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  • richardmitnick 2:56 pm on December 23, 2017 Permalink | Reply
    Tags: , Axion theory, , Cosmic Axion Spin-Precession Experiment (CASPEr), , International Linear Collider in Japan, Large Underground Xenon (LUX) dark matter experiment, LBNL LZ project at SURF Lead SD USA, MACHOs, SIMPs, ,   

    From UC Berkeley: “MACHOs are Dead. WIMPs are a No-Show. Say Hello to SIMPs” 

    UC Berkeley

    UC Berkeley

    December 4, 2017
    Robert Sanders

    The intensive, worldwide search for dark matter, the missing mass in the universe, has so far failed to find an abundance of dark, massive stars or scads of strange new weakly interacting particles, but a new candidate is slowly gaining followers and observational support.

    Fundamental structures of a pion (left) and a proposed SIMP (strongly interacting massive particle). Pions are composed of an up quark and a down antiquark, with a gluon (g) holding them together. A SIMP would be composed of a quark and an antiquark held together by an unknown type of gluon (G). (Kavli IPMU graphic)

    Called SIMPs – strongly interacting massive particles – they were proposed three years ago by UC Berkeley theoretical physicist Hitoshi Murayama, a professor of physics and director of the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) in Japan, and former UC Berkeley postdoc Yonit Hochberg, now at Hebrew University in Israel.

    Murayama says that recent observations of a nearby galactic pile-up [Nature] could be evidence for the existence of SIMPs, and he anticipates that future particle physics experiments will discover one of them.

    Murayama discussed his latest theoretical ideas about SIMPs and how the colliding galaxies support the theory in an invited talk Dec. 4 at the 29th Texas Symposium on Relativistic Astrophysics in Cape Town, South Africa.

    Astronomers have calculated that dark matter, while invisible, makes up about 85 percent of the mass of the universe. The solidest evidence for its existence is the motion of stars inside galaxies: Without an unseen blob of dark matter, galaxies would fly apart. In some galaxies, the visible stars are so rare that dark matter makes up 99.9 percent of the mass of the galaxy.

    Theorists first thought that this invisible matter was just normal matter too dim to see: failed stars called brown dwarfs, burned-out stars or black holes. Yet so-called massive compact halo objects – MACHOs – eluded discovery, and earlier this year a survey of the Andromeda galaxy by the Subaru Telescope basically ruled out any significant undiscovered population of black holes.

    NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA,4,207 m (13,802 ft) above sea level

    The researchers searched for black holes left over from the very early universe, so-called primordial black holes, by looking for sudden brightenings produced when they pass in front of background stars and act like a weak lens. They found exactly one – too few to contribute significantly to the mass of the galaxy.

    This Hubble Space Telescope image of the galaxy cluster Abell 3827 shows the ongoing collision of four bright galaxies and one faint central galaxy, as well as foreground stars in our Milky Way galaxy and galaxies behind the cluster (Arc B and Lensed image A) that are distorted because of normal and dark matter within the cluster. SIMPs could explain why the dark matter, unseen but detectable because of the lensing, lags behind the normal matter in the collision.

    “That study pretty much eliminated the possibility of MACHOs; I would say it is pretty much gone,” Murayama said.

    WIMPs — weakly interacting massive particles — have fared no better, despite being the focus of researchers’ attention for several decades. They should be relatively large – about 100 times heavier than the proton – and interact so rarely with one another that they are termed “weakly” interacting. They were thought to interact more frequently with normal matter through gravity, helping to attract normal matter into clumps that grow into galaxies and eventually spawn stars.

    SIMPs interact with themselves, but not others.

    SIMPs, like WIMPs and MACHOs, theoretically would have been produced in large quantities early in the history of the universe and since have cooled to the average cosmic temperature. But unlike WIMPs, SIMPs are theorized to interact strongly with themselves via gravity but very weakly with normal matter. One possibility proposed by Murayama is that a SIMP is a new combination of quarks, which are the fundamental components of particles like the proton and neutron, called baryons. Whereas protons and neutrons are composed of three quarks, a SIMP would be more like a pion in containing only two: a quark and an antiquark.

    Conventional WIMP theories predict that dark matter particles rarely interact. Murayama and Hochberg predict that dark matter SIMPs, comprised of a quark and an antiquark, would collide and interact, producing noticeable effects when the dark matter in galaxies collide. (Kavli IPMU graphic)

    The SIMP would be smaller than a WIMP, with a size or cross section like that of an atomic nucleus, which implies there are more of them than there would be WIMPs. Larger numbers would mean that, despite their weak interaction with normal matter – primarily by scattering off of it, as opposed to merging with or decaying into normal matter – they would still leave a fingerprint on normal matter, Murayama said.

    He sees such a fingerprint in four colliding galaxies within the Abell 3827 cluster, where, surprisingly, the dark matter appears to lag behind the visible matter. This could be explained, he said, by interactions between the dark matter in each galaxy that slows down the merger of dark matter but not that of normal matter, basically stars.

    “One way to understand why the dark matter is lagging behind the luminous matter is that the dark matter particles actually have finite size, they scatter against each other, so when they want to move toward the rest of the system they get pushed back,” Murayama said. “This would explain the observation. That is the kind of thing predicted by my theory of dark matter being a bound state of new kind of quarks.”

    SIMPs also overcome a major failing of WIMP theory: the ability to explain the distribution of dark matter in small galaxies.

    Conventional WIMP theories predict a highly peaked distribution, or cusp, of dark matter in a small area in the center of every galaxy. SIMP theory predicts a spread of dark matter in the center, which is more typical of dwarf galaxies. (Kavli IPMU graphic based on NASA, STScI images)

    “There has been this longstanding puzzle: If you look at dwarf galaxies, which are very small with rather few stars, they are really dominated by dark matter. And if you go through numerical simulations of how dark matter clumps together, they always predict that there is a huge concentration towards the center. A cusp,” Murayama said. “But observations seem to suggest that concentration is flatter: a core instead of a cusp. The core/cusp problem has been considered one of the major issues with dark matter that doesn’t interact other than by gravity. But if dark matter has a finite size, like a SIMP, the particles can go ‘clink’ and disperse themselves, and that would actually flatten out the mass profile toward the center. That is another piece of ‘evidence’ for this kind of theoretical idea.”

    Ongoing searches for WIMPs and axions

    Ground-based experiments to look for SIMPs are being planned, mostly at accelerators like the Large Hadron Collider at CERN in Geneva, where physicists are always looking for unknown particles that fit new predictions.


    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    Another experiment at the planned International Linear Collider in Japan could also be used to look for SIMPs.

    ILC schematic, being planned for the Kitakami highland, in the Iwate prefecture of northern Japan

    As Murayama and his colleagues refine the theory of SIMPs and look for ways to find them, the search for WIMPs continues. The Large Underground Xenon (LUX) dark matter experiment in an underground mine in South Dakota has set stringent limits on what a WIMP can look like, and an upgraded experiment called LZ will push those limits further. Daniel McKinsey, a UC Berkeley professor of physics, is one of the co-spokespersons for this experiment, working closely with Lawrence Berkeley National Laboratory, where Murayama is a faculty senior scientist.

    Lux Dark Matter 2 at SURF, Lead, SD, USA

    LBNL LZ project at SURF, Lead, SD, USA

    Physicists are also seeking other dark matter candidates that are not WIMPs. UC Berkeley faculty are involved in two experiments looking for a hypothetical particle called an axion, which may fit the requirements for dark matter. The Cosmic Axion Spin-Precession Experiment (CASPEr), led by Dmitry Budker, a professor emeritus of physics who is now at the University of Mainz in Germany, and theoretician Surjeet Rajendran, a UC Berkeley professor of physics, is planning to look for perturbations in nuclear spin caused by an axion field. Karl van Bibber, a professor of nuclear engineering, plays a key role in the (ADMX-HF), which seeks to detect axions inside a microwave cavity within a strong magnetic field as they convert to photons.

    ADMX Axion Dark Matter Experiment at the University of Washington

    “Of course we shouldn’t abandon looking for WIMPs,” Murayama said, “but the experimental limits are getting really, really important. Once you get to the level of measurement, where we will be in the near future, even neutrinos end up being the background to the experiment, which is unimaginable.”

    Neutrinos interact so rarely with normal matter that an estimated 100 trillion fly through our bodies every second without our noticing, something that makes them extremely difficult to detect.

    “The community consensus is kind of, we don’t know how far we need to go, but at least we need to get down to this level,” he added. “But because there are definitely no signs of WIMPs appearing, people are starting to think more broadly these days. Let’s stop and think about it again.”

    Murayama’s research is supported by the U.S. Department of Energy, National Science Foundation and Japanese Ministry of Education, Culture, Sports, Science and Technology. Murayama is also collaborating with Eric Kuflik of Hebrew University, Tomer Volansky of Tel Aviv University and Jay Wacker of Quora Inc. in Mountain View, California, and Stanford University.

    See the full article here .

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  • richardmitnick 12:35 pm on June 17, 2017 Permalink | Reply
    Tags: Axion theory, , , , , Helen Quinn and Roberto Peccei, Peccei-Quinn symmetry, , ,   

    From Quanta: “Roberto Peccei and Helen Quinn, Driving Around Stanford in a Clunky Jeep” 

    Quanta Magazine
    Quanta Magazine

    June 15, 2017
    Thomas Lin
    Olena Shmahalo, Art Director
    Lucy Reading-Ikkanda, graphics

    Ryan Schude for Quanta Magazine
    Helen Quinn and Roberto Peccei walking toward Stanford University’s new science and engineering quad. Behind them is the main quad, the oldest part of the campus. “If you look at a campus map,” said Quinn, who along with Peccei proposed Peccei-Quinn symmetry, “you will see the axis that goes through the middle of both quadrangle areas. We are on that line between the two.”

    Four decades ago, Helen Quinn and Roberto Peccei took on one of the great problems in theoretical particle physics: the strong charge-parity (CP) problem. Why does the symmetry between matter and antimatter break in weak interactions, which are responsible for nuclear decay, but not in strong interactions, which hold matter together?

    “The academic year 1976-77 was particularly exciting for me because Helen Quinn and Steven Weinberg were visiting the Stanford department of physics,” Peccei told Quanta in an email. “Helen and I had similar interests and we soon started working together.”

    Encouraged by Weinberg, who would go on to win a Nobel Prize in physics in 1979 for his work on the unification of electroweak interactions, Quinn and Peccei zeroed in on a CP-violating interaction whose strength can be characterized by an angular variable, theta. They knew theta had to be small, but no one had an elegant mechanism for explaining its smallness.

    “Steve liked to discuss physics over lunch, and Helen and I often joined him,” Peccei said. “Steve invariably brought up the theta problem in our lunch discussions, urging us to find a natural solution for why it was so small.”

    Quinn said by email that she and Peccei knew two things: The problem goes away if any quarks have zero mass (which seems to make theta irrelevant), and “in the very early hot universe all the quarks have zero mass.” They wondered how it could be that “theta is irrelevant in the early universe but matters once it cools enough that the quarks get their masses?”

    They proceeded to draft a “completely wrong paper based on conclusions we drew from this set of facts,” Quinn said. They went to Weinberg, whose comments helped clarify their thinking and, she said, “put us on the right track.”

    They realized they could naturally arrive at a zero value for theta by requiring a new symmetry, now known as the Peccei-Quinn mechanism. Besides being one of the popular proposed solutions to the strong CP problem, Peccei-Quinn symmetry also predicts the existence of a hypothetical “axion” particle, which has become a mainstay in theories of supersymmetry and cosmic inflation and has been proposed as a candidate for dark matter.

    Peccei and Quinn discussing their proposed symmetry with the aid of a sombrero. Ryan Schude for Quanta Magazine

    That year at Stanford, Quinn and Peccei regularly interacted with the theory group at the Stanford Linear Accelerator Center (SLAC) as well as with another group from the University of California, Santa Cruz.

    “We formed a large and active group of theorists, which created a wonderful atmosphere of open discussion and collaboration,” Quinn said, adding that she recalls “riding with Roberto back and forth from Stanford to SLAC in his yellow and clunky Jeep, talking physics ideas as we went.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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.

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