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  • richardmitnick 8:57 am on October 18, 2017 Permalink | Reply
    Tags: , , , DAMA LIBRA Dark Matter Experiment, Dark Matter candidates, , , , NIST PROSPECT detector, , , ,   

    From COSMOS: “Closing in on dark matter” 

    Cosmos Magazine bloc

    COSMOS Magazine

    18 October 2017
    Cathal O’Connell

    1
    Dark matter can’t be detected but it glues galaxies together. It outweighs ordinary matter by five to one. Maltaguy1/Getty Images

    One Saturday I hired a metal detector and drove four hours to the historic gold-rush town of Bright in Victoria, Australia, where my wedding ring lies lost, somewhere on the bed of the Ovens River. I spent the evening wading through the icy waters in gumboots, uncovering such treasures as a bottle cap, a fisher’s lead weight and a bracelet caked in rust. I did not uncover the ring. But that doesn’t mean the ring is not there.

    Like me, physicists around the world are in the midst of an important search that has so far proven fruitless. Their quarry is nothing less than most of the matter in the universe, so-called “dark matter”.

    So far their most sensitive detectors have found – to be pithy – nada. Despite the lack of results, scientists aren’t giving up. “The frequency with which articles show up in the popular press saying ‘maybe dark matter isn’t real’ massively exceeds the frequency with which physicists or astronomers find any reason to re-examine that question,” says Katie Mack, a theoretical astrophysicist at the University of Melbourne.

    In many respects, the quest for dark matter has only just begun. We can expect quite a few more null results before the real treasure turns up. So here is where we stand, and what we can expect from the next few years.

    Imagine a toddler sitting on one end of a seesaw and launching her father, at the other end, high into the air. It’s a weird and unsettling image, yet we regularly observe this kind of ‘impossible’ behaviour in the universe at large. Like the little girl on the seesaw, galaxies behave as if they have four or five times the mass we can see.

    Our first inkling of this discrepancy came in the 1930s, when the Swiss astronomer Fritz Zwicky noticed odd movements among the Coma cluster of galaxies.

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    Fritz Zwicky: The Father of Dark Matter. https://www.youtube.com/watch?v=TV0c1EFIKy4

    Zwicky’s anomaly was largely ignored until the 1970s, when astrophysicist Vera Rubin, based at the Carnegie Institute in Washington, noticed that the way galaxies spin did not tally with the laws of physics.

    3
    Astronomer Vera Rubin in 1974, with her “measuring engine” used to examine photographic plates. Credit: Courtesy of Carnegie Institution of Washington

    The meticulous observations by Rubin (who passed away in December 2016) convinced most of the astronomical community something was amiss. There were two possible answers to the problem: either galaxies were a lot heavier than they appeared, or our theory of gravity was kaput when it came to galaxy-scale movements.

    From the outset, astronomers preferred the first explanation. At first they thought the missing matter was probably nothing too weird – just regular astronomical objects (like planets, black holes and stars) too dim for us to see. But as we surveyed the sky with ever bigger telescopes, these so-called ‘massive compact halo objects’ (or MACHOs) never turned up in the numbers needed to explain all the extra mass.

    Other astrophysicists, such as the Mordehai Milgrom at Israel’s Weizmann Institute, explored models where gravity behaved differently at cosmic scales. [See https://sciencesprings.wordpress.com/2017/05/18/from-nautilus-the-physicist-who-denies-dark-matter/%5D

    5
    Mordehai Milgrom. Cosmos on Nautilus

    They were not successful.

    Slowly astronomers realised they had something radically different on their hands – a new kind of stuff they called ‘dark matter’, which must outweigh the universe’s regular matter by about five to one. “Certainly, when all the evidence is taken together,” Mack says, “there’s no competing idea right now that comes anywhere close to explaining it as well.”

    We know four main facts about dark matter. First, it has gravity. Second, it doesn’t emit, absorb or reflect light. Third, it moves slowly. Fourth, it doesn’t seem to interact with anything, even itself.

    Like detectives in a TV murder mystery, physicists have compiled a list of suspects. Topping the list are three hypothetical particles already wanted on other charges: axions, sterile neutrinos and WIMPs. Besides nailing dark matter, each would help explain a grand mystery of their own.

    The axion is a particle proposed by Roberto Peccei and Helen Quinn back in 1977 to explain a quirk of the strong force (namely, why it can’t distinguish left from right, the way the weak force does). Thirty years on, axions are still our best explanation for that puzzle.

    Axions could have any mass, but if – and it is a big ‘if’ – they have a mass about 100 billion times lighter than an electron, theorists have calculated they would have been created in the Big Bang in such vast numbers that they could account for the universe’s dark matter. Like detectives with a dragnet, physicists are searching through different possible masses in an attempt to close in from both ends and corner the axion.

    The Axion Dark Matter eXperiment (ADMX), based at the University of Washington, is dragging the lightest end of the range.

    U Washington ADMX


    U Washington ADMX Axion Dark Matter Experiment

    Since 2010 the project has been trying to catch axions by turning them into photons using strong magnetic fields. So far ADMX has ruled out the featherweight mass range between 150 to 270 billion times lighter than the electron.

    The CERN Axion Solar Telescope (CAST) is dragging the heavyweight end of the range looking for axions that are a few tens of millions to about a million times lighter than the electron.

    CERN CAST Axion Solar Telescope

    The theorised source of these hefty axions is the Sun, where they might be created by X-rays in the presence of strong electric fields. In an example of recycling at its big-science best, CAST was assembled from a piece of the Large Hadron Collider -– a giant test magnet. It aims to detect solar axions by turning them back into X-rays. It has been running since 2003. The search goes on.

    4
    Hypothetical particles known as axions could explain dark matter. Physicists at CERN have taken a giant magnet from the Large Hadron Collider and turned it into an axion detector, the CERN Axion Solar Telescope. Howard Cunningham/Getty Images

    Sterile neutrinos are the hypothetical heavier, lazier brothers of neutrinos – the ghostly, fast-moving particles created in nuclear reactions and in the centre of the Sun. They are called ‘sterile’ or ‘inactive’ because they only interact via gravity.

    Besides being a dark-matter candidate, sterile neutrinos would plug a number of holes in 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.

    which, like a subatomic version of the periodic table, has had great success in predicting the properties of the fundamental building blocks of the universe. For instance, sterile neutrinos could explain why neutrinos are so light, and why every neutrino we’ve ever seen has a ‘left-handed’ spin; sterile neutrinos would be the missing ‘right-handed’ partners.

    Physicists are trying to detect sterile neutrinos in different ways, including searching deep space for the X-rays emitted when they decay. NASA’s Chandra X-ray telescope has picked up an excess of X-rays from the Perseus cluster of galaxies, which is so far unexplained.

    NASA/Chandra Telescope

    6
    Perseus cluster. NASA

    Meanwhile, regular neutrino detectors based at nuclear reactors, such as Daya Bay in China, have noticed anomalies that might be explained by sterile neutrinos.

    Daya Bay, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China

    “Like Elvis, people see hints of the sterile neutrino everywhere,” quipped Francis Halzen in August 2016, when he and his colleagues at the IceCube Neutrino Observatory announced the disappointing results of their own search.


    U Wisconsin ICECUBE neutrino detector at the South Pole

    Their detector, buried up to 2.5 km deep in ice near the South Pole, found no evidence of the elusive sterile neutrino – a result that seems to rule out the Daya Bay reactor sightings. For a conclusive answer, we’ll need to wait for the next neutrino searches, such as the Precision Reactor Antineutrino Oscillation and Spectrum Measurement (PROSPECT) under construction at the US National Institute of Standards and Technology (NIST) in Maryland.

    8
    The PROSPECT detector will consist of an 11 x 14 array of long skinny cells filled with liquid scintillator, which is designed to sense antineutrinos emanating from the reactor core. If a sterile neutrino flavor exists, then PROSPECT will see waves of antineutrinos that appear and disappear with a period determined by their energy. Composition not drawn to scale. NIST.

    The third and most popular suspect is WIMPs – weakly interacting massive particles. The name covers a broad range of hypothetical particles that would interact via the weak force. They pop naturally out of the ideas of supersymmetry, an extension proposed to tidy up the loose ends of the Standard Model.

    Physicists calculate that the simplest possible WIMP, with a mass of about 100 billion electron volts, would have been created in the Big Bang at just the right numbers to explain dark matter: the so-called ‘WIMP miracle’.

    WIMP detectors are typically deep underground, watching for a telltale flash given out when a particle of dark matter bumps into an atomic nucleus.

    The most sensitive WIMP experiment yet is LUX, a bathtub-sized vat holding 370 kg of liquid xenon at the Sanford Underground Research Facility [SURF] in South Dakota. In 2016, the LUX team announced it had discovered no dark matter signals during its first 20-month-long search. Undeterred, the LUX team plan to upgrade to a 7,000-kg vat, LUX-ZEPLIN, by 2020.

    LBNL Lux Zeplin project at SURF

    The most intriguing dark matter result so far comes from the DAMA/LIBRA experiment in Italy. Using a detector made of highly purified sodium-iodide crystals, 1.5 km beneath Italy’s Gran Sasso mountain, scientists believe they have seen evidence of dark matter every year for the past 14 years (see Cosmos 65, p60). Their evidence comes in an annual rise and fall in background detections. Such a pattern might reflect the Earth’s relative speed through the dark-matter cloud that surrounds the Milky Way; while our planet moves around the Sun at 30 km/s, the Solar System as a whole is travelling at 230 km/s around the centre of the Milky Way.

    DAMA LIBRA Dark Matter Experiment, 1.5 km beneath Italy’s Gran Sasso mountain

    Gran Sasso LABORATORI NAZIONALI del GRAN SASSO, located in L’Aquila, Italy

    For half of the year the Earth’s orbital speed would add to the speed of the Solar System, increasing the rate of dark-matter interactions. For the other half, the speeds would subtract and the rate of interactions decrease. The problem is that lots of other things change with the seasons too, such as the thickness of the atmosphere. To rule out terrestrial effects, astronomers are setting up two identical detectors, called SABRE, in opposite hemispheres – so that one is collecting data in winter and the other in summer.

    One detector will be based at Gran Sasso, the other in Australia, in an abandoned gold mine near Stawell, Victoria. Each detector will be made of 50 kg of sodium iodide, and have noise levels 10 times lower than DAMA/LIBRA. Construction on each is under way, and could be finished this year.

    Rather than detecting dark matter, others are trying to make it. The closest we can get to the conditions of the Big Bang – where dark matter was presumably created – is in the collision chambers of the Large Hadron Collider, CERN’s 27-km long particle smasher. These chambers are ringed by sensors that can pick up the energies of millions of particles generated in each smash-up, and tally this against the known collision energy. If some energy is missing, it might indicate the creation of a particle that could not be detected by any sensors: dark matter.

    So far, notwithstanding a brief, hallucinatory blip in late 2015, the LHC has not discovered anything that might constitute a dark matter particle such as a WIMP. But the LHC has only collected about 1% of the data it is due to produce before it is retired in 2025. So it is too early to throw in the towel on producing dark matter yet. Plans are afoot for the LHC’s successor, which will be able to probe far higher energies.

    Snowmelt from the Alpine ranges had swelled the Ovens River. I had to hug the shore with my metal detector, where the water was shallow and easy to sweep. I searched those parts that I could search as thoroughly as possible. If I did not find my prize, I wanted to at least be able to point to the map and say with confidence where the ring was not.

    The map that physicists search has coordinates of energy levels and interaction strengths. Each new search sweeps out a new territory, so even a null result is valuable information. So far, in our search for the three primary candidates – axions, sterile neutrinos and WIMPs – we have only probed the most shallow, accessible waters. “There’s nothing really that says they have to be easy to detect,” Mack says. “It may just be that their interactions with our detectors are smaller than expected.”

    It took almost 50 years for the Higgs boson to be discovered. Gravitational waves took almost a century. Let’s not give up on dark matter just yet.

    I certainly won’t be giving up my own search. Next summer, when the Ovens dries, I will return to Bright and sweep the next unprobed area of the riverbed. I’d say wish me luck, but the point is to be so rigorous that luck has nothing to do with it.

    See the full article here .

    Please help promote STEM in your local schools.

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    Stem Education Coalition

     
  • richardmitnick 4:11 pm on September 1, 2017 Permalink | Reply
    Tags: , Dark Matter candidates, DEAP3600, , , ,   

    From TRIUMF: “New results surface from world’s most sensitive argon dark matter experiment” 

    TRIUMF

    31. August 2017

    1

    Argon in its natural form is a colourless, odorless, and non-flammable gas. It is also utterly unreactive – chemists and physicists have long wielded argon to formulate nonreactive and inert conditions. These qualities earned this noble gas its name, derived from the Greek word for ‘inactive.’

    What use, then, is a 3600-kilogram sphere of liquid argon, buried under two kilometers of Ontario bedrock?

    If you ask Dr. Pietro Giampa, a newly-joined TRIUMF scientist and recipient of the Otto Hausser Postdoctoral Fellowship, the simple answer (accompanied by a knowing smile) is: “Possibly changing our entire understanding of physics beyond the Standard Model, but also potentially the entire universe.” He delivers this response with the ease of repetition, a common trait among dark matter physicists. And while it may seem like a lofty claim, for Giampa and a dedicated team of particle physicists, astrophysicists, and astronomers at SNOLAB in Sudbury, ON, the proof may very well be in the depths of liquid argon.

    SNOLAB, Sudbury, Ontario, Canada.

    Deeper understanding

    The sphere of argon is a dark matter detector, and the central component of a state-of-the-art system called DEAP-3600: ‘Dark Matter Experiment using Argon Pulse-shape’ (with the argon weighing in at just over 3600 kilograms). Giampa and the DEAP-3600 team are working to characterize the fundamental properties of dark matter, a nebulous substance that makes up 23% of the mass of our universe and which we know next to nothing about.

    DEAP-3600 is in search of a host of particles widely considered the most viable candidates for dark matter: weakly interacting massive particles, or WIMPs. WIMPs behave similarly to the building-block particles of our universe like protons and neutrons, except that they don’t interact via any forces other than the electroweak and gravitational. This means that most WIMPs pass through our world without any interaction with atoms, subatomic particles, or nearly anything else.

    DEAP-3600 works by listening for collisions between dark matter and the nuclei of argon atoms. The impacts will be faint, and the apparatus can only listen in on one bandwidth at a time. Theoretical models beyond the Standard Model point to a WIMP of mass 100 gigaelectronvolts (GeV) or greater, a range DEAP is uniquely capable of investigating.

    Essentially, the detector provides a small sphere of space where collision events between WIMPs and the nuclei of argon atoms can be quietly recorded. Inactive argon, which undergoes no radioactive decay unless perturbed, is the perfect target for incoming dark matter particles; situating the argon sphere 2070 meters below Earth’s surface only heightens DEAP’s senses, eliminating the white noise of WIMP-like cosmic rays and muons. With a sufficiently large detector space and a sufficiently sensitive detection apparatus, there’s a chance that we’ll bear witness to the first WIMP ever observed as it glances off an argon atom.

    2
    DEAP-3600 takes a long, hard listen; silence.

    The DEAP team’s first results have surfaced: a new paper published by the group on August 1st, 2017 describes preliminary results from the experiment, and conclusions gleaned from just four and a half days of data-taking immediately following the completion of the detector system in August 2016. The paper details an extremely sensitive system, and a similarly sensitive, high-performance mathematical model for discriminating between the energy signals of WIMPs of different masses near the 100 GeV range.

    The experiment didn’t observe any dark matter-argon collisions during its initial monitoring period, but this absence of signal is itself a telling sign. While the number of potential WIMP-argon collisions is as large as the diversity of WIMP masses, it is finite – by ruling out different masses of WIMPs, Giampa and the DEAP team are honing in on the mass of the WIMP that may interact with an argon nucleus.

    Finding such a particle would be a boon for the field of particle physics. While WIMPS were chosen because they fit snugly into current theoretical models as potential dark matter particles, their discovery would have vast ramifications that extend beyond our current understanding of particle physics. Our entire concept of the universe would undergo a dramatic, tectonic shift.

    With this lofty goal as their north star, the DEAP team (including TRIUMF scientists Pierre-Andre Amadruz, Ben Smith, Thomas Lidner, and TRIUMF team leader Fabrice Retiere) will continue their search, re-calibrating and tuning into different bandwidths of potential collisions. Further data-taking has been ongoing since August 2016, and it’s possible that more results will surface soon.

    “We’re very excited to have proven the precision and sensitivity of the detector apparatus. While we’re but one of the many experiments around the world investigating the identity of dark matter, we can’t help but think that we are now one step closer to making this remarkable discovery.” – Dr. Pietro Giampa

    To keep tabs on the DEAP team or to learn more about the experiment, visit: http://deap3600.ca/

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition
    Triumf Campus
    Triumf Campus
    World Class Science at Triumf Lab, British Columbia, Canada
    Canada’s national laboratory for particle and nuclear physics
    Member Universities:
    University of Alberta, University of British Columbia, Carleton University, University of Guelph, University of Manitoba, Université de Montréal, Simon Fraser University,
    Queen’s University, University of Toronto, University of Victoria, York University. Not too shabby, eh?

    Associate Members:
    University of Calgary, McMaster University, University of Northern British Columbia, University of Regina, Saint Mary’s University, University of Winnipeg, How bad is that !!

     
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