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Space Science

Cosmic Antimatter Excess Confirmed 113

sciencehabit writes "In 2008, the Italian satellite PAMELA picked up an unusual signal: a spike in antimatter particles whizzing through space. The discovery, controversial at the time, hinted that physicists might be coming close to detecting dark matter, an enigmatic substance thought to account for 85% of the matter in the universe. Now, new data from NASA's Fermi Gamma-ray Space Telescope confirm the spike (abstract)."
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Cosmic Antimatter Excess Confirmed

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  • by buchner.johannes ( 1139593 ) on Wednesday November 23, 2011 @05:18AM (#38146636) Homepage Journal

    The answer is in the second paragraph of the article.

  • by Anonymous Coward on Wednesday November 23, 2011 @05:32AM (#38146698)

    goatse alert

  • by justin12345 ( 846440 ) on Wednesday November 23, 2011 @05:43AM (#38146756)
    "Theorists generally believe that when two dark matter particles collide, they should annihilate each other to produce ordinary particles, such as an electron and its antimatter twin, a positron. Thanks to Einstein's iconic equivalence between energy and mass, E=mc2, each of those particles should emerge with an energy essentially equal to the mass of the original dark matter particle."

    I suspect that the author doesn't know that "dark matter" isn't a synonym for "antimatter". The above paragraph, if true, would make the universe a very explode-y place.
  • by sheepe2004 ( 1029824 ) on Wednesday November 23, 2011 @06:17AM (#38146892) Homepage
    Why is this modded +4 informative? The quoted text doesn't confuse dark matter and antimatter. The universe isn't explode-y because (if the theorists are right) dark matter interacts very weakly and so collisions are very rare.
  • by muon-catalyzed ( 2483394 ) on Wednesday November 23, 2011 @06:31AM (#38146942)
    > It doesn't matter
    yes it matters, particle physics is very important as are cosmic radiation studies, recetly Soudan 2 (underground proton decay particle detector) measure that 10000 relativistic muons are hitting every 1m2 of earth per minute (avg.), now we got this new type of cold fusion confirmed [], exciting times!
  • by sheepe2004 ( 1029824 ) on Wednesday November 23, 2011 @06:50AM (#38146994) Homepage
    I'm guessing you didn't RTFA? They are not saying that antimatter is dark matter.

    They have detected a large and unexpected amount of antimatter.
    Dark matter collisions (theoretically) can create large amounts of antimatter.
    So one possible explanation for the antimatter is that two dark matter particles collided.
  • by miketee ( 513478 ) on Wednesday November 23, 2011 @07:01AM (#38147030)
    Clearly, the author DOES differentiate between Dark matter, and antimatter (and matter). They use the terms to refer to different things: 2 Dark Matter particles BEFORE a collision, and a matter + antimatter particle AFTER it. If you meant that the particle/antiparticle pair would instantly annihilate (and the large amount of DM would cause many such annihilations), remember that the DM particles *collided*. IANA Physcist, but wouldn't momentum be conserved, and the 2 new particles move apart with the conserved momentum, preventing annihilation? Also there is the weak interactivity that others have mentioned.
  • by MrZilla ( 682337 ) on Wednesday November 23, 2011 @07:53AM (#38147256) Homepage

    I think you are confusing "massive" and "strongly interacting".

    The whole point of "dark matter" is that it interacts gravitationally with ordinary matter, but almost never in any other way. So, having massive dark matter particles means a higher gravitational field around them, but nothing else.

    I agree with your other point however, that having two of these dark matter particles annihilating directly to a electron/positron pair seems.. strange. Normal matter/antimatter annihilations always (afaik) produce "energy" (i.e. photons).

    But a good thing is that if annihilation of dark matter produces electron/positron pairs, then smashing electrons and positrons together in an accelerator should produce dark matter.

  • by Anonymous Coward on Wednesday November 23, 2011 @08:06AM (#38147338)

    Anything which can produce photons also can produce electron-positron pairs, just with lower probability. However dark matter particles should not produce photons directly because they don't interact electromagnetically (the defining property of dark matter!), and annihilating (directly) into photons would be an electromagnetic interaction (photons are not "pure energy", no matter how often you read that). Rather as weakly interacting particles, I'd expect them to produce virtual Z0s which then could decay into (real) electron-positron pairs, assuming sufficient energy (I'd expect the dominant decay channel to be into neutrinos, though).

  • by Stirling Newberry ( 848268 ) on Wednesday November 23, 2011 @08:58AM (#38147738) Homepage Journal
    The most promising wimp is a particle known as the neutralino. This is a hypothesized particle which would exist in either super-symmetrical theories. Super-symmetry says that in an unbroken general theory, every boson - a particle like a photon with an integer spin - has a fermion - 1/2 spin - partner, with the difference being that the fermion has a spin of 1/2. Since we don't seen bosons and fermions of the same energy, if, and it is still if, there was super-symmetry, it is a broken symmetry.

    The neutralino would be a composite particle, composed of the super-partners of the guage bosons and the higgs - that is wino (w partner), higgsino (higgs parnter), bino (partner of the weak hypercharge). Since the symmetry is broken, we don't see the original super-partners, only their super-imposed forms with the same mass eigenstate.

    When particles annihilate, they produce a set of particles that have a quantum number of 0. Any particles with the same mass-energy as the original colliding pair of particle and anti-particle can be produced. If mass energies are low, this means that the result will be mostly photons, because photons have no mass, and are only energy. That is, they have a low total mass energy. But any particles can be produced, so long as the result totals to 0, and has the same mass energy.

    Neutralinos, as you would guess, from the term WIMP, are weakly interacting, and massive. That means that when a neutralino annihilates another, particles with greater mass energy can be produced.

    In a 1994 paper Drees et al [] calculated neutralino decay into gluons. One of the co-authors here Kamionkowski went on to publish more on dark matter and neutralinos. There have been other papers on other possible decay products from neutralino annihilation, because, of course, if annihilation produces unstable particles, or anti-particle pairs, it can keep going until it reaches an end state of stable products. However, not all anti-particle pairs produce annihilate, and if the products are stable, they go bouncing on their merry way.

    This means that anti-protons and positrons above the background, and at certain energy levels could be the signature of neutralino dark matter.

    Or to roll things back: one of the few ways, other than gravity, we can detect WIMPS is from their annihilations. To determine if, and if so, what, WIMPs are composed of, we have to look at the decay products of those events. The Pamela data shows that there is an excess of positrons, however, it does not show that this excess is from WIMP annihilation. The search for this spectrum is important for both large and small reasons: large because cosmology evolves based on mass, and small because neutralinos, if detected, tell us about the final broken super-symmetrical extensions to the Standard Model, and in turn tell us about the super-partners, and, in turn, about the partners. For example, we have not seen a higgs boson, but a neutralino is an eigenstate of a higgsino fermion, which implies a higgs boson to be partnered with. Back in the 1990's Drees et al published

  • by Surt ( 22457 ) on Wednesday November 23, 2011 @11:04AM (#38149138) Homepage Journal

    It was mainly invented to explain why the amount of gravitational effects observed exceed the amount of mass visible. If dark matter has normal gravity, but interacts with other matter in an otherwise very limited fashion, then, no, there wouldn't need to be more of it.

  • by Pope ( 17780 ) on Wednesday November 23, 2011 @11:43AM (#38149634)

    Eight light minutes, actually.

  • by Roger W Moore ( 538166 ) on Wednesday November 23, 2011 @01:29PM (#38150692) Journal

    If the article would report everything, it would be talking about momentum, and virtual particles too.

    It would not report on virtual particles because the annihilation takes place in the galactic core where the densities of DM are highest and virtual particles can only exist for the tiniest fractions of an instant not the ~50k years needed to make it from the core.

    The question you should be asking is where are all the anti-protons? Since DM particles generally need to have masses roughly ~100 or more times the mass of the proton their annihilations should be capable of producing all stable anti-particles below this. Hence most models predict an excess of anti-protons as well as positrons but no satellite has seen any evidence of this. So if this positron excess is due to DM (and that is a BIG if!) we may have to start looking at some of the more exotic DM models (e.g. Arkani-Hamed et al. Phys Rev D (2009) vol. 79 (1) pp. 015014) which some of us are already looking for with the LHC.

"Well I don't see why I have to make one man miserable when I can make so many men happy." -- Ellyn Mustard, about marriage