Tagged: Sam Ting Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 3:21 pm on September 28, 2017 Permalink | Reply
    Tags: AMS, , , , Sam Ting   

    From Quantum Diaries: “Latest news from outer space on dark matter” 

    9.28.17
    Pauline Gagnon

    Pauline Gagnon

    To celebrate the first five years of operation on board the International Space Station, Professor Sam Ting, the spokesperson for the Alpha Magnetic Spectrometer (AMS-02) Collaboration just presented their latest results at a recent seminar held at CERN. With a sample of 90 million events collected in cosmic rays, they now have the most precise data on a wide range of particles found in outer space.

    CERN Alpha Magnetic Spectrometer

    2
    Sam Ting, CERN and MIT

    Many physicists wonder if the AMS Collaboration will resolve the enigma on the origin of the excess of positrons found in cosmic rays. Positrons are the antimatter of electrons. Given that we live in a world made almost uniquely of matter, scientists have been wondering for more than a decade where these positrons come from. It is well known that some positrons are produced when cosmic rays interact with the interstellar material. What is puzzling is that more positrons are observed than what is expected from this source alone.

    Various hypotheses have been formulated to explain the origin of these extra positrons. One particularly exciting possibility is that these positrons could emanate from the annihilation of dark matter particles. Dark matter is some form of invisible matter that is observed in the Universe mostly through its gravitational effects. Regular matter, everything we know on Earth but also everything found in stars and galaxies, emits light when heated up, just like a piece of heated metal glows.

    Dark matter emits no light, hence its name. It is five times more prevalent than regular matter. Although no one knows, we suspect dark matter, just like regular matter, is made of particles but no one has yet been able to capture a particle of dark matter. However, if dark matter particles exist, they could annihilate with each other and produce an electron and a positron, or a proton and antiproton pair. This would at long last establish that dark matter particles exist and reveal some clues on their characteristics.

    An alternative but less exotic explanation would be that the observed excess of positrons comes from pulsars. Pulsars are neutron stars with a strong magnetic field that emit pulsed light. But light is made of photons and photons can also decay into an electron and a positron. So both the pulsar and the dark matter annihilation provide a plausible explanation on the source of these positrons.

    To tell the difference, one must measure the energy of all positrons found in cosmic rays and see how many are found at high energy. This is what AMS has done and their data are shown on the left plot below, where we see the flux of positrons (vertical axis) found at different energies (horizontal axis). The flux combines the number of positrons found with their energy cube. The green curve gives how many positrons are expected from cosmic rays hitting the interstellar material (ISM).

    If the excess of positrons were to come from dark matter annihilation, no positron would be found with an energy exceeding the mass of the dark matter particle. They would have an energy distribution similar to the brown curve on the plot below as expected for dark matter particles having a mass of 1 TeV, a thousand times heavier than a proton. In that case, the positrons energy distribution curve would drop off sharply. The red dots represent the AMS data with their experimental errors shown by the vertical bars. If, on the other end, the positrons came from pulsars, the drop at high energy would be less pronounced.

    2
    source: AMS Collaboration

    The name of the game is therefore to figure out precisely what is happening at high energy. But there are much fewer positrons there, making it very difficult to see what is happening as indicated by the large error bars attached to the data points at higher energy. These indicate the size of the experimental errors.

    But by looking at the fraction of positrons found in all data collected for electrons and positrons (right plot above), some of the experimental errors cancel out. AMS has collected over a million positrons and 16 million electrons. The red dots on the right plot show the fraction of positrons found in their sample as a function of energy. Given the actual precision of these measurements, it is still not completely clear if this fraction is really falling off at higher energy or not.

    The AMS Collaboration hopes however to have enough data to distinguish the two hypotheses by 2024 when the ISS will cease operation. These projections are shown on the next two plots both for the positrons flux (left) and the positron fraction (right). As it stands today, both hypotheses are still possible given the size of the experimental errors.

    3
    source: AMS Collaboration

    There is another way to test the dark matter hypothesis. By interacting with the interstellar material, cosmic rays produce not only positrons, but also antiprotons. And so would dark matter annihilations but pulsars cannot produce antiprotons. If there were also an excess of antiprotons in outer space that could not be accounted for by cosmic rays, it would reinforce the dark matter hypothesis. But this entails knowing precisely how cosmic rays propagate and interact with the interstellar medium.

    Using the AMS large sample of antiprotons, Prof. Sam Ting claimed that such excess already exists. He showed the following plot giving the fraction of antiprotons found in the total sample of protons and antiprotons as a function of their energy. The red dots represent the AMS measurements, the brown band, some theoretical calculation for cosmic rays, and the blue band, what could be coming from dark matter.

    4
    source: AMS Collaboration

    This plot clearly suggests that more antiprotons are found than what is expected from cosmic rays interacting with the interstellar material (ISM). But both Dan Hooper and Ilias Cholis, two theorists and experts on this subject, strongly disagree, saying that the uncertainty on this calculation is much larger. They say that the following plot (from Cuoco et al.) is by far more realistic. The pink dots represent the AMS data for the antiproton fraction. The data seem in good agreement with the theoretical prediction given by the black line and grey bands. So there are no signs of a large excess of antiprotons here. We need to wait for a few more years before the AMS data and the theoretical estimates are precise enough to determine if there is an excess or not.

    5
    source: Cuoco, Krämer and Korsmeier, arXiv:1610.03071v1

    The AMS Collaboration could have another huge surprise is stock: discovering the first antiatoms of helium in outer space. Given that anything more complex than an antiproton is much more difficult to produce, they will need to analyze huge amounts of data and further reduce all their experimental errors before such a discovery could be established.

    Will AMS discover antihelium atoms in cosmic rays, establish the presence of an excess of antiprotons or even solve the positron enigma? AMS has lots of exciting work on its agenda. Well worth waiting for it!

    Pauline Gagnon

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Participants in Quantum Diaries:

    Fermilab

    Triumf

    US/LHC Blog

    CERN

    Brookhaven Lab

    KEK

     
  • richardmitnick 2:23 pm on December 8, 2016 Permalink | Reply
    Tags: Alpha Magnetic Spectrometer experiment, , , , , , Sam Ting   

    From Symmetry: “A syllabus in cosmic rays” 

    Symmetry Mag

    Symmetry

    12/08/16
    Kathryn Jepsen

    What have scientists learned in five years of studying cosmic rays with the Alpha Magnetic Spectrometer experiment?

    1
    NASA

    On May 19, 2011, astronauts used a remote-controlled robotic arm to attach a nearly 17,000-pound payload to the side of the International Space Station. That payload was the Alpha Magnetic Spectrometer, or AMS-02, an international experiment sponsored by the US Department of Energy and NASA.

    AMS was designed to detect cosmic rays, highly energetic particles and nuclei that bombard the Earth from space. Since its installation, AMS has collected data from more than 90 billion cosmic ray events, experiment lead Sam Ting reported today in a colloquium at the experiment’s headquarters, CERN European research center.

    Ting, a Nobel Laureate and Thomas Dudley Cabot Professor of Physics at the Massachusetts Institute of Technology, shared a mix of new and recent results during his talk. Together they spelled out the persistent message of the AMS experiment: We have a lot left to learn from cosmic rays.

    For one, cosmic rays could tell us about the imbalance between matter and antimatter in the universe.

    Because matter and antimatter particles are created in pairs, scientists think the Big Bang should have produced half of each. But those evenly matched partners would have annihilated one another, and we would not exist.

    The generally accepted theory is that this imbalance came about thanks to processes in the very young universe that favor matter over antimatter. But an alternative idea is that a large amount of antimatter is still out there; it just hasn’t had a chance to collide with our matter-filled universe.

    One clue that this is the case would be finding an antimatter nucleus in the wild.

    With the negligible amount of antimatter that exists in our universe, “it’s almost impossible to make anything bigger than a proton,” says AMS Deputy Principal Investigator Mike Capell of MIT. “Getting the antimatter together to collide into an antihelium or anticarbon nucleus is not very probable.”

    AMS scientists do not claim to have detected antihelium, but they did announce that they have not ruled out “a few” candidate events.

    “Given the success of the standard cosmological model and the absence of gamma rays from hypothetical matter-antimatter interfaces, I think it’s very implausible that there’d be whole galaxies made of antimatter,” says theoretical astrophysicist Roger Blandford of the Kavli Institute for Particle Astrophysics and Cosmology, a joint institute of Stanford University and SLAC National Accelerator Laboratory. “But it’s the sort of investigation that could still give us a surprising discovery.”

    Cosmic rays could also tell us something about dark matter, which has never been detected directly.

    Cosmic rays can consist of a variety of particles, such as electrons or their antimatter counterparts, positrons. In previous measurements, AMS detected a surprising number of positrons on the higher end of its energy range. It is possible that collisions between dark matter particles created this excess of antimatter particles.

    An updated analysis—this one using almost double the number of electrons and positrons—continues to show this excess. But dark matter isn’t the only possible cause, Blandford says.

    “One interpretation is that one is seeing the annihilation of dark matter particles,” he says. “But there might be equally reasonable explanations associated with traditional astrophysics that could make the same sort of signal.”

    Pulsars are a particularly difficult alternative source to rule out. But AMS scientists anticipate that they will collect enough data to better discriminate between models by 2024, Ting said in his presentation.

    Cosmic rays could tell us about their history.

    As particles in cosmic rays approach light speed, time effectively slows down for them, as Albert Einstein predicted in his theory of relativity. We can see evidence of time dilation in the extended lifetimes of particles traveling near light speed.

    In a forthcoming AMS result, scientists look at just how much the lifetimes of isotopes of beryllium stretch as they travel in cosmic rays. Based on that measurement, they estimate the cosmic rays we see in our galaxy are about 12 million years old.

    Cosmic rays could tell us about what they go through on their trip to Earth.

    Both observation and theory have a ways to go in this area, Blandford says. “They are both works in progress and, despite great advances, we still do not understand how cosmic rays propagate from their sources—mainly supernova remnants—to Earth. ”

    When cosmic rays get into collisions, they can produce secondary cosmic rays, which are made up of different ingredients. In a recently published result studying the ratio of boron (found only in secondary cosmic rays) to carbon (found in primary cosmic rays) at different energies, AMS scientists found possible evidence of turbulence in the cosmic rays’ path to our planet—but nothing that would explain the positron excess.

    Finally, cosmic rays could tell us that we don’t know what we think we know.

    In an unpublished analysis, AMS scientists found that their measurements of the spectra and ratios of different nuclei—protons, lithium and helium—did not fit well with predictions. This could mean that scientists’ assumptions about cosmic rays need to be reexamined.

    AMS scientists want to help with that. They plan to collect data from hundreds of billions of primary cosmic rays in the coming years as their experiment continues its orbit about 240 miles above the Earth.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: