≡ Menu

Positron Signaling For Dark Matter Inconclusive

The Fermi Gamma-ray Space Telescope (formerly called GLAST). Credit: NASA

A couple of years ago, the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics, PAMELA, sent us back some curious information… an overload of anti-matter in the Milky Way. Why does this member of the cosmic ray spectrum have interesting implications to the scientific community? It could mean the proof needed to confirm the existence of dark matter.

By employing the Fermi Large Area Telescope, researchers with the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University were able to verify the results of PAMELA’s findings. What’s more, by being in the high energy end of the spectrum, these abundances seem to verify current thinking on dark matter behavior and how it might produce positrons.

“There are various theories, but the basic idea is that if a dark matter particle were to meet its anti-particle, both would be annihilated. And that process of annihilation would generate new particles, including positrons.” says Stephan Funk, an assistant professor at Stanford and member of KIPAC. “When the PAMELA experiment looked at the spectrum of positrons, which means sampling positrons across a range of energy levels, it found more than would be expected from already understood astrophysics processes. The reason PAMELA generated such excitement is that it’s at least possible the excess positrons are coming from annihilation of dark matter particles.”

But there has been a glitch in what might have been a smooth solution. Current thinking has the positron signal dropping off when it reaches a specific level – a finding which wasn’t verified and led the researchers to feel the results were inconclusive. But the research just didn’t end there. The team consisting of Funk, Justin Vandenbroucke, a postdoc and Kavli Fellow and avli-supported graduate student Warit Mitthumsiri, came up with some creative solutions. While the Fermi Gamma-ray Space Telescope can’t distinguish between negatively charged electrons and positively charged positrons without a magnet – the group came up with their needs just a few hundred miles away.

Earth’s own magnetic field…

This illustration shows how the electron-positron sky appears to the Large Area Telescope. The purple region contains positrons while electrons are blocked by the Earth's bulk, the orange region contains electrons but is inaccessible to positrons, and the green region is completely out of the Earth's shadow for both positrons and electrons. Image courtesy Justin Vandenbroucke, Fermi-LAT collaboration.

That’s right. Our very own planet is capable of bending the paths of these highly charged particles. Now it was time for the research team to start a study on geophysics maps and figure out precisely how the Earth was sifting out the previously detected particles. It was a new way of filtering findings, but could it work?

“The thing that was most fun about this analysis for me is its interdisciplinary nature. We absolutely could not have made the measurement without this detailed map of the Earth’s magnetic field, which was provided by an international team of geophysicists. So to make this measurement, we had to understand the Earth’s magnetic field, which meant poring over work published for entirely different reasons by scientists in another discipline altogether.” said Vandenbroucke. “The big takeaway here is how valuable it is to measure and understand the world around us in as many ways as possible. Once you have this basic scientific knowledge, it’s often surprising how that knowledge can be useful.”

Oddly enough, they still came up with more than the expected amount of antimatter positrons as previously reported in Nature. But again, the findings didn’t show the theoretical drop-off that was to be expected if dark matter were involved. Despite these inconclusive results, it’s still a unique way of looking at difficult studies and making the most of what’s at hand.

“I find it to be fascinating to try to get the most out of an astrophysical instrument and I think we did that with this measurement. It was very satisfying that our approach, novel as it was, seemed to work so well. Also, you really have to go where the science takes you.” says Funk. “Our motivation was to confirm the PAMELA results because they are so exciting and unexpected. And as far as understanding what the Universe is actually trying to tell us here, I think it was important that PAMELA results were confirmed by a completely different instrument and technique.”

Original Story Source: Kavli Foundation News Release. For Further Reading: Measurement of separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope.

About 

Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.

Comments on this entry are closed.

  • Anonymous November 29, 2011, 5:40 PM

    This is a bit disappointing, but noteworthy. I am presuming the drop off initially reported is an artifact of the motion of positrons in the Earth’s magnetic field.

    LC

  • Torbjörn Larsson November 29, 2011, 8:51 PM

    Awesome experiment!

    But as for the field, I am trying to understand the rationale here and it makes me believe this is a long shot.

    First, pair production et cetera creates a background. As I remember it many signals have already been written off as dispersing from our SMBH and what not. If so the likelihood of success diminishes over time.

    Second, it dawned on me today that there are two huge assumptions here.

    Standard matter gives a relatively low background. And why is that? Because matter and antimatter exist in different amounts. This is after all why we are all here instead of having a universe filled with radiation.

    Standard matter is often claimed to have undergone what is called a spontaneous symmetry breaking. And that happened early on in our universe.

    That isn’t what happened for dark matter in the positron excess model then. Instead it could be that dark matter is different from standard matter, being a so called Majorana fermion. The absence of charges in dark matter allows that. So that is the first assumption.

    But note that we start to add up oddities.

    Fine, a particle that is its antiparticle can’t undergo symmetry breaking. And the absence of charges may make a reasonable annihilation rate.

    However, there is as of yet no known Majorana fermion. As noted, all Standard Model fermions are different, so called Dirac fermions. It may be that neutrinos are Majorana, which would explain their low mass. But this also means they are not acquiring mass as Standard Model particles, from Higgs particles. (See the 2nd link.)

    The second assumption that shores up the first is that the supersymmetric neutralino, which is a dark matter candidate, is Majorana. (See the 1st link.)

    And now we go through the catalog of oddities again.

    There is as of yet no known supersymmetric particle. Again, all Standard Model particles are completely different as I understand it. Supersymmetry means mixing internal and external degrees of freedom to get around a requirement on so called gauge theories of particle fields. But this is different from, say, mixing different field particles to get a physical particle (as is described in the 2nd link).

    It could happen, it just seems so tenuous for an outsider.

    Also, these experiments don’t seem to use the protocol of blinding et cetera that particle physicists use to get around tenuous spurious results? So how would we know when it is sensible to stop searching? (Of the two normal options “when we run out of new physics” may be eliminated and the remaining would be “when we run out of new funding”.)

    • Anonymous November 30, 2011, 1:19 PM

      Neutrinos might compose one part of dark matter. These particles weigh in at a few electron volts and could sum up to equal the matter bound up in stars. This would be an ?_? = 10^{-3}, which is still a small component of the entire luminous matter contribution ?_m = .045. This is rather surprising, but most luminous matter is in nebula and interstellar gas. The dark matter component is ?_dm = .23, which is more than two orders of magnitude larger than what neutrinos might contribute.

      Another contribution is the hypothetical axion particle. This is a scalar particle field associated with the nuclear force or QCD in a Lagrangian L = (?/4)F_{??}F^{??} where the field terms are contained in the component of the tensor F_{??}. The field term ? is a putative field which exhibits CP violations, so the weak and strong nuclear forces have the same parity violating physics. The axion decouples from the rest of quantum fields early in the universe and is then at near zero temperature. These could conceivably account for some portion of dark matter.

      The neutralino is the best theoretical candidate for DM. This is a condensate state for the supersymmetric partner to the photon, Z^0 and the neutral component of the Higgs doublet. All of these end up with the same quantum numbers and then form a composite quantum state with Majorana statistics.

      There are some speculations that stable supersymmetric partners form some component of DM as well. There are then ideas about there being an alternate world right in our midst with interactions between these superpartners that are very weakly coupled to luminous matter.

      LC

  • Anonymous November 29, 2011, 11:28 PM

    Can anyone explain that illustration better? Half the sky is filled with antimatter? I don’t get why earth is squished or how that would be a view from an earth based telescope at all.
    Also the word “with” needs to be inserted in the first sentence of the second last paragraph.

  • Anonymous November 29, 2011, 11:30 PM

    Oh, another question…Why do we think we know anything at all about Dark matter? Okay, gravity affects it and it doesn’t interact with normal baryonic matter but that doesn’t lead me very far.

    • Torbjörn Larsson November 30, 2011, 2:35 PM

      Because it shows up in standard cosmology as a prediction, and has been used successfully for structure formation prediction from the scale of the universe down to dwarf galaxies.

      It is also directly detected by observations of its gravitational effects in gravitational lenses of galactic scale and gravitational wells of universe scale, and indirectly by cluster behavior (say, the Bullet cluster observations).

      Just this week a lower limit constraint on DM particle mass was achieved for the first time (?) from observations. (Though I believe the Majorana assumption was made in order to arrive at the limit.)

    • Torbjörn Larsson November 30, 2011, 2:35 PM

      Because it shows up in standard cosmology as a prediction, and has been used successfully for structure formation prediction from the scale of the universe down to dwarf galaxies.

      It is also directly detected by observations of its gravitational effects in gravitational lenses of galactic scale and gravitational wells of universe scale, and indirectly by cluster behavior (say, the Bullet cluster observations).

      Just this week a lower limit constraint on DM particle mass was achieved for the first time (?) from observations. (Though I believe the Majorana assumption was made in order to arrive at the limit.)

  • Anonymous November 29, 2011, 11:30 PM

    Oh, another question…Why do we think we know anything at all about Dark matter? Okay, gravity affects it and it doesn’t interact with normal baryonic matter but that doesn’t lead me very far.

  • HeadAroundU November 30, 2011, 9:53 AM

    A bit off-topic. What’s up with AMS on ISS? Anybody has some news?

  • HeadAroundU November 30, 2011, 9:53 AM

    A bit off-topic. What’s up with AMS on ISS? Anybody has some news?

  • Anonymous November 30, 2011, 10:20 AM

    I absolutely LOVE make-do, DIY maverick scientific exploration like this! How fun. Great article.

  • Anonymous November 30, 2011, 10:20 AM

    I absolutely LOVE make-do, DIY maverick scientific exploration like this! How fun. Great article.

  • zetetic elench November 30, 2011, 2:55 PM

    ah now there’s another source of power for robotic probes around gas giants.
    deploy big loops of wire to generate induced current as it zings through the flux.

  • zetetic elench November 30, 2011, 2:55 PM

    ah now there’s another source of power for robotic probes around gas giants.
    deploy big loops of wire to generate induced current as it zings through the flux.

  • Otto Krog December 5, 2011, 7:11 PM

    My take on dark matter is, that it is a miscalculating, stemming from our presumption that the speed of light is constant.

    What if the speed of light varies through time and space?

    That creates some interesting theory, at least I think so.

    Antimatter is the mind and consciousness of all living entities.

    You are your own universe.

    Reality is where the minds (antimatter) meets the physical universe.

    Interested? Then read my philosophical multiverse theory.

    Google crestroyer theory and find it instantly

    http://crestroyertheory.com/the-theory/

hide