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  • richardmitnick 1:31 pm on July 15, 2018 Permalink | Reply
    Tags: , , PandaX - Particle and Astrophysical Xenon Detector and PandaX-II, , SIDM - self-interacting dark matter model,   

    From UC Riverside: “In Search of Dark Matter” 

    UC Riverside bloc

    From UC Riverside

    July 12, 2018
    Iqbal Pittalwala

    Researchers, including a UC Riverside particle physicist, interpret new experimental data aimed at showing dark matter interacts with ordinary matter — an unmet challenge in modern physics.

    1
    Particle physicist Hai-Bo Yu is an assistant professor of physics and astronomy at UC Riverside. Photo credit: I. Pittalwala, UC Riverside.

    An international team of scientists that includes University of California, Riverside, physicist Hai-Bo Yu has imposed conditions on how dark matter may interact with ordinary matter — constraints that can help identify the elusive dark matter particle and detect it on Earth.

    Dark matter — nonluminous material in space — is understood to constitute 85 percent of the matter in the universe. Unlike normal matter, it does not absorb, reflect, or emit light, making it difficult to detect.

    Physicists are certain dark matter exists, having inferred this existence from the gravitational effect dark matter has on visible matter. What they are less certain of is how dark matter interacts with ordinary matter — or even if it does.

    In the search for direct detection of dark matter, the experimental focus has been on WIMPs, or weakly interacting massive particles, the hypothetical particles thought to make up dark matter.

    But Yu’s international research team invokes a different theory to challenge the WIMP paradigm: the self-interacting dark matter model, or SIDM, a well-motivated framework that can explain the full range of diversity observed in the galactic rotation curves. First proposed in 2000 by a pair of eminent astrophysicists, SIDM has regained popularity in both the particle physics and the astrophysics communities since around 2009, aided, in part, by work Yu and his collaborators did.

    Yu, a theorist in the Department of Physics and Astronomy at UCR, and Yong Yang, an experimentalist at Shanghai Jiaotong University in China, co-led the team analyzing and interpreting the latest data collected in 2016 and 2017 at PandaX-II, a xenon-based dark matter direct detection experiment in China (PandaX refers to Particle and Astrophysical Xenon Detector; PandaX-II refers to the experiment). Should a dark matter particle collide with PandaX-II’s liquefied xenon, the result would be two simultaneous signals: one of photons and the other of electrons.

    PandaX Experiment, located in the China Jin-Ping underground Laboratory

    PandaX II Dark Matter experiment at Jin-ping Underground Laboratory (CJPL) in Sichuan, China

    Yu explained that PandaX-II assumes dark matter “talks to” normal matter — that is, interacts with protons and neutrons — by means other than gravitational interaction (just gravitational interaction is not enough). The researchers then search for a signal that identifies this interaction. In addition, the PandaX-II collaboration assumes the “mediator particle,” mediating interactions between dark matter and normal matter, has far less mass than the mediator particle in the WIMP paradigm.

    “The WIMP paradigm assumes this mediator particle is very heavy — 100 to 1000 times the mass of a proton — or about the mass of the dark matter particle,” Yu said. “This paradigm has dominated the field for more than 30 years. In astrophysical observations, we don’t, however, see all its predictions. The SIDM model, on the other hand, assumes the mediator particle is about 0.001 times the mass of the dark matter particle, inferred from astrophysical observations from dwarf galaxies to galaxy clusters. The presence of such a light mediator could lead to smoking-gun signatures of SIDM in dark matter direct detection, as we suggested in an earlier theory paper [http://iopscience.iop.org/article/10.1088/1475-7516/2015/10/055/meta JCAP]. Now, we believe PandaX-II, one of the world’s most sensitive direct detection experiments, is poised to validate the SIDM model when a dark matter particle is detected.”

    The international team of researchers reports July 12 in Physical Review Letters the strongest limit on the interaction strength between dark matter and visible matter with a light mediator. The journal has selected the research paper as a highlight, a significant honor.

    “This is a particle physics constraint on a theory that has been used to understand astrophysical properties of dark matter,” said Flip Tanedo, a dark matter expert at UCR, who was not involved in the research. “The study highlights the complementary ways in which very different experiments are needed to search for dark matter. It also shows why theoretical physics plays a critical role to translate between these different kinds of searches. The study by Hai-Bo Yu and his colleagues interprets new experimental data in terms of a framework that makes it easy to connect to other types of experiments, especially astrophysical observations, and a much broader range of theories.”

    PandaX-II is located at the China Jinping Underground Laboratory, Sichuan Province, where pandas are abundant. The laboratory is the deepest underground laboratory in the world. PandaX-II had generated the largest dataset for dark matter detection when the analysis was performed. One of only three xenon-based dark matter direct detection experiments in the world, PandaX-II is one of the frontier facilities to search for extremely rare events where scientists hope to observe a dark matter particle interacting with ordinary matter and thus better understand the fundamental particle properties of dark matter.

    Particle physicists’ attempts to understand dark matter have yet to yield definitive evidence for dark matter in the lab.

    “The discovery of a dark matter particle interacting with ordinary matter is one of the holy grails of modern physics and represents the best hope to understand the fundamental, particle properties of dark matter,” Tanedo said.

    For the past decade, Yu, a world expert on SIDM, has led an effort to bridge particle physics and cosmology by looking for ways to understand dark matter’s particle properties from astrophysical data. He and his collaborators have discovered a class of dark matter theories with a new dark force that may explain unexpected features seen in the systems across a wide range, from dwarf galaxies to galaxy clusters. More importantly, this new SIDM framework serves as a crutch for particle physicists to convert astronomical data into particle physics parameters of dark matter models. In this way, the SIDM framework is a translator for two different scientific communities to understand each other’s results.

    Now with the PandaX-II experimental collaboration, Yu has shown how self-interacting dark matter theories may be distinguished at the PandaX-II experiment.

    “Prior to this line of work, these types of laboratory-based dark matter experiments primarily focused on dark matter candidates that did not have self-interactions,” Tanedo said. “This work has shown how dark forces affect the laboratory signals of dark matter.”

    Yu noted that this is the first direct detection result for SIDM reported by an experimental collaboration.

    “With more data, we will continue to probe the dark matter interactions with a light mediator and the self-interacting nature of dark matter,” he said.

    Yu was joined in the study by researchers from institutes in China, including Shanghai Jiaotong University, Xinjiang University, Yalong River Hydropower Development Company, Chinese Academy of Sciences, Shangdong University, Tsung-Dao Lee Institute, Peking University, and University of Shanghai for Science and Technology; and from the University of Maryland, College Park, USA. The spokesperson for the PandaX-II collaboration is Xiangdong Ji.

    A grant from the U.S. Department of Energy supported Yu.

    See the full article here .

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    Please help promote STEM in your local schools.

    Stem Education Coalition

    UC Riverside Campus

    The University of California, Riverside is one of 10 universities within the prestigious University of California system, and the only UC located in Inland Southern California.

    Widely recognized as one of the most ethnically diverse research universities in the nation, UCR’s current enrollment is more than 21,000 students, with a goal of 25,000 students by 2020. The campus is in the midst of a tremendous growth spurt with new and remodeled facilities coming on-line on a regular basis.

    We are located approximately 50 miles east of downtown Los Angeles. UCR is also within easy driving distance of dozens of major cultural and recreational sites, as well as desert, mountain and coastal destinations.

     
  • richardmitnick 12:55 pm on September 14, 2017 Permalink | Reply
    Tags: , , , , , , Dark matter may have strong self-interactions, Physicists Offer Explanation for Diverse Galaxy Rotations, SIDM - self-interacting dark matter model,   

    From UC Riverside: “Physicists Offer Explanation for Diverse Galaxy Rotations” 

    UC Riverside bloc

    UC Riverside

    September 14, 2017
    Iqbal Pittalwala

    1
    Hai-Bo Yu is an assistant professor of theoretical particle physics and astrophysics at UC Riverside. Photo credit: I. Pittalwala, UC Riverside.

    Identical twins are similar to each other in many ways, but they have different experiences, friends, and lifestyles.

    This concept is played out on a cosmological scale by galaxies. Two galaxies that appear at first glance to be very similar and effectively identical can have inner regions rotating at very different rates – the galactic analog of twins with different lifestyles.

    A team of physicists, led by Hai-Bo Yu of the University of California, Riverside, has found a simple and viable explanation for this diversity.

    Every galaxy sits within a dark matter halo that forms the gravitational scaffolding holding it together.

    Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

    The distribution of dark matter in this halo can be inferred from the motion of stars and gas particles in the galaxy.

    Yu and colleagues report in Physical Review Letters that diverse galactic-rotation curves, a graph of rotation speeds at different distances from the center, can be naturally explained if dark matter particles are assumed to strongly collide with one another in the inner halo, close to the galaxy’s center – a process called dark matter self-interaction.

    “In the prevailing dark matter theory, called Cold Dark Matter or CDM, dark matter particles are assumed to be collisionless, aside from gravity,” said Yu, an assistant professor of theoretical particle physics and astrophysics, who led the research. “We invoke a different theory, the self-interacting dark matter model or SIDM, to show that dark matter self-interactions thermalize the inner halo, which ties ordinary matter and dark matter distributions together so that they behave like a collective unit. The self-interacting dark matter halo then becomes flexible enough to accommodate the observed diverse rotation curves.”

    Yu explained that the dark matter collisions take place in the dense inner halo, where the luminous galaxy is located. When the particles collide, they exchange energy and thermalize. For low-luminous galaxies, the thermalization process heats up the inner dark matter particles and pushes them out of the central region, reducing the density, analogous to a popcorn machine in which kernels hit each other as they pop, causing them to fly up from the bottom of the machine. For high-luminous galaxies such as the Milky Way, thermalization pulls the particles into the deep potential well of the luminous matter and increases the dark matter density. In addition, the cosmological assembly history of halos also plays a role in generating the observed diversity.

    “Our work demonstrates that dark matter may have strong self-interactions, a radical deviation from the prevailing theory,” Yu said. “It well explains the observed diversity of galactic rotating curves, while being consistent with other cosmological observations.”

    Dark matter makes up about 85 percent of matter in the universe, but its nature remains largely unknown despite its unmistakable gravitational imprint on astronomical and cosmological observations. The conventional way to study dark matter is to assume that it has some additional, nongravitational interaction with visible matter that can be studied in the lab. Physicists do not know, however, if such an interaction between dark and visible matter even exists.

    Over the last decade, Yu has pioneered a new line of research based on the following premise: Setting aside whether dark matter interacts with visible matter, what happens if dark matter interacts with itself through some new dark force?

    Yu posited the new dark force would affect the dark matter distribution in each galaxy’s halo. He realized that there is indeed a discrepancy between CDM and astronomical observations that could be solved if dark matter is self-interacting.

    “The compatibility of this hypothesis with observations is a major advance in the field,” said Flip Tanedo, an assistant professor of theoretical particle physics at UC Riverside, who was not involved in the research. “The SIDM paradigm is a bridge between fundamental particle physics and observational astronomy. The consistency with observations is a big hint that this proposal has a chance of being correct and lays the foundation for future observational, experimental, numerical, and theoretical work. In this way, it is paving the way to new interdisciplinary research.”

    SIDM was first proposed in 2000 by a pair of eminent astrophysicists. It experienced a revival in the particle physics community around 2009, aided in part by key work by Yu and collaborators.

    “This is a special time for this type of research because numerical simulations of galaxies are finally approaching a precision where they can make concrete predictions to compare the observational predictions of the self-interacting versus cold dark matter scenarios,” Tanedo said. “In this way, Hai-Bo is the architect of modern self-interacting dark matter and how it merges multiple different fields: theoretical high-energy physics, experimental high-energy physics, observational astronomy, numerical simulations of astrophysics, and early universe cosmology and galaxy formation.”

    The research paper is included by Physical Review Letters as a “Editor’s Suggestion” and featured also in APS Physics.

    Yu was joined in the research by Ayuki Kamada, a postdoctoral researcher at UCR; and UC Irvine’s Manoj Kaplinghat and Andrew B. Pace.

    Yu’s research was supported by grants from the U.S. Department of Energy and the Hellman Fellows Fund. The National Science Foundation provided the research team with additional funding.

    See the full article here .

    [This article would have been helped with examples of galaxies.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    UC Riverside Campus

    The University of California, Riverside is one of 10 universities within the prestigious University of California system, and the only UC located in Inland Southern California.

    Widely recognized as one of the most ethnically diverse research universities in the nation, UCR’s current enrollment is more than 21,000 students, with a goal of 25,000 students by 2020. The campus is in the midst of a tremendous growth spurt with new and remodeled facilities coming on-line on a regular basis.

    We are located approximately 50 miles east of downtown Los Angeles. UCR is also within easy driving distance of dozens of major cultural and recreational sites, as well as desert, mountain and coastal destinations.

     
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