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  • richardmitnick 10:22 am on July 23, 2017 Permalink | Reply
    Tags: , , , , , , , Dark Matter, FDM-Fuzzy Dark Matter, Lyman-alpha forest   

    From Astro Watch: “Flashes of Light on the Dark Matter” 

    Astro Watch bloc

    Astro Watch

    July 23, 2017
    No writer credit found

    1

    A web that passes through infinite intergalactic spaces, a dense cosmic forest illuminated by very distant lights and a huge enigma to solve. These are the picturesque ingredients of a scientific research – carried out by an international team composed of researchers from the International School for Adavnced Studies (SISSA) and the Abdus Salam International Center for Theoretical Physics (ICTP) in Trieste, the Institute of Astronomy of Cambridge and the University of Washington – that adds an important element for understanding one of the fundamental components of our Universe: the dark matter.

    In order to study its properties, scientists analyzed the interaction of the “cosmic web” – a network of filaments made up of gas and dark matter present in the whole Universe – with the light coming from very distant quasars and galaxies. Photons interacting with the hydrogen of the cosmic filaments create many absorption lines defined “Lyman-alpha forest”. This microscopic interaction succeeds in revealing several important properties of the dark matter at cosmological distances. The results further support the theory of Cold Dark Matter, which is composed of particles that move very slowly. Moreover, for the first time, they highlight the incompatibility with another model, i.e. the Fuzzy Dark Matter, for which dark matter particles have larger velocities. The research was carried out through simulations performed on international parallel supercomputers and has recently been published in Physical Review Letters.

    Although constituting an important part of our cosmos, the dark matter is not directly observable, it does not emit electromagnetic radiation and it is visible only through gravitational effects. Besides, its nature remains a deep mystery. The theories that try to explore this aspect are various. In this research, scientists investigated two of them: the so-called Cold Dark Matter, considered a paradigm of modern cosmology, and an alternative model called Fuzzy Dark Matter (FDM), in which the dark matter is deemed composed of ultralight bosons provided with a non-negligible pressure at small scales. To carry out their investigations, scientists examined the cosmic web by analyzing the so-called Lyman-alpha forest. The Lyman-alpha forest consists of a series of absorption lines produced by the light coming from very distant and extremely luminous sources, that passes through the intergalactic space along its way toward the earth’s telescopes. The atomic interaction of photons with the hydrogen present in the cosmic filaments is used to study the properties of the cosmos and of the dark matter at enormous distances.

    Through simulations carried out with supercomputers, researchers reproduced the interaction of the light with the cosmic web. Thus they were able to infer some of the characteristics of the particles that compose the dark matter. More in particular, evidence showed for the first time that the mass of the particles, which allegedly compose the dark matter according to the FDM model, is not consistent with the Lyman-alpha Forest observed by the Keck telescope (Hawaii, US) and the Very Large Telescope (European Southern Observatory, Chile).


    Keck Observatory, Maunakea, Hawaii, USA

    ESO/VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

    Basically, the study seems not to confirm the theory of the Fuzzy Dark Matter. The data, instead, support the scenario envisaged by the model of the Cold Dark Matter.

    The results obtained – scientists say – are important as they allow to build new theoretical models for describing the dark matter and new hypotheses on the characteristics of the cosmos. Moreover, these results can provide useful indications for the realization of experiments in laboratories and can guide observational efforts aimed at making progress on this fascinating scientific theme.

    See the full article here .

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  • richardmitnick 1:16 pm on July 12, 2017 Permalink | Reply
    Tags: , Dark Matter, HAYSTAC - Haloscope at Yale Sensitive To Axion Cold Dark Matter, , ,   

    From Yale: “Needle in a HAYSTAC” 

    Yale University bloc

    Yale University

    July 5, 2017
    Elizabeth Ruddy

    1
    No image caption or credit.

    Imagine searching for a needle in a haystack. The needle weighs about 100 billion times less than an electron and has no charge. It acts like a wave rather than a particle, and the haystack is the size of our universe. Needles like this may exist in the tens of trillions in every cubic centimeter of space—the trick is proving that they’re there.

    That is the mission of the HAYSTAC Project at Yale, which stands for the Haloscope at Yale Sensitive To Axion Cold Dark Matter. HAYSTAC is a collaboration between Yale University, University of California, Berkeley and University of Colorado, Boulder. The project is based here in the Wright Laboratory, lead by Professor Steve Lamoreaux and a team of Yale scientists and graduate students. The scientists began their project about five years ago and released their first results this past February in The Physics Review Letters. The first author was Yale graduate student Ben Brubaker.

    “The goals of the experiment are to detect dark matter, or failing that, to at least rule out some possible models for what dark matter is,” explained Brubaker. “In simplest terms, dark matter started out as an astrophysics question: that is, there is more mass in the universe than can be accounted for by the mass we can see [through] all the wavelengths we can detect: visible light, radio waves, ultraviolet.” Dark matter is the “invisible” matter.

    The HAYSTAC project is dedicated specifically to the detection of the axion, a subatomic particle that was proposed in 1983 as a likely candidate for dark matter. Like the aforementioned needle, axions are theorized to have almost miniscule mass, no charge, and no spin. Based on the gravitational movement of stars and galaxies, we know that 80 percent of the matter in our universe is dark matter, but axions interact with other matter so weakly they become almost impossible to detect. Because they are so light, they have very little energy and behave more like waves than particles. As a result, the scientists must employ an unusual identification strategy to find them.

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    The HAYSTAC axion detector probes the universe for axions, a potential candidate for dark matter. No image credit.

    The HAYSTAC detection device essentially produces a magnetic field that converts the axions to photons. The frequency of oscillation of the photons is determined by the mass of the axion. Therefore, when the detector is tuned in to one specific frequency at a time, it can amplify these oscillations to make them detectable.

    “Our detector is in essence a tunable radio receiver, and we painstakingly tune the receiving frequency looking for an increase in noise. It is like driving through a desert looking for a station on the car radio: you tune slowly in hopes of finding something,” said Professor Lamoreaux, the head of the project.

    In the February report, the team demonstrated its recent breakthroughs in design: they had achieved sufficient sensitivity to test out much higher frequencies in the potential mass range than ever before. By incorporating technology from other fields such as quantum electronics, Lamoreaux and his colleagues have made the detector colder and quieter than any of its contemporaries, eliminating as much of the background noise as possible. According to Brubaker, the device is kept at about 0.1 degree Celsius above absolute zero, the unattainable temperature at which atoms physically stop moving. Freezing temperatures are critical for sensitivity because a major source of noise is thermal radiation: photons being shed by matter and interfering with the detection of axions.

    According to Professor Lamoreaux, their detector is currently the most sensitive radio receiver ever built. “Imagine a match lit on the surface of the Moon…the rate of energy entering the pupil of your eye, when the match is viewed from the Earth, is about the level of sensitivity we achieve.”

    The size of the detector scales inversely with the mass range being tested, so the Wright Lab instrument will only be able to search a small portion of the wide range of possible dark matter masses. However, the team has proven they have a design with the sensitivity capability necessary to perform these sweeps. Their design is a pioneering model for the future.

    See the full article here .

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    Yale University Campus

    Yale University comprises three major academic components: Yale College (the undergraduate program), the Graduate School of Arts and Sciences, and the professional schools. In addition, Yale encompasses a wide array of centers and programs, libraries, museums, and administrative support offices. Approximately 11,250 students attend Yale.

     
  • richardmitnick 1:18 pm on July 6, 2017 Permalink | Reply
    Tags: , , , Chasing the invisible, Dark Matter, , ,   

    From ATLAS: “Chasing the invisible” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    6th July 2017
    ATLAS Collaboration

    1
    Figure 1: The second highest ETmiss monojet event in the 2016 ATLAS data. A jet with pT of 1707 GeV is indicated by the green and yellow bars corresponding to the energy deposition in the electromagnetic and hadronic calorimeters respectively. The ETmiss of 1735 GeV is shown as the white dashed line in the opposite side of the detector. No additional jets with pT above 30 GeV are found. (Image: ATLAS Collaboration/CERN)

    Cosmological and astrophysical observations based on gravitational interactions indicate that the matter described by the Standard Model of particle physics constitutes only a small fraction of the entire known Universe. These observations infer the existence of Dark Matter, which, if of particle nature, would have to be beyond the Standard Model.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Although the existence of Dark Matter is well-established, its nature and properties still remain one of the greatest unsolved puzzles of fundamental physics. Excellent candidates for Dark Matter particles are weakly interacting massive particles (WIMPs). These “invisible” particles cannot be detected directly by collision experiments.

    At the LHC, most collisions of protons produce sprays of energetic particles that bundle together into so-called “jets”. Momentum conservation requires that if particles are reconstructed in one part of the detector there have to be recoiling particles in the opposite direction. However, if WIMPs are produced they will leave no trace in the detector, causing a momentum imbalance called “missing transverse momentum” (ETmiss). However, a pair of WIMPs can be produced together with a quark or gluon that is radiated from an incoming parton (a generic constituent of the proton) producing a jet which allows to tag this kind of events.

    The jets+ETmiss search looks at final states where a highly energetic jet is produced in association with large ETmiss. Many beyond the Standard Model theories can be probed by looking for an excess of events with large missing transverse momentum compared to the Standard Model expectation. Among those theories, Supersymmetry and models which foresee the existence of Large Extra Spatial Dimensions (LED), predict additional particles that are invisible to collider experiments. These theories could give an elegant explanation to several anomalies still unsolved in the Standard Model.

    2
    Figure 2: Missing transverse momentum distribution after the jets+ETmiss selection in data and in the Standard Model predictions. The different background processes are shown in different colors. The expected spectra of LED, Supersymmetric and WIMP scenarios are also illustrated with dashed lines. (Image: ATLAS Collaboration/CERN)

    The combination of data-driven techniques and high-precision theoretical calculations has allowed ATLAS to predict the main Standard Model background processes with great precision. The shape of the ETmiss spectrum is used to increase the discovery potential of the analysis and increase the discrimination power between signals and background.

    The figure shows the missing transverse momentum spectrum compared to the measurement with the Standard Model expectation. Since no significant excess is observed, the level of agreement between data and the prediction is translated into limits on unknown parameters of the Dark Matter, Supersymmetry and LED models.

    In the WIMP scenario, the latest analysis using data collected in 2015 and 2016 in a specific interaction model are able to exclude Dark Matter masses up to 440 GeV and interaction mediators up to 1.55 TeV. Under the considered model, these represent competitive results when compared with other experiments using different detection approaches.

    Over the next two years the LHC aims to increase the data available by a factor of three. This will be a unique opportunity for ATLAS to investigate the energy frontier and the jets+ETmiss channel will continue to hold the potential to profoundly revise our understanding of the universe.

    Links:

    Search for dark matter and other new phenomena in events with an energetic jet and large missing transverse momentum using the ATLAS detector (ATLAS-CONF-2017-060): link coming soon
    EPS 2017 presentation by Shin-Shan Yu: Dark matter searches at colliders
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .

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

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  • richardmitnick 5:53 pm on July 2, 2017 Permalink | Reply
    Tags: , , , , Dark Matter, ,   

    From Waterloo: ‘Waterloo researchers capture first image of a dark matter web that connects galaxies” 

    U Waterloo bloc

    University of Waterloo

    April 12, 2017 [Missed this one.]
    Media Contact:
    Matthew Grant
    matthew.grant@uwaterloo.ca
    University of Waterloo
    519-888-4451
    http://www.uwaterloo.ca/news
    @uWaterlooNews

    1
    Dark matter filaments (shown in red) bridge the space between galaxies (shown in white) on this false colour map. No image credit.

    Researchers at the University of Waterloo have been able to capture the first composite image of a dark matter bridge that connects galaxies together.

    The composite image, which combines a number of individual images, confirms predictions that galaxies across the universe are tied together through a cosmic web connected by dark matter that has until now remained unobservable.

    Dark matter, a mysterious substance that comprises around 25 per cent of the universe, doesn’t shine, absorb or reflect light. It has traditionally been largely undetectable, except through gravity.

    “For decades, researchers have been predicting the existence of dark-matter filaments between galaxies that act like a web-like superstructure connecting galaxies together,” said Mike Hudson, a professor of astronomy at the University of Waterloo. “This image moves us beyond predictions to something we can see and measure.”

    As part of their research, Hudson and co-author Seth Epps, a former master’s student at the University of Waterloo, used a technique called weak gravitational lensing.

    It’s an effect that causes the images of distant galaxies to warp slightly under the influence of an unseen mass such as a planet, a black hole, or in this case, dark matter. The effect was measured in images from a multi-year sky survey at the Canada-France-Hawaii Telescope.


    CFHT Telescope, Mauna Kea, Hawaii, USA

    They combined lensing images from more than 23,000 galaxy pairs located 4.5 billion light-years away to create a composite image or map that shows the presence of dark matter between the two galaxies. Results show the dark matter filament bridge is strongest between systems less than 40 million light-years apart.

    “By using this technique, we’re not only to able to see that these dark matter filaments in the universe exist, we’re able to see the extent to which these filaments connect galaxies together,” said Epps.

    Hudson and Epps’ research appears in the Monthly Notices of the Royal Astronomical Society.

    See the full article here .

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    U Waterloo campus

    In just half a century, the University of Waterloo, located at the heart of Canada’s technology hub, has become a leading comprehensive university with nearly 36,000 full- and part-time students in undergraduate and graduate programs.

    Consistently ranked Canada’s most innovative university, Waterloo is home to advanced research and teaching in science and engineering, mathematics and computer science, health, environment, arts and social sciences. From quantum computing and nanotechnology to clinical psychology and health sciences research, Waterloo brings ideas and brilliant minds together, inspiring innovations with real impact today and in the future.

    As home to the world’s largest post-secondary co-operative education program, Waterloo embraces its connections to the world and encourages enterprising partnerships in learning, research, and commercialization. With campuses and education centres on four continents, and academic partnerships spanning the globe, Waterloo is shaping the future of the planet.

     
  • richardmitnick 11:39 am on June 30, 2017 Permalink | Reply
    Tags: , , Dark Matter, ,   

    From aeon: “In the dark” 

    1

    aeon

    6.29.17
    Alexander B Fry

    1
    Photomultiplier array at LUX, SURF, South Dakota. Photo courtesy of Luxdarkmatter.org

    Dark matter is the commonest, most elusive stuff there is. Can we grasp this great unsolved problem in physics?

    Lux Dark Matter Experiment

    SURF building in Lead SD USA

    LUX Xenon experiment at SURF, Lead, SD, USA


    I’m sitting at my desk at the University of Washington trying to conserve energy. It isn’t me who’s losing it; it’s my computer simulations. Actually, colleagues down the hall might say I was losing it as well. When I tell people I’m working on speculative theories about dark matter, they start to speculate about me. I don’t think everyone who works in the building even believes in it.

    In presentations, I point out how many cosmological puzzles it helps to solve. Occam’s Razor is my silver bullet: the fact that just one posit can explain so much. Then I talk about the things that standard dark matter doesn’t fix. There don’t seem to be enough satellite galaxies around our Milky Way. The inner shapes of small galaxies are inconsistent. I invoke Occam’s Razor again and argue that you can resolve these issues by adding a weak self-interaction to standard dark matter, a feeble scattering pattern when its particles collide. Then someone will ask me if I really believe in all this stuff. Tough question.

    The world we see is an illusion, albeit a highly persistent one. We have gradually got used to the idea that nature’s true reality is one of uncertain quantum fields; that what we see is not necessarily what is. Dark matter is a profound extension of this concept. It appears that the majority of matter in the universe has been hidden from us. That puts physicists and the general public alike in an uneasy place. Physicists worry that they can’t point to an unequivocal confirmed prediction or a positive detection of the stuff itself. The wider audience finds it hard to accept something that is necessarily so shadowy and elusive. The situation, in fact, bears an ominous resemblance to the aether controversy of more than a century ago.

    In the late-1800s, scientists were puzzled at how electromagnetic waves (for instance, light) could pass through vacuums. Just as the most familiar sort of waves are constrained to water — it’s the water that does the waving — it seemed obvious that there had to be some medium in which electromagnetic waves were ripples. Hence the notion of ‘aether’, an imperceptible field that was thought to permeate all of space.

    The American scientists Albert Michelson and Edward Morley carried out the most famous experiment to probe the existence of aether in 1887. If light needed a medium to propagate, they reasoned, then the Earth ought to be moving through this same medium. They set up an ingenious apparatus to test the idea: a rigid optics table floating on a cushioning vat of liquid mercury such that the table could rotate in any direction. The plan was to compare the wavelengths of light beams travelling in different relative directions, as the apparatus rotated or as the Earth swung around the sun. As our planet travelled along its orbit in an opposite direction to the background aether, light beams should be impeded, compressing their wavelength. Six months later, the direction of the impedance should reverse and the wavelength would expand. But to the surprise of many, the wavelengths were the same no matter what direction the beams travelled in. There was no sign of the expected medium. Aether appeared to be a mistake.

    This didn’t rule out its existence in every physicist’s opinion. Disagreement about the question rumbled on until at least some of the aether proponents died. Morley himself didn’t believe his own results. Only with perfect hindsight is the Michelson-Morley experiment seen as evidence for the absence of aether and, as it turned out, confirmation of Albert Einstein’s more radical theory of relativity.

    Dark matter, dark energy, dark money, dark markets, dark biomass, dark lexicon, dark genome: scientists seem to add dark to any influential phenomenon that is poorly understood and somehow obscured from direct perception. The darkness, in other words, is metaphorical. At first, however, it was intended quite literally. In the 1930s, the Swiss astronomer Fritz Zwicky observed a cluster of galaxies, all gravitationally bound to each other and orbiting one another much too fast. Only the gravitational pull of a very large, unseen mass seemed capable of explaining why they did not simply spin apart. Zwicky postulated the presence of some kind of ‘dark’ matter in the most casual sense possible: he just thought there was something he couldn’t see. But astronomers have continued to find the signature of unseen mass throughout the cosmos. For example, the stars of galaxies also rotate too fast. In fact, it looks as if dark matter is the commonest form of matter in our universe.

    It is also the most elusive. It does not interact strongly with itself or with the regular matter found in stars, planets or us. Its presence is inferred purely through its gravitational effects, and gravity, vexingly, is the weakest of the fundamental forces. But gravity is the only significant long-range force, which is why dark matter dominates the universe’s architecture at the largest scales.

    In the past half-century, we have developed a standard model of cosmology that describes our observed universe quite well.

    The standard cosmology model, ΛCDM model Cosmic pie chart after Planck Big Bang and inflation

    In the beginning, a hot Big Bang caused a rapid expansion of space and sowed the seeds for fluctuations in the density of matter throughout the universe. Over the next 13.7 billion years, those density patterns were scaled up thanks to the relentless force of gravity, ultimately forming the cosmic scaffolding of dark matter whose gravitational pull suspends the luminous galaxies we can see.

    This standard model of cosmology is supported by a lot of data, including the pervasive radiation field of the universe, the distribution of galaxies in the sky, and colliding clusters of galaxies. These robust observations combine expertise and independent analysis from many fields of astronomy. All are in strong agreement with a cosmological model that includes dark matter. Astrophysicists who try to trifle with the fundamentals of dark matter tend to find themselves cut off from the mainstream. It isn’t that anybody thinks it makes for an especially beautiful theory; it’s just that no other consistent, predictively successful alternative exists. But none of this explains what dark matter actually is. That really is a great, unsolved problem in physics.

    So the hunt is on. Particle accelerators sift through data, detectors wait patiently underground, and telescopes strain upwards. The current generation of experiments has already placed strong constraints on viable theories. Optimistically, the nature of dark matter could be understood within a few decades. Pessimistically, it might never be understood.

    We are in an era of discovery. A body of well-confirmed theory governs the assortment of fundamental particles that we have already observed. The same theory allows the existence of other, hitherto undetected particles. A few decades ago, theorists realised that a so-called Weakly Interacting Massive Particle (WIMP) might exist. This generic particle would have all the right characteristics to be dark matter, and it would be able to hide right under our noses. If dark matter is indeed a WIMP, it would interact so feebly with regular matter that we would have been able to detect it only with the generation of dark matter experiments that are just now coming on stream. The most promising might be the Large Underground Xenon (LUX) experiment in South Dakota, the biggest dark matter detector in the world. The facility opened in a former gold mine this February and is receptive to the most elusive of subatomic particles. And yet, despite LUX’s exquisite sensitivity, the hunt for dark matter itself has been something of a waiting game. So far, the only particles to turn up in the detector’s trap are bits of cosmic noise: nothing more than a nuisance.

    The past success of standard paradigms in theoretical physics leads us to hunt for a single generic dark matter particle — the dark matter. Arguably, though, we have little justification for supposing that there is anything to be found at all; as the English physicist John D Barrow said in 1994: ‘There is no reason that the universe should be designed for our convenience.’ With that caveat in mind, it appears the possibilities are as follows. Either dark matter exists or it doesn’t. If it exists, then either we can detect it or we can’t. If it doesn’t exist, either we can show that it doesn’t exist or we can’t. The observations that led astronomers to posit dark matter in the first place seem too robust to dismiss, so the most common argument for non-existence is to say there must be something wrong with our understanding of gravity – that it must not behave as Einstein predicted. That would be a drastic change in our understanding of physics, so not many people want to go there. On the other hand, if dark matter exists and we can’t detect it, that would put us in a very inconvenient position indeed.

    But we are living through a golden age of cosmology. In the past two decades, we have discovered so much: we have measured variations in the relic radiation of the Big Bang, learnt that the universe’s expansion is accelerating, glimpsed black holes and spotted the brightest explosions ever in the universe. In the next decades, we are likely to observe the first stars in the universe, map nearly the entire distribution of matter, and hear the cataclysmic merging of black holes through gravitational waves. Even among these riches, dark matter offers a uniquely inviting prospect, sitting at a confluence of new observations, theory, technology and (we hope) new funding.

    The various proposals to get its measure tend to fall into one of three categories: artificial creation (in a particle accelerator), indirect detection, and direct detection. The last, in which researchers attempt to catch WIMPs in the wild, is where the excitement is. The underground LUX detector is one of the first in a new generation of ultra-sensitive experiments. It counts on the WIMP interacting with the nucleus of a regular atom. These experiments generally consist of a very pure detector target, such as pristine elemental Germanium or Xenon, cooled to extremely low temperatures and shielded from outside particles. The problem is that stray particles tend to sneak in anyway. Interloper interactions are carefully monitored. Noise reduction, shielding and careful statistics are the only way to confirm real dark-matter interaction events from false alarms.

    Theorists have considered a lot of possibilities for how the real thing might work with the standard WIMP. Actually, the first generation of experiments has already ruled out the so-called z-boson scattering interaction. What is left is Higgs boson-mediated scattering, which would involve the same particle that the Large Hadron Collider discovered in Geneva in November last year.

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event


    Higgs Always the last place your look.

    That implies a very weak interaction, but it would be perfectly matched to the current sensitivity threshold of the new generation of experiments.

    Then again, science is less about saying what is than what is not, and non-detections have placed relatively interesting constraints on dark matter. They have also, in a development that is strikingly reminiscent of the aether controversy, thrown out some anomalies that need to be cleared up. Using a different detector target to LUX, the Italian DAMA (short for ‘DArk MAtter’) experiment claims to have found an annual modulation of their dark matter signal.

    DAMA-LIBRA at Gran Sasso

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

    Detractors dispute whether they really have any signal at all. Just like with the aether, we expected to see this kind of yearly variation, as the Earth orbits the Sun, sometimes moving with the larger galactic rotation and sometimes against it. The DAMA collaboration measured such an annual modulation. Other competing projects (XENON, CDMS, Edelweiss and ZEPLIN, for example) didn’t, but these experiments cannot be compared directly, so we should probably reserve judgment.

    XENON1T at Gran Sasso

    LBNL SuperCDMS, at SNOLAB (Vale Inco Mine, Sudbury, Canada)

    Edelweiss Dark Matter Experiment, located at the Modane Underground Laboratory in France

    Lux Zeplin project at SURF

    Nature can be cruel. Physicists could take non-detection as a hint to give up, but there is always the teasing possibility that we just need a better experiment. Or perhaps dark matter will reveal itself to be almost as complex as regular matter. Previous experiments imposed quite strict limitations on just how much complexity we can expect — there’s no prospect of dark-matter people, or even dark-matter chemistry, really — but it could still come in multiple varieties. We might find a kind of particle that explains only a fraction of the expected total mass of dark matter.

    In a sense, this has already occurred. Neutrinos are elusive but widespread (60 billion of them pass through an area the size of your pinky every second). They hardly ever interact with regular matter, and until 1998 we thought they were entirely massless. In fact, neutrinos make up a tiny fraction of the mass budget of the universe, and they do act like an odd kind of dark matter. They aren’t ‘the’ dark matter, but perhaps there is no single type of dark matter to find.

    To say that we are in an era of discovery is really just to say that we are in an era of intense interest. Physicists say we would have achieved something if we determine that dark matter is not a WIMP. Would that not be a discovery? At the same time, the field is burgeoning with ideas and rival theories. Some are exploring the idea that dark matter has interactions, but we will never be privy to them. In this scenario, dark matter would have an interaction at the smallest of scales which would leave standard cosmology unchanged. It might even have an exotic universe of its own: a dark sector. This possibility is at once terrifying and entrancing to physicists. We could posit an intricate dark matter realm that will always escape our scrutiny, save for its interaction with our own world through gravity. The dark sector would be akin to a parallel universe.

    It is rather easy to tinker with the basic idea of dark matter when you make all of your modifications very feeble. And so this is what all dark matter theorists are doing. I have run with the idea that dark matter might have self-interactions and worked that into supercomputer simulations of galaxies. On the largest scales, where cosmology has made firm predictions, this modification does nothing, but on small scales, where the theory of dark matter shows signs of faltering, it helps with several issues. The simulations are pretty to look at and they make acceptable predictions. There are too many free parameters, though — what scientists call fine-tuning — such that the results can seem tailored to fit the observations. That’s why I reserve judgement, and you would be well advised to do the same.

    We will probably never know for certain whether dark matter has self-interactions. At best, we might put an upper limit on how strong such interactions could be. So, when people ask me if I think self-interacting dark matter is the correct theory, I say no. I am constraining what is possible, not asserting what is. But this is kind of disappointing, isn’t it? Surely cosmology should hold some deep truth that we can hope to grasp.

    One day, perhaps, LUX or one of its competitors might discover just what they are looking for. Or maybe on some unassuming supercomputer, I will uncover a hidden truth about dark matter. Regardless, such a discovery will feel removed from us, mediated as it will be through several layers of ghosts in machines. The dark matter universe is part of our universe, but it will never feel like our universe.

    Nature plays an epistemological trick on us all. The things we observe each have one kind of existence, but the things we cannot observe could have limitless kinds of existence. A good theory should be just complex enough. Dark matter is the simplest solution to a complicated problem, not a complicated solution to simple problem. Yet there is no guarantee that it will ever be illuminated. And whether or not astrophysicists find it in a conceptual sense, we will never grasp it in our hands. It will remain out of touch. To live in a universe that is largely inaccessible is to live in a realm of endless possibilities, for better or worse.

    See the full article here .

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  • richardmitnick 5:00 pm on June 13, 2017 Permalink | Reply
    Tags: A different kind of dark matter could help to resolve an old celestial conundrum, , , , , Dark Matter, , Dark matter superfluid, Dark matter vortices, , Kent Ford, , ,   

    From Quanta: “Dark Matter Recipe Calls for One Part Superfluid” 

    Quanta Magazine
    Quanta Magazine

    June 13, 2017
    Jennifer Ouellette

    A different kind of dark matter could help to resolve an old celestial conundrum.

    1
    Markos Kay for Quanta Magazine

    For years, dark matter has been behaving badly. The term was first invoked nearly 80 years ago by the astronomer Fritz Zwicky, who realized that some unseen gravitational force was needed to stop individual galaxies from escaping giant galaxy clusters. Later, Vera Rubin and Kent Ford used unseen dark matter to explain why galaxies themselves don’t fly apart.

    Yet even though we use the term “dark matter” to describe these two situations, it’s not clear that the same kind of stuff is at work. The simplest and most popular model holds that dark matter is made of weakly interacting particles that move about slowly under the force of gravity. This so-called “cold” dark matter accurately describes large-scale structures like galaxy clusters. However, it doesn’t do a great job at predicting the rotation curves of individual galaxies. Dark matter seems to act differently at this scale.

    In the latest effort to resolve this conundrum, two physicists have proposed that dark matter is capable of changing phases at different size scales. Justin Khoury, a physicist at the University of Pennsylvania, and his former postdoc Lasha Berezhiani, who is now at Princeton University, say that in the cold, dense environment of the galactic halo, dark matter condenses into a superfluid — an exotic quantum state of matter that has zero viscosity. If dark matter forms a superfluid at the galactic scale, it could give rise to a new force that would account for the observations that don’t fit the cold dark matter model. Yet at the scale of galaxy clusters, the special conditions required for a superfluid state to form don’t exist; here, dark matter behaves like conventional cold dark matter.

    “It’s a neat idea,” said Tim Tait, a particle physicist at the University of California, Irvine. “You get to have two different kinds of dark matter described by one thing.” And that neat idea may soon be testable. Although other physicists have toyed with similar ideas, Khoury and Berezhiani are nearing the point where they can extract testable predictions that would allow astronomers to explore whether our galaxy is swimming in a superfluid sea.

    Impossible Superfluids

    Here on Earth, superfluids aren’t exactly commonplace. But physicists have been cooking them up in their labs since 1938. Cool down particles to sufficiently low temperatures and their quantum nature will start to emerge. Their matter waves will spread out and overlap with one other, eventually coordinating themselves to behave as if they were one big “superatom.” They will become coherent, much like the light particles in a laser all have the same energy and vibrate as one. These days even undergraduates create so-called Bose-Einstein condensates (BECs) in the lab, many of which can be classified as superfluids.

    Superfluids don’t exist in the everyday world — it’s too warm for the necessary quantum effects to hold sway. Because of that, “probably ten years ago, people would have balked at this idea and just said ‘this is impossible,’” said Tait. But recently, more physicists have warmed to the possibility of superfluid phases forming naturally in the extreme conditions of space. Superfluids may exist inside neutron stars, and some researchers have speculated that space-time itself may be a superfluid. So why shouldn’t dark matter have a superfluid phase, too?

    To make a superfluid out of a collection of particles, you need to do two things: Pack the particles together at very high densities and cool them down to extremely low temperatures. In the lab, physicists (or undergraduates) confine the particles in an electromagnetic trap, then zap them with lasers to remove the kinetic energy and lower the temperature to just above absolute zero.

    2
    Lucy Reading-Ikkanda/Quanta Magazine

    The dark matter particles that would make Khoury and Berezhiani’s idea work are emphatically not WIMP-like. WIMPs should be pretty massive as fundamental particles go — about as massive as 100 protons, give or take. For Khoury’s scenario to work, the dark matter particle would have to be a billion times less massive. Consequently, there should be billions of times as many of them zipping through the universe — enough to account for the observed effects of dark matter and to achieve the dense packing required for a superfluid to form. In addition, ordinary WIMPs don’t interact with one another. Dark matter superfluid particles would require strongly interacting particles.

    The closest candidate is the axion, a hypothetical ultralight particle with a mass that could be 10,000 trillion trillion times as small as the mass of the electron. According to Chanda Prescod-Weinstein, a theoretical physicist at the University of Washington, axions could theoretically condense into something like a Bose-Einstein condensate.

    But the standard axion doesn’t quite fit Khoury and Berezhiani’s needs. In their model, particles would need to experience a strong, repulsive interaction with one another. Typical axion models have interactions that are both weak and attractive. That said, “I think everyone thinks that dark matter probably does interact with itself at some level,” said Tait. It’s just a matter of determining whether that interaction is weak or strong.

    Cosmic Superfluid Searches

    The next step for Khoury and Berezhiani is to figure out how to test their model — to find a telltale signature that could distinguish this superfluid concept from ordinary cold dark matter. One possibility: dark matter vortices. In the lab, rotating superfluids give rise to swirling vortices that keep going without ever losing energy. Superfluid dark matter halos in a galaxy should rotate sufficiently fast to also produce arrays of vortices. If the vortices were massive enough, it would be possible to detect them directly.

    Inside galaxies, the role of the electromagnetic trap would be played by the galaxy’s gravitational pull, which could squeeze dark matter together enough to satisfy the density requirement. The temperature requirement is easier: Space, after all, is naturally cold.

    Outside of the “halos” found in the immediate vicinity of galaxies, the pull of gravity is weaker, and dark matter wouldn’t be packed together tightly enough to go into its superfluid state. It would act as dark matter ordinarily does, explaining what astronomers see at larger scales.

    But what’s so special about having dark matter be a superfluid? How can this special state change the way that dark matter appears to behave? A number of researchers over the years have played with similar ideas. But Khoury’s approach is unique because it shows how the superfluid could give rise to an extra force.

    In physics, if you disturb a field, you’ll often create a wave. Shake some electrons — for instance, in an antenna — and you’ll disturb an electric field and get radio waves. Wiggle the gravitational field with two colliding black holes and you’ll create gravitational waves. Likewise, if you poke a superfluid, you’ll produce phonons — sound waves in the superfluid itself. These phonons give rise to an extra force in addition to gravity, one that’s analogous to the electrostatic force between charged particles. “It’s nice because you have an additional force on top of gravity, but it really is intrinsically linked to dark matter,” said Khoury. “It’s a property of the dark matter medium that gives rise to this force.” The extra force would be enough to explain the puzzling behavior of dark matter inside galactic halos.

    A Different Dark Matter Particle

    Dark matter hunters have been at work for a long time. Their efforts have focused on so-called weakly interacting massive particles, or WIMPs. WIMPs have been popular because not only would the particles account for the majority of astrophysical observations, they pop out naturally from hypothesized extensions of the Standard Model of particle physics.

    Yet no one has ever seen a WIMP, and those hypothesized extensions of the Standard Model haven’t shown up in experiments either, much to physicists’ disappointment.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    With each new null result, the prospects dim even more, and physicists are increasingly considering other dark matter candidates. “At what point do we decide that we’ve been barking up the wrong tree?” said Stacy McGaugh, an astronomer at Case Western Reserve University.

    Unfortunately, this is unlikely to be the case: Khoury’s most recent computer simulations suggest that vortices in the dark matter superfluid would be “pretty flimsy,” he said, and unlikely to offer researchers clear-cut evidence that they exist. He speculates it might be possible to exploit the phenomenon of gravitational lensing to see if there are any scattering effects, similar to how a crystal will scatter X-ray light that passes through it.

    Gravitational Lensing NASA/ESA

    Astronomers could also search for indirect evidence that dark matter behaves like a superfluid. Here, they’d look to galactic mergers.

    The rate that galaxies collide with one another is influenced by something called dynamical friction. Imagine a massive body passing through a sea of particles. Many of the small particles will get pulled along by the massive body. And since the total momentum of the system can’t change, the massive body must slow down a bit to compensate.

    That’s what happens when two galaxies start to merge. If they get sufficiently close, their dark matter halos will start to pass through each other, and the rearrangement of the independently moving particles will give rise to dynamical friction, pulling the halos even closer. The effect helps galaxies to merge, and works to increase the rate of galactic mergers across the universe.

    But if the dark matter halo is in a superfluid phase, the particles move in sync. There would be no friction pulling the galaxies together, so it would be more difficult for them to merge. This should leave behind a telltale pattern: rippling interference patterns in how matter is distributed in the galaxies.

    Perfectly Reasonable Miracles

    While McGaugh is mostly positive about the notion of superfluid dark matter, he confesses to a niggling worry that in trying so hard to combine the best of both worlds, physicists might be creating what he terms a “Tycho Brahe solution.” The 16th-century Danish astronomer invented a hybrid cosmology in which the Earth was at the center of the universe but all the other planets orbited the sun. It attempted to split the difference between the ancient Ptolemaic system and the Copernican cosmology that would eventually replace it. “I worry a little that these kinds of efforts are in that vein, that maybe we’re missing something more fundamental,” said McGaugh. “But I still think we have to explore these ideas.”

    Tait admires this new superfluid model intellectually, but he would like to see the theory fleshed out more at the microscopic level, to a point where “we can really calculate everything and show why it all works out the way it’s supposed to. At some level, what we’re doing now is invoking a few miracles” in order to get everything to fit into place, he said. “Maybe they’re perfectly reasonable miracles, but I’m not fully convinced yet.”

    One potential sticking point is that Khoury and Berezhiani’s concept requires a very specific kind of particle that acts like a superfluid in just the right regime, because the kind of extra force produced in their model depends upon the specific properties of the superfluid. They are on the hunt for an existing superfluid — one created in the lab — with those desired properties. “If you could find such a system in nature, it would be amazing,” said Khoury, since this would essentially provide a useful analog for further exploration. “You could in principle simulate the properties of galaxies using cold atoms in the lab to mimic how superfluid dark matter behaves.”

    While researchers have been playing with superfluids for many decades, particle physicists are only just beginning to appreciate the usefulness of some of the ideas coming from subjects like condensed matter physics. Combining tools from those disciplines and applying it to gravitational physics might just resolve the longstanding dispute on dark matter — and who knows what other breakthroughs might await?

    “Do I need superfluid models? Physics isn’t really about what I need,” said Prescod-Weinstein. “It’s about what the universe may be doing. It may be naturally forming Bose-Einstein condensates, just like masers naturally form in the Orion nebula. Do I need lasers in space? No, but they’re pretty cool.”

    See the full article here .

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 5:10 pm on May 26, 2017 Permalink | Reply
    Tags: , Dark Matter, Dark photons, Dark sector particles, First results from search for a dark light, Jlab CEBAF accelerator,   

    From Symmetry: “First results from search for a dark light” 

    Symmetry Mag

    Symmetry

    05/26/17
    Manuel Gnida

    1
    Chris Smith, SLAC National Accelerator Laboratory

    The Heavy Photon Search at Jefferson Lab is looking for a hypothetical particle from a hidden “dark sector.”

    JLab campus

    2

    In 2015, a group of researchers installed a particle detector just half of a millimeter away from an extremely powerful electron beam. The detector could either start them on a new search for a hidden world of particles and forces called the “dark sector”—or its sensitive parts could burn up in the beam.

    Earlier this month, scientists presented the results from that very first test run at the Heavy Photon Search collaboration meeting at the US Department of Energy’s Thomas Jefferson National Accelerator Facility. To the scientists’ delight, the experiment is working flawlessly.

    Dark sector particles could be the long-sought components of dark matter, the mysterious form of matter thought to be five times more abundant in the universe than regular matter. To be specific, HPS is looking for a dark-sector version of the photon, the elementary “particle of light” that carries the fundamental electromagnetic force in the Standard Model of particle physics.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Analogously, the dark photon would be the carrier of a force between dark-sector particles. But unlike the regular photon, the dark photon would have mass. That’s why it’s also called the heavy photon.

    To search for dark photons, the HPS experiment uses a very intense, nearly continuous beam of highly energetic electrons from Jefferson Lab’s CEBAF accelerator. When slammed into a tungsten target, the electrons radiate energy that could potentially produce the mystery particles. Dark photons are believed to quickly decay into pairs of electrons and their antiparticles, positrons, which leave tracks in the HPS detector.

    4
    JLab CEBAF accelerator

    “Dark photons would show up as an anomaly in our data—a very narrow bump on a smooth background from other processes that produce electron-positron pairs,” says Omar Moreno from SLAC National Accelerator Laboratory, who led the analysis of the first data and presented the results at the collaboration meeting.

    The challenge is that, due to the large beam energy, the decay products are compressed very narrowly in beam direction. To catch them, the detector must be very close to the electron beam. But not too close—the smallest beam movements could make the beam swerve into the detector. Even if the beam doesn’t directly hit the HPS apparatus, electrons interacting in the target can scatter into the detector and cause unwanted signals.

    The HPS team implemented a number of precautions to make sure their detector could handle the potentially destructive beam conditions. They installed and carefully aligned a system to intercept any large beam motions, made the detector’s support structure movable to bring the detector close to the beam and measure the exact beam position, and installed a feedback system that would shut the beam down if its motions were too large. They also placed their whole setup in vacuum because interactions of the beam with gas molecules would create too much background. Finally, they cooled the detector to negative 30 degrees Fahrenheit to reduce the effects of radiation damage. These measures allowed the team to operate their experiment so close to the beam.

    “That’s maybe as close as anyone has ever come to such a particle beam,” says John Jaros, head of the HPS group at SLAC, which built the innermost part of the HPS detector, the Silicon Vertex Tracker. “So, it was fairly exciting when we gradually decreased the distance between the detector and the beam for the first time and saw that everything worked as planned. A large part of that success lies with the beautiful beams Jefferson Lab provided.”

    SLAC’s Mathew Graham, who oversees the HPS analysis group, says, “In addition to figuring out if we can actually do the experiment, the first run also helped us understand the background signals in the experiment and develop the data analysis tools we need for our search for dark photons.”

    So far, the team has seen no signs of dark photons. But to be fair, the data they analyzed came from just 1.7 days of accumulated running time. HPS collects data in short spurts when the CLAS experiment, which studies protons and neutrons using the same beam line, is not in use.

    A second part of the analysis is still ongoing: The researchers are also closely inspecting the exact location, or vertex, from which an electron-positron pair emerges.

    “If a dark photon lives long enough, it might make it out of the tungsten target where it was produced and travel some distance through the detector before it decays into an electron-positron pair,” Moreno says. The detector was specifically designed to observe such a signal.

    Jefferson Lab has approved the HPS project for a total of 180 days of experimental time. Slowly but surely, HPS scientists are finding chances to use it.

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 1:15 pm on May 25, 2017 Permalink | Reply
    Tags: , , , , Dark Matter, , , , ,   

    From U Chicago: “World’s most sensitive dark matter detector releases first results” 

    U Chicago bloc

    University of Chicago

    May 18, 2017
    news@uchicago.edu
    (773) 702-8360
    News media only

    UChicago scientists part of international XENON collaboration

    1
    XENON1T installation in the underground hall of Laboratori Nazionali del Gran Sasso. The three story building on the right houses various auxiliary systems. The cryostat containing the LXeTPC is located inside the large water tank on the left. Photo by Roberto Corrieri and Patrick De Perio

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

    2
    Scientists assembling the XENON1T time projection chamber. Photo by Enrico Sacchetti

    Scientists behind XENON1T, the largest dark matter experiment of its kind ever built, are encouraged by early results, describing them as the best so far in the search for dark matter.

    Dark matter is one of the basic constituents of the universe, five times more abundant than ordinary matter. Several astronomical measurements have corroborated the existence of dark matter, leading to an international effort to observe it directly. Scientists are trying to detect dark matter particle interacting with ordinary matter through the use of extremely sensitive detectors. Such interactions are so feeble that they have escaped direct detection to date, forcing scientists to build detectors that are more and more sensitive and have extremely low levels of radioactivity.

    On May 18, the XENON Collaboration released results from a first, 30-day run of XENON1T, showing the detector has a record low radioactivity level, many orders of magnitude below surrounding material on earth.

    “The care that we put into every single detail of the new detector is finally paying back,” said Luca Grandi, assistant professor in physics at the University of Chicago and member of the XENON Collaboration. “We have excellent discovery potential in the years to come because of the huge dimension of XENON1T and its incredibly low background. These early results already are allowing us to explore regions never explored before.”

    The XENON Collaboration consists of 135 researchers from the United States, Germany, Italy, Switzerland, Portugal, France, the Netherlands, Israel, Sweden and the United Arab Emirates, who hope to one day confirm dark matter’s existence and shed light on its mysterious properties.

    Located deep below a mountain in central Italy, XENON1T features a 3.2-ton xenon dual-phase time projection chamber. This central detector sits fully submersed in the middle of the water tank, in order to shield it from natural radioactivity in the cavern. A cryostat helps keep the xenon at a temperature of minus-95 degrees Celsius without freezing the surrounding water. The mountain above the laboratory further shields the detector, preventing it from being perturbed by cosmic rays.

    But shielding from the outer world is not enough, since all materials on Earth contain tiny traces of natural radioactivity. Thus extreme care was taken to find, select and process the materials making up the detector to achieve the lowest possible radioactive content. This allowed XENON1T to achieve record “silence” necessary to detect the very weak output of dark matter.

    A particle interaction in the one-ton central core of the time projection chamber leads to tiny flashes of light. Scientists record and study these flashes to infer the position and the energy of the interacting particle—and whether it might be dark matter.

    Despite the brief 30-day science run, the sensitivity of XENON1T has already overcome that of any other experiment in the field probing unexplored dark matter territory.

    “For the moment we do not see anything unexpected, so we set new constraints on dark matter properties,” Grandi said. “But XENON1T just started its exciting journey and since the end of the 30-day science run, we have been steadily accumulating new data.”

    UChicago central to international collaboration

    Grandi’s group is very active within XENON1T, and it is contributing to several aspects of the program. After its initial involvement in the preparation, assembly and early operations of the liquid xenon chamber, the group shifted its focus in the last several months to the development of the computing infrastructure and to data analysis.

    “Despite its low background, XENON1T is producing a large amount of data that needs to be continuously processed,” said Evan Shockley, a graduate student working with Grandi. “The raw data from the detector are directly transferred from Gran Sasso Laboratory to the University of Chicago, serving as the unique distribution point for the entire collaboration.”

    The framework, developed in collaboration with a group led by Robert Gardner, senior fellow at the Computation Institute, allows for the processing of data, both on local and remote resources belonging to the Open Science Grid. The involvement of UChicago’s Research Computing Center including Director Birali Runesha allows members of the collaboration all around the world to access processed data for high-level analyses.

    Grandi’s group also has been heavily involved in the analysis that led to this first result. Christopher Tunnell, a fellow at the Kavli Institute for Cosmological Physics, is one of the two XENON1T analysis coordinators and corresponding author of the result. Recently, UChicago hosted about 25 researchers for a month to perform the analyses that led to the first results.

    “It has been a large, concentrated effort and seeing XENON1T back on the front line makes me forget the never-ending days spent next to my colleagues to look at plots and distributions,“ Tunnell said. “There is no better thrill than leading the way in our knowledge of dark matter for the coming years.”

    See the full article here .

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    An intellectual destination

    One of the world’s premier academic and research institutions, the University of Chicago has driven new ways of thinking since our 1890 founding. Today, UChicago is an intellectual destination that draws inspired scholars to our Hyde Park and international campuses, keeping UChicago at the nexus of ideas that challenge and change the world.

     
  • richardmitnick 12:05 pm on May 23, 2017 Permalink | Reply
    Tags: , Bubble chamber, Dark Matter, FNAL PICO, , ,   

    From FNAL: “Sleuths use bubbles to look for WIMPs” 

    FNAL II photo

    FNAL Art Image
    FNAL Art Image by Angela Gonzales

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

    May 22, 2017
    Dan Garisto

    Invisible, imperceptible and yet far more common than ordinary matter, dark matter makes up an astounding 85 percent of the universe’s mass. Physicists are slowly but steadily tracking down the nature of this unidentified substance. The latest result from the PICO experiment places some of the best limits yet on the properties of certain types of dark matter.

    PICO searches for WIMPs (weakly interacting massive particles), a hypothesized type of dark matter particle that would interact only rarely, which makes them difficult to find.

    1
    FNAL PICO. 6,800 feet underground, PICO-60 is installed into its pressure vessel, which sits in a water tank. Photo: Dan Baxter

    In this extreme cosmic game of “Where’s Waldo?” the newest, most technologically complex detectors are usually considered the most promising. Many of these dark matter experiments rely on hundreds if not thousands of electrical channels and require racks of computer servers just to store the data they collect.

    But PICO relies on a simple phenomenon and a fairly low-key detector: bubbles, and a bubble chamber. At its core, PICO’s apparatus is simply a glass jar filled with fluid in which bubbles can form and be monitored by a video camera.

    Reinventing the bubble

    PICO had its beginnings in 2005 as a collaboration between the University of Chicago and the U.S. Department of Energy’s Fermilab. (The experiment started under a different name, COUPP, and later merged with the PICASSO experiment to form PICO.) In the experiment’s early days, much of Fermilab scientists’ work was devoted simply to developing bubble chamber technology. Because while the bubble chamber was hardly new — it was invented in 1952 — the technology had also been out of use for 20 years.

    Bubble chambers are designed to convert the energy deposited by a subatomic particle into a bubble that can be observed. In a liquid such as room temperature water, particle collisions do nothing noticeable. To achieve sensitivity to particles, the fluid inside bubble chambers is heated to just above its boiling point, so the slightest disruption could tip the fluid to a boiling state, creating a bubble.

    “You can actually watch the chamber and see the bubble form,” said Fermilab physicist Hugh Lippincott, a collaborator on PICO. In typical particle physics experiments, information about particle interactions is given solely through computer interfaces. In PICO, the interactions are visible to the naked eye as bubbles.

    “It’s great to press your face up against the glass and just … pop!” said Fermilab physicist Andrew Sonnenschein, also a collaborator on PICO.

    If WIMPs exist, they should occasionally interact with fluid in PICO’s bubble chamber, creating a certain number of bubbles every year.

    It was a return to old-school, low-tech particle physics when Fermilab collaborators began engineering the PICO bubble chamber, which is installed 2 kilometers underground at the Canadian laboratory SNOLAB.

    SNOLAB, Sudbury, Ontario, Canada.

    Bubble chambers of decades past had been used to track millions of charged particles such as protons and electrons, which would leave long, winding tracks in the fluid.

    “Old bubble chambers had a great run, but it ended in the ’80s,” Sonnenschein said. “They were too slow to keep up with experiments that had much larger data rates.”

    As a result, bubble chambers were phased out when modern particle colliders such as Fermilab’s Tevatron and CERN’s Large Hadron Collider took over. Using complex electronics, detectors at these colliders were able to collect millions of times more data than bubble chambers.

    FNAL Tevatron

    FNAL/Tevatron map


    FNAL/Tevatron DZero detector


    FNAL/Tevatron CDF detector

    LHC

    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    In fact, bubble chambers had been out of commission for so long that PICO’s founders had to go back to the drawing board, return to some of the papers of the original bubble chamber pioneers, and effectively reinvent the technology for detecting dark matter.

    “After the early bubble chamber designers figured out how to make them work to track high-energy particles with trails of bubbles, the basic ingredients of the recipe didn’t change. We’re looking for low-energy particles that make only single bubbles, so many things are different,” Sonnenschein said.

    The new design to allow bubble chambers to detect dark matter still preserves many of the elements from older bubble chamber detectors.

    “The thing that makes PICO interesting is that we’re using a relatively simple detector design compared to the other dark matter experiments,” said Dan Baxter, a Northwestern University graduate student and Fermilab fellow who was PICO’s latest run coordinator.

    Unlike traditional charged-particle-detecting bubble chambers, PICO’s bubble chamber is designed to look for elusive, neutrally charged WIMPs that might take years to make an appearance.

    “It’s using it in a different way,” Lippincott said. “In the old days, you would never expect to use a bubble chamber by just letting it sit there without anything happening.”

    3
    PICO-60’s inner vessel is cleaned to remove even microscopic particles. Photo: Dan Baxter

    A WIMPy bubble

    The weak force that governs WIMPs lives up to its name. For comparison, it’s about 10,000 times weaker than the electromagnetic force. Particles that interact through the weak force, such as WIMPs and neutrinos, don’t interact often, making them hard to capture. But even a slow-moving WIMP can deposit enough energy to be visible in a detector.

    By carefully calibrating heat and pressure in PICO’s bubble chamber fluid, scientists were able to make the detector sensitive only to the interactions from massive particles like WIMPs. PICO researchers were able to avoid much of the standard background, such as signals from electrons and gamma rays, that plague other dark matter detectors.

    Mastering the technology to do this took years. Predecessors to PICO started off as little more than test tubes filled with a few teaspoons of liquid. Gradually, the vessels grew larger. Then researchers added sound monitoring to their detectors to capture the “pops” from bubbles created by WIMPs.

    “We see a sound chirp,” Sonnenschein said, referring to the bubbles popping. “It turns out that if you look at the frequency content of the sound chirp and the amplitude, you can tell the difference between different kinds of particle interactions.”

    If a WIMP created a bubble, PICO would be able to not only see evidence of dark matter, but hear it as well. Using this acoustic technology, researchers were able to effectively veto bubbles that could not have been created by WIMPs, allowing them to eliminate background.

    As it turns out, PICO did not see any bubbles from WIMPs, so they were able to place limits on both WIMP masses and the likelihood that they will interact with matter — two factors that influence the number of bubbles WIMPs produce.

    Placing limits on these factors — mass and interaction rate — can tell physicists where they should look next for dark matter.

    Where no bubble has gone before

    “We don’t know what dark matter is, and so there’s a lot of theories about what it could be and about how it could interact with normal matter,” Baxter said.

    The variety of theories calls for a variety of different experiments. Other experiments search for different sources of dark matter, such as particles called axions or sterile neutrinos. PICO’s search for WIMPs has a specific focus on so-called spin-dependent WIMPs.

    “We don’t know what the WIMPs are,” Lippincott said. “But broadly speaking their interactions with normal matter would fall into two categories: one that isn’t sensitive to the spin of the nucleus, and one that is.”

    Spin, like charge, is an intrinsic quantity carried by particles and atomic nuclei. PICO looks primarily for WIMP interactions that are sensitive to the spin of the nucleus. To boost their resolution of these interactions, the researchers use a fluid with a liquid containing fluorine, which has a relatively large nuclear spin. With this method, PICO increased their ability to see spin-sensitive WIMPs by a factor of 17.

    Essentially, PICO’s result is that these spin-sensitive WIMPs, if they exist, must interact extremely infrequently — otherwise PICO would have seen more bubbles.

    This result, which is by far the best yet for spin-sensitive WIMPs interacting with protons, does not rule out the existence of WIMPs. There are many other places left to still look for dark matter, but thanks to PICO, fewer places for it to hide.

    The PICO collaboration currently has a proposal in to the Canada Foundation for Innovation to build the next generation of PICO chamber, and physicists like Lippincott and Sonnenschein remain optimistic because of the technology’s potential to scale up.

    “They’re pretty cheap once the engineering is done, mainly because they’re very simple mechanically. The fiddly bits are not very fiddly,” Lippincott said. “There’s a good chance that bubble chambers will continue to play a role in the hunt for dark matter.”

    PICO comprises about 50 physicists at 20 institutions in the Canada, Europe, India, Mexico and the United States and receives support from the U.S. Department of Energy Office of Science and National Science Foundation.

    See the full article here .

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    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. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
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  • richardmitnick 8:26 pm on May 21, 2017 Permalink | Reply
    Tags: , Dark Matter, Interactions.org, , , ,   

    From interactions.org: “XENON1T, the most sensitive detector on Earth searching for WIMP dark matter, releases its first result” 

    Interactions.org

    Laboratori Nazionali del Gran Sasso – INFN

    18 May 2017
    Contact:

    XENON spokesperson
    Prof. Elena Aprile, Columbia University, New York, US.
    Tel. +39 3494703313
    Tel. +1 212 854 3258
    age@astro.columbia.edu

    INFN spokesperson
    Roberta Antolini
    + 39 0862 437216
    Roberta.antolini@lngs.infn.it

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

    INFN Gran Sasso ICARUS, since moved to FNAL

    “The best result on dark matter so far! … and we have just started!”

    This is how scientists behind XENON1T, now the most sensitive dark matter experiment world-wide, hosted in the INFN Laboratori Nazionali del Gran Sasso, Italy, commented on their first result from a short 30-day run presented today to the scientific community.

    XENON1T at Gran Sasso

    Dark matter is one of the basic constituents of the Universe, five times more abundant than ordinary matter. Several astronomical measurements have corroborated the existence of dark matter, leading to a world-wide effort to observe directly dark matter particle interactions with ordinary matter in extremely sensitive detectors, which would confirm its existence and shed light on its properties. However, these interactions are so feeble that they have escaped direct detection up to this point, forcing scientists to build detectors that are more and more sensitive. The XENON Collaboration, that with XENON100 led the field for years in the past, is now back on the frontline with XENON1T. The result from a first short 30-day run shows that this detector has a new record low radioactivity level, many orders of magnitude below surrounding materials on Earth. With a total mass of about 3200 kg, XENON1T is at the same time the largest detector of this type ever built. The combination of significantly increased size with much lower background implies an excellent discovery potential in the years to come.

    The XENON Collaboration consists of 135 researchers from the US, Germany, Italy, Switzerland, Portugal, France, the Netherlands, Israel, Sweden and the United Arab Emirates. The latest detector of the XENON family has been in science operation at the LNGS underground laboratory since autumn 2016. The only things you see when visiting the underground experimental site now are a gigantic cylindrical metal tank, filled with ultra-pure water to shield the detector at his center, and a three-story-tall, transparent building crowded with equipment to keep the detector running, with physicists from all over the world. The XENON1T central detector, a so-called Liquid Xenon Time Projection Chamber (LXeTPC), is not visible. It sits within a cryostat in the middle of the water tank, fully submersed, in order to shield it as much as possible from natural radioactivity in the cavern. The cryostat allows keeping the xenon at a temperature of -95°C without freezing the surrounding water.

    The mountain above the laboratory further shields the detector, preventing it to be perturbed by cosmic rays. But shielding from the outer world is not enough since all materials on Earth contain tiny traces of natural radioactivity. Thus extreme care was taken to find, select and process the materials making up the detector to achieve the lowest possible radioactive content. Laura Baudis, professor at the University of Zürich and professor Manfred Lindner from the Max-Planck-Institute for Nuclear Physics in Heidelberg emphasize that this allowed XENON1T to achieve record “silence”, which is necessary to listen with a larger detector much better for the very weak voice of dark matter.

    A particle interaction in liquid xenon leads to tiny flashes of light. This is what the XENON scientists are recording and studying to infer the position and the energy of the interacting particle and whether it might be dark matter or not. The spatial information allows to select interactions occurring in the central 1 ton core of the detector. The surrounding xenon further shields the core xenon target from all materials which already have tiny surviving radioactive contaminants. Despite the shortness of the 30-day science run the sensitivity of XENON1T has already overcome that of any other experiment in the field, probing un-explored dark matter territory.

    “WIMPs did not show up in this first search with XENON1T, but we also did not expect them so soon!” says Elena Aprile, Professor at Columbia University and spokesperson of the project. “The best news is that the experiment continues to accumulate excellent data which will allow us to test quite soon the WIMP hypothesis in a region of mass and cross-section with normal atoms as never before. A new phase in the race to detect dark matter with ultra-low background massive detectors on Earth has just began with XENON1T. We are proud to be at the forefront of the race with this amazing detector, the first of its kind.”

    Further information:
    http://www.xenon1t.org
    http://www.lngs.infn.it

    See the full article here .

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