neutrinos, maybe dark matter will occasionally experience an interaction with ordinary matter and be detected.

    So why would scientists postulate that dark matter experiences the weak force? One answer is that if it doesn’t, we’ll never detect it. But that’s not a very good answer. A better answer involves the Higgs boson. Because the Higgs field gives mass to ordinary matter, maybe it also gives mass to dark matter. Further, since the Higgs field was invented to solve a problem with early attempts to unify the weak and electromagnetic forces, maybe the interaction of the Higgs boson with dark matter also ties dark matter to the weak force. And this would be great, as we know from experience that we can detect weak force interactions.

    So this leads us to the meaning of the term “WIMP.” It is a weakly interacting massive particle — the name is quite literal. It is not necessary that dark matter interact via the weak force, and dark matter may not be a WIMP. If dark matter does not interact via the weak force, we’ll probably never detect it directly. In short, the success of all direct dark matter searches depends crucially on dark matter being WIMP-y.

    —Don Lincoln

    See the full article here.

    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.

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  • richardmitnick 8:53 am on November 2, 2013 Permalink | Reply
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    From Livermore Lab: “First results from LUX world’s most sensitive dark matter detector” 


    Lawrence Livermore National Laboratory

    10/31/2013
    Stephen P Wampler, LLNL, (925) 423-3107, wampler1@llnl.gov

    After its first run of more than three months, operating a mile underground in the Black Hills of South Dakota, a new experiment named LUX has proven itself the most sensitive dark matter detector in the world.

    device
    LUX researchers, seen here in a clean room on the surface at the Sanford Lab, work on the interior of the detector, before it is inserted into its titanium cryostat.

    cryo
    Photomultiplier tubes capable of detecting as little as a single photon of light line the top and bottom of the LUX dark matter detector. They will record the position and intensity of collisions between dark matter particles and xenon nuclei.

    “LUX is blazing the path to illuminate the nature of dark matter,” says Brown University physicist Rick Gaitskell, co-spokesperson for LUX with physicist Dan McKinsey of Yale University. LUX stands for Large Underground Xenon experiment.

    Gaitskell and McKinsey announced the LUX first-run results, on behalf of the collaboration, at a seminar Wednesday at the Sanford Underground Research Facility (Sanford Lab) in Lead, S.D. The Sanford Lab is a state-owned facility, and the U.S. Department of Energy (DOE) supports its operation. The LUX scientific collaboration, which is supported by the National Science Foundation and DOE, includes 17 research universities and national laboratories in the United States, the United Kingdom and Portugal.

    Three researchers from Lawrence Livermore National Laboratory — principal investigator Adam Bernstein and staff scientists Peter Sorensen and Kareem Kazkaz, all from the Lab’s Rare Event Detection Group in Physics Division — have been closely involved with the LUX project since its inception.

    Dark matter, so far observed only by its gravitational effects on galaxies and clusters of galaxies, is the predominant form of matter in the universe. Weakly interacting massive particles, or WIMPs – so-called because they rarely interact with ordinary matter except through gravity — are the leading theoretical candidates for dark matter. The mass of WIMPs is unknown, but theories and results from other experiments suggest a number of possibilities.

    LUX has a peak sensitivity at a WIMP mass of 33 GeV/c2 (see **below), with a sensitivity limit three times better than any previous experiment. LUX also has a sensitivity that is more than 20 times better than previous experiments for low-mass WIMPs, whose possible detection has been suggested by other experiments. Three candidate low-mass WIMP events recently reported in ultra-cold silicon detectors would have produced more than 1,600 events in LUX’s much larger detector, or one every 80 minutes in the recent run. No such signals were seen.

    “This is only the beginning for LUX,” McKinsey said. “Now that we understand the instrument and its backgrounds, we will continue to take data, testing for more and more elusive candidates for dark matter.”

    See the full article here.

    Operated by Lawrence Livermore National Security, LLC, for the Department of Energy’s National Nuclear Security
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  • richardmitnick 2:10 pm on May 22, 2013 Permalink | Reply
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    From SLAC: “KIPAC Theorists Weigh In on Where to Hunt Dark Matter” 

    May 21, 2013
    Lori Ann White

    Theorists from the Kavli Institute for Particle Astrophysics and Cosmology are helping dark matter sleuths decide where to start their search.

    “Now that it looks like the hunt for the Higgs boson is over, particles of dark matter are at the top of the physics ‘Most Wanted’ list. Dozens of experiments have been searching for them, but often come up with contradictory results.

    Theorists from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint SLAC-Stanford institute, believe they’ve come up with an algorithm – a mathematical description of how the individual particles behave – that could help narrow the search for these elusive particles, which are thought to make up more than 25 percent of the matter and energy in the universe.

    It starts with assumptions, said Yao-Yuan Mao, lead author of a paper published in The Astrophysical Journal that outlines their new search tool. Assumptions are a good starting point when you don’t know where to look. A popular assumption about dark matter is that it’s made up of WIMPs, Weakly Interacting Massive Particles. The “M” in WIMP accounts for gravity’s ability to herd these particles around; the “P” and “I” hint at why they’re so hard to detect otherwise.

    three
    KIPAC theorists (l to r) Louis Strigari, Risa Wechsler and Yao-Yuan Mao discussing dark matter velocity distributions. (Credit: Luis Fernandez.)

    Most dark matter detectors are based on the assumption that, every once in a while, a WIMP must smack into the nucleus of an atom of visible matter, making the nucleus vibrate and releasing a signal. Such disruptions can be detected. But what that disruption looks like and how often it happens depends on yet more assumptions. How heavy is the dark matter particle? How fast is it moving?

    box
    Left panel: Air molecules whiz around at a variety of speeds, and some are very fast. When they collide with both heavy and light elements – for example, xenon (purple) and silicon (orange) – these fast moving particles have enough momentum to affect both nuclei. Right panel: Dark matter particles are moving more slowly and are less able to affect the heavy xenon nucleus. As a result, detectors made from lighter materials like silicon may prove to be more effective at picking up signals of dark matter. (Credit: Greg Stewart/SLAC National Accelerator Laboratory)

    Another common assumption that touches on these issues, said Mao, is that collections of WIMPs behave as an ideal gas, a collection of particles that hang out together and occasionally bounce off each other. Sometimes a lucky bounce gives a particle more energy, sending it zooming off at a greater speed. How often particles pick up more energy and more speed depends on how much you turn up the heat or put on the pressure.

    But, as far as scientists can tell, turning up the heat and putting on the pressure doesn’t affect WIMPs. Only gravity does.

    “The Ideal Gas Law doesn’t describe a system of particles, like dark matter particles, that don’t seem to transfer energy to each other,” said Mao. This incorrect description can distort the carefully built picture upon which a search for WIMPs is based. In particular, it means predictions of their velocities can be off by a significant amount, but velocities affect what a detector will see.

    Mao and his colleagues have used simulations to provide new insight into how fast WIMPs are expected to move.”

    See the full article here.

    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.

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  • richardmitnick 9:32 am on April 11, 2012 Permalink | Reply
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    From Fermilab today: “Next generation of dark matter” 

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    ch
    Craig Hogan, director of the Center for Particle Astrophysics, wrote this week’s column.

    Wednesday, April 11, 2012

    “Scientists gathered at a Fermilab workshop last Friday to discuss the hunt for cosmic dark matter. They agreed that we may finally be closing in on a long-sought quarry. Fermilab theorist Dan Hooper succinctly expressed their sense of hopeful anticipation in the figure shown below. Technology is improving rapidly – faster than Moore’s law for computer speed – and theorists expect a discovery soon.

    image
    Dan Hooper’s schematic plot shows how dark-matter experiments are becoming more sensitive to weaker and weaker particle interactions over time. If cosmic dark matter is made of Weakly Interacting Massive Particles [WIMPS], we should find them in the next decade.

    The Department of Energy and the National Science Foundation recently announced that they will competitively fund advanced dark-matter searches with Generation 2 detectors. Fermilab projects will be part of this new initiative, and one of them could be the first to detect this new form of matter.”

    See the full article here.

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.

     
  • richardmitnick 8:35 am on September 6, 2011 Permalink | Reply
    Tags: , , WIMPS   

    From Quantum Diaries: Richard Ruiz on a Bunch of Topics 

    Check out Richard Ruiz’latest post, WIMPs – The Most Ubiquitous Term in the ‘verse

    Participants in Quantum Diaries:

    Fermilab

    Triumf

    US/LHC Blog


    CERN

    Brookhaven Lab

    KEK

     
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