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  • richardmitnick 1:43 pm on July 26, 2016 Permalink | Reply
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    From Sky & Telescope: “No Dark Matter from LUX Experiment” 

    SKY&Telescope bloc

    Sky & Telescope

    An underground detector reports zero detections of weakly interacting massive particles (WIMPs), the top candidate for mysterious dark matter.

    The Davis cavern, deep within what used to be the Homestake Mine, before the placement of the LUX experiment.

    SURF logo

    Sanford Underground levels
    Sanford Underground levels

    LUX/Dark matter experiment at SURF
    LUX/Dark matter experiment at SURF

    Founded in 1876, the town of Lead in South Dakota hummed along as a mining community for more than a century. Homestake Mine employed thousands in the largest, deepest, and most productive gold mine in the Western Hemisphere.

    Now scientists are using it to mine for gold of a darker kind.

    More than a mile underground, where miners once accessed precious ore, sits a 3-foot-tall, dodecagonal cylinder of liquid xenon. The 122 photomultiplier tubes at the container’s top and bottom await the glitter of light that would signal an elusive dark matter shooting through the cylinder and interacting with one of the xenon atoms.

    But after more than a year of data collecting, the Large Underground Xenon (LUX) experiment announced last week at the Identification of Dark Matter 2016 conference that they’re still coming up empty-handed.

    A Physicist’s Gold Mine

    Weakly interacting massive particles (WIMPs) are the top candidates for dark matter, the invisible stuff that makes up about 84% of the universe’s matter. By definition, dark matter doesn’t interact with light, nor does it interact via the strong force that holds nuclei together. And while we know it interacts with gravity, that interaction leaves only indirect evidence of its existence, such as its effect on galaxy rotation.

    This bottom view shows the photomultiplier tube holders in the LUX experiment.

    But WIMP theory says dark matter particles should also interact via the weak force, a fundamental force that governs nature on a subatomic level — including the fusion within the Sun. So a WIMP particle should very rarely smash into a heavy nucleus, generating a flash of light. The chance for a direct hit is very, very low, but 350 kilograms (770 pounds) of liquid xenon in the LUX experiment should have good odds.

    After just three months of operation, in 2013 the LUX experiment had already reported a null result. At the time, the experiment had probed with a sensitivity 20 times that of previous experiments (check out the graph here to see how three months of LUX ruled out numerous WIMP scenarios).

    A new 332-day run began in September 2014, and the preliminary analysis announced last week probes four times deeper than the results before. Yet despite a longer run time, increased sensitivity, and better statistical analysis, the LUX team still hasn’t found any WIMPs.

    Simply put: either WIMPs don’t exist at all, or the WIMPs that do exist really, really don’t like interacting with normal matter.

    It’s also worth noting that LUX isn’t just looking for WIMPs. The WIMP scenario is the primary one it’s testing, and the one that last week’s announcement focused on. But more results are forthcoming about LUX results on dark matter alternatives, such as axions and axion-like particles.

    Not All That’s Gold Glitters

    The non-finding may not win any Nobel Prizes, but in a way it’s great news for physicists. Numerous experiments (such as CDMS II, CoGeNT, and CRESST) had found glimmers of WIMP detections, but none had found results statistically significant enough to be claimed as a real detection. The LUX results have been helpful in ruling out those hints of low-mass WIMPs.

    For the technically minded, this is the result that was presented at the Identification of Dark Matter conference in Sheffield, UK. The plot shows the possibilities for dark matter in terms of its cross-section — the bigger the value, the more easily it interacts with normal matter — and its mass. (The mass is given in gigaelectron volts per speed of light squared, which translates to teeny tiny units of 1.9 x 10-27 kg.) LUX’s most recent results rule out any dark matter particles with mass and cross-section that place them above the solid black line. The upshot is that LUX, the most sensitive dark matter experiment to date, is narrowing the playing field, especially for low-mass WIMP scenarios.

    “It turns out there is no experiment we can think of so far that can eliminate the WIMP hypothesis entirely,” says Dan McKinsey (University of California, Berkeley). “But if we don’t detect WIMPs with the experiments planned in the next 15 years or so . . . physicists will likely conclude that dark matter isn’t made of WIMPs.”

    That’s why — despite not finding any WIMPs this time around — the LUX team continues to work on the next-gen experiment: LUX-ZEPLIN. Its 7 tons of liquid xenon should begin awaiting flashes from dark matter interactions by 2020.

    Lux Zeplin project
    Lux Zeplin project at SURF

    Three years of data from LUX-ZEPLIN will probe WIMP scenarios down to fundamental limits from the cosmic ray background. In other words, if LUX-ZEPLIN doesn’t detect WIMPs, they don’t exist — or they’re beyond our detection capabilities altogether.

    See the full article here (More …)

  • richardmitnick 1:29 pm on November 11, 2015 Permalink | Reply
    Tags: , , WIMPS, XENON1T experiment   

    From Symmetry: “Dark matter’s newest pursuer” 


    Mike Ross

    Scientists have inaugurated the new XENON1T experiment at Gran Sasso National Laboratory in Italy.


    Researchers at a laboratory deep underneath the tallest mountain in central Italy have inaugurated XENON1T, the world’s largest and most sensitive device designed to detect a popular dark matter candidate.

    Gran Sasso XENON1T
    XENON1T tank

    “We will be the biggest game in town,” says Columbia University physicist Elena Aprile, spokesperson for the XENON collaboration, which has over the past decade designed, built and operated a succession of ever-larger experiments that use liquid xenon to look for evidence of weakly interacting massive dark matter particles or WIMPs, at the Gran Sasso National Laboratory.

    Gran Sasso National Laboratory

    Interactions with these dark matter particles are expected to be rare: Just one a year for every 1000 kilograms of xenon. As a result, larger experiments have a better chance of intercepting a WIMP as it passes through the Earth.

    XENON1T’s predecessors—XENON 10 (2006 to 2009) and XENON 100 (2010 to the present)—held 25 and 160 kilograms of xenon, respectively. The new XENON11 experiment’s detector measures 1 meter high and 1 meter in diameter and contains 3500 kilograms of liquid xenon, nearly 10 times as much as the next-biggest xenon-filled dark matter experiment, the Large Underground Xenon [LUX] experiment.

    LUX Dark matter

    Looking for WIMPs

    Should a WIMP collide with a xenon atom, kicking its nucleus or knocking out one of its electrons, the result is a burst of fast ultraviolet light and a bunch of free electrons. Scientists built a strong electric field in the XENON1T detector to direct these freed particles to the top of the chamber, where they will create a second burst of light. The relative timing and brightness of the two flashes will help the scientists determine the type of particle that created them.

    “Since our detectors can detect even a single electron or photon, XENON1T will be sensitive to even the most feeble particle interactions,” says Rafael Lang, a Purdue University physicist on the XENON collaboration.

    Scientists cool the xenon to minus 163 degrees Fahrenheit to turn it into a liquid three times denser than water. One oddity of xenon is that its boiling temperature is only 7 degrees Fahrenheit above its melting temperature. So “we have to control our temperature and pressure precisely,” Aprile says.

    The experiment is shielded from other particles such as cosmic rays by separate layers of water, lead, polyethylene and copper—not to mention 1400 meters of Apennine rock that lie above the Gran Sasso lab’s underground tunnels.

    Keeping the xenon free of contaminants is essential to the detector’s sensitivity. Oxygen, for example, can trap electrons. And the decay of some radioactive krypton isotopes, which are difficult to separate from xenon, can obscure a WIMP signal. The XENON collaboration’s solution is to continuously circulate and filter 100 liters of xenon gas every minute from the top of the detector through a filtering system before chilling it and returning it to service.
    A matter of scale

    XENON researchers hope that their new experiment will finally be the one to see definitive evidence of WIMPs. But just in case, XENON1T was designed to accommodate a swift upgrade to 7000 kilograms of xenon in its next iteration. (At the same time, the LUX and UK-based Zeplin groups joined forces to design a similar-scale xenon detector, LZ.)

    “If we see nothing with XENON1T, it will still be worth it to move up to the 7000-kilogram device, since it will be relatively easy to do that,” Aprile says. “If we do see a few events with XENON1T—and we’re sure they are from the dark matter particle—then the best way to prove that it’s real is to confirm that result with a larger, more sensitive experiment.

    “In any case,” Aprile says, “we should know whether the WIMP is real or not before 2020.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 2:09 pm on July 15, 2015 Permalink | Reply
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    From Symmetry: “Miraculous WIMPs” 


    July 15, 2015
    Manuel Gnida

    Artwork by Sandbox Studio, Chicago with Ana Kova

    What are WIMPs, and what makes them such popular dark matter candidates?

    Invisible dark matter accounts for 85 percent of all matter in the universe, affecting the motion of galaxies, bending the path of light and influencing the structure of the entire cosmos. Yet we don’t know much for certain about its nature.

    Most dark matter experiments are searching for a type of particles called WIMPs, or weakly interacting massive particles.

    “Weakly interacting” means that WIMPs barely ever “talk” to regular matter. They don’t often bump into other matter and also don’t emit light—properties that could explain why researchers haven’t been able to detect them yet.

    Created in the early universe, they would be heavy (“massive”) and slow-moving enough to gravitationally clump together and form structures observed in today’s universe.

    Scientists predict that dark matter is made of particles. But that assumption is based on what they know about the nature of regular matter, which makes up only about 4 percent of the universe.

    WIMPs advanced in popularity in the late 1970s and early 1980s when scientists realized that particles that naturally pop out in models of Supersymmetry could potentially explain the seemingly unrelated cosmic mystery of dark matter.

    Supersymmetry, developed to fill gaps in our understanding of known particles and forces, postulates that each fundamental particle has a yet-to-be-discovered superpartner. It turns out that the lightest one of the bunch has properties that make it a top contender for dark matter.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    “The lightest supersymmetric WIMP is stable and is not allowed to decay into other particles,” says theoretical physicist Tim Tait of the University of California, Irvine. “Once created in the big bang, many of these WIMPs would therefore still be around today and could have gone unnoticed because they rarely produce a detectable signal.”

    When researchers use the properties of the lightest supersymmetric particle to calculate how many of them would still be around today, they end up with a number that matches closely the amount of dark matter experimentally observed—a link referred to as the “WIMP miracle.” Many researchers believe it could be more than coincidence.

    “But WIMPs are also popular because we know how to look for them,” says dark matter hunter Thomas Shutt of Stanford University and SLAC National Accelerator Laboratory. “After years of developments, we finally know how to build detectors that have a chance of catching a glimpse of them.”


    Shutt is co-founder of the LUX experiment and one of the key figures in the development of the next-generation LUX-ZEPLIN experiment. He is one member of the group of scientists trying to detect WIMPs as they traverse large, underground detectors.

    Lux Dark Matter 2

    Lux Zeplin project

    Other scientists hope to create them in powerful particle collisions at CERN’s Large Hadron Collider.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    “Most supersymmetric theories estimate the mass of the lightest WIMP to be somewhere above 100 gigaelectronvolts, which is well within LHC’s energy regime,” Tait says. “I myself and others are very excited about the recent LHC restart. There is a lot of hope to create dark matter in the lab.”


    A third way of searching for WIMPs is to look for revealing signals reaching Earth from space. Although individual WIMPs are stable, they decay into other particles when two of them collide and annihilate each other. This process should leave behind detectable amounts of radiation. Researchers therefore point their instruments at astronomical objects rich in dark matter such as dwarf satellite galaxies orbiting our Milky Way or the center of the Milky Way itself.


    “Dark matter interacts with regular matter through gravitation, impacting structure formation in the universe,” says Risa Wechsler, a researcher at Stanford and SLAC. “If dark matter is made of WIMPs, our predictions of the distribution of dark matter based on this assumption must also match our observations.”

    Wechsler and others calculate, for example, how many dwarf galaxies our Milky Way should have and participate in research efforts under way to determine if everything predicted can also be found experimentally.

    So how would researchers know for sure that dark matter is made of WIMPs? “We would need to see conclusive evidence for WIMPs in more than one experiment, ideally using all three ways of detection,” Wechsler says.

    In the light of today’s mature detection methods, dark matter hunters should be able to find WIMPs in the next five to 10 years, Shutt, Tait and Wechsler say. Time will tell if scientists have the right idea about the nature of dark matter.

    See the full article here.

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.

  • 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.


    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

    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.

    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.

    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.

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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.

    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?

    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.

    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.

    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:



    US/LHC Blog


    Brookhaven Lab


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