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CERN | Geneva | Switzerland

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Grey matter confronted to dark matter

After 18 years spent building the experiment and nearly two years taking data from the International Space Station, the Alpha Magnetic Spectrometer or AMS-02 collaboration showed its first results on Wednesday to a packed audience at CERN. But Prof. Sam Ting, one of the 1976 Nobel laureates and spokesperson of the experiment, only revealed part of the positron energy spectrum measured so far by AMS-02.

Positrons are the antimatter of electrons. Given we live in a world where matter dominates, it is not easy to explain where this excess of positrons comes from. There are currently two popular hypotheses: either the positrons come from pulsars or they originate from the annihilation of dark matter particles into a pair of electron and positron.  To tell these two hypotheses apart, one needs to see exactly what happens at the high-energy end of the spectrum. But this is where fewer positrons are found, making it extremely difficult to achieve the needed precision. Yesterday, we learned that AMS-02 might indeed be able to reach the needed accuracy.

The fraction of positrons (measured with respect to the sum of electrons and positrons) captured by AMS-02 as a function of their energy is shown in red. The vertical bars indicate the size of the uncertainty. The most important part of this spectrum is the high-energy part (above 100 GeV or 102) where the results of two previous experiments are also shown: Fermi in green and PAMELA in blue. Note that the AMS-02 precision exceeds the one obtained by the other experiments. The spectrum also extends to higher energy. The big question now is to see if the red curve will drop sharply at higher energy or not. More data is needed before the AMS-02 can get a definitive answer.

Only the first part of the story was revealed yesterday. The data shown clearly demonstrated the power of AMS-02. That was the excellent news delivered at the seminar: AMS-02 will be able to measure the energy spectrum accurately enough to eventually be able to tell where the positrons come from.

But the second part of the story, the punch line everyone was waiting for, will only be delivered at a later time. The data at very high energy will reveal if the observed excess in positrons comes from dark matter annihilation or from “simple” pulsars.  How long will it take before the world gets this crucial answer from AMS-02? Prof. Ting would not tell. No matter how long, the whole scientific community will be waiting with great anticipation until the collaboration is confident their measurement is precise enough. And then we will know.

If AMS-02 does manage to show that the positron excess has a dark matter origin, the consequences would be equivalent to discovering a whole new continent. As it stands, we observe that 26.8% of the content of the Universe comes in the form of a completely unknown type of matter called dark matter but have never been able to catch any of it. We only detect its presence through its gravitational effects. If AMS-02 can prove dark matter particles can annihilate and produce pairs of electrons and positrons, it would be a complete revolution.


Here are two plots to show how different the positron fraction spectrum (i.e. the curve showing the fraction of positrons as a function of energy) would differ at high energy (the rightmost part of the plot) if the positrons come from the sum of all pulsars around or if it comes from dark matter annihilation. Note they are not on the same scale and difficult to compare, but they still give some idea. It will be easier once theorists update their plots with the new AMS-02 data points on them and of course, once AMS-02 releases further information at high energy.

This is one theoretical prediction of what the positron fraction spectrum should look like if the positrons come from dark matter particles like neutralinos (represented by the symbol χ). Two curves are shown, depending on the hypothetical mass of the neutralino (mχ) at 400 GeV or 800 GeV. In each case, the maximum energy the positrons can get is roughly equal to the the mass of the neutralino, such that the curve ends close to the neutralino mass. Note the logarithmic scale on both axes.

Here is the expected spectrum if the positrons come from the sum of all pulsars. Three hypotheses were shown but only the middle one seemed to fit the PAMELA experimental results. The important feature is that this curve comes down smoothly, and not sharply at neutralino mass as with the dark matter hypothesis. Again, this curve only represents one theoretical prediction as done by Dan Hooper and his colleagues. The data point in red are from the PAMELA experiment and stop around 100 GeV. The hope is that AMS-02 will be able to provide accurate measurements at higher energies, up to several hundred GeV.

Pauline Gagnon

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  • You know too much physics to buy into the false “dark matter or pulsars” dichotomy, as well as the “looks like it could be dark matter” hype.

    Would have appreciated a much more measured post than this.

  • Thanks for sharing your thoughts on this. I guess we will have to agree to disagree on this point… As I said, these are the two most popular options.

    Cheers, Pauline

  • John

    Thanks for the post. I was looking for something that provided some explanation beyond the standard lay press release. This leaves me with more questions than answer unfortunately. Maybe you help with some:

    – Can you elaborate a little more on the what will be the discriminator between pulsar-related positrons and dark-matter related positrons?

    – I presume a sharp positron dropoff above 300 GeV will be indicative of pulsar positrons? Why? Does this assumption presuppose constraints on the WIMP mass?

    Thanks for clarifying…

  • Dark Matter and Dark Energy are Mirage http://arxiv.org/pdf/1004.4496

  • Hello,

    you have a very good point. I wanted to include these but did not want to make the text too long plus I could not find completely satisfactory plots. But please see the addendum I just posted on the blog. I hope it helps you see what I meant. The curve at high energy should have a different shape if the positrons come from pulsars or from dark matter annihilation.

    I hope this helps. Pauline

  • I completely agree with you on one point: it sure smells of ether… I think the important point is that something is wrong in our current understanding of the Universe. Either dark matter and dark energy exist, in which case we need to find out more about them, or it is a mirage as you said, which would mean that something else in our current understanding is completely wrong. Either way, we are bound to learn something new. Plus, we cannot shove under the cosmic rug 95% of the universe!

    Cheers, Pauline

  • Ting demands dark matter is greater than 175 GeV/c^2 stable non-baryonic particles (top quark, 172.9 GeV/c^2). Cryogenic collision detectors and Juan Collar’s bubble chambers detect no “hafnium atoms” flitting about.

    Assumed fermionic matter vacuum mirror symmetry suffers parity violations (patched by symmetry breakings). Vacuum is trace chiral anisotropic toward matter. Noether couples vacuum isotropy with angular momentum conservation. Matter leaks 1.2×10^(-10) m/s^2 Milgrom acceleration. Racemic molecular rotors’ temperatures diverge when traversing a vacuum chiral background, opposite shoes fit to one foot. Dark matter is falsifiable within minutes.

    A supersonic vacuum expansion ~5 kelvin racemic molecular beam enters a chirped pulse FT microwave spectrometer. Given Eötvös’ 5×10^(-14) mass/mass threshold, D_3-trishomocuban-4-one rotational energy divergence is [(/_\mc^2)(MW)]/[(molecules/mole)(Boltzmann’s constant)],

    [(4.49378 J/g)(166.212 g/mol)]/[(6.02214×10^23/mole)(1.380651×10^(-23) J/kelvin)] = 90 kelvin/molecule divergence

    30 g ketone obtained in seven steps

    Somebody should look. The worst it can do is succeed.

  • Dear Uncle Al,
    you are entitled to your opinion but nothing you say here makes sense to me…

  • John

    Pauline – thanks so much for your addendum. The addendum definitely clarifies my understanding. Unfortunately with that understanding comes other questions. Please indulge me,
    – does the AMS-02 have anisotropic detection capabilities? Can the insrument resolve source origin (which pulsar might be contributing to the positron levels) if not isotropic?
    – if WIMPs are majorana particles and are their own anti-particles, won’t this imply equal numbers of particle and antiparticle annihilation productions? Why is the ratio not 0.5? Is AMS-02 counting electrons that are NOT the result of WIMP annihilation?

    Thanks! John

  • cb

    I agree with Ethan Siegel and I would add that there is probably no such thing as “simple” pulsars…

  • I completely agree with you too. This was precisely what I meant and this is why I put simple between quotes. Of course there is no such things as simple pulsars. This was meant as a joke. Sorry if it was not clear.


  • dipankar chandra

    It has to be a modified gravity, not dark matter. My modified gravity does not have to be identical to the MOND theory. Gravity may be “quantized”, by changing its dependence on the distance systematically, and decreasing weakly with increasing distance for very large distances, and increasing strongly with decreasing distance for very short distances.

  • Dear John,

    glad you found the information useful. There is a very interesting paper that came out this morning talking about the anisotropy measurement. AMS-02 has some capability and showed their measurement last Wednesday. But apparently, there exists some data already taken b some telescopes that could determine this anisotropy with much greater accuracy. And indeed, this is a crucial test to distinguish between the pulsars and the dark matter hypothesis. See this paper: http://arxiv.org/pdf/1304.1791v1.pdf. They show there that the new AMS measurements are compatible with the pulsar hypothesis so far but a better measurement of the anisotropy would do a much better job at nailing the coffin on this hypothesis. what is great is the possibility that other data could be used to get more information on this anisotropy.

    Regarding your second point. There are way more sources of electrons contributing to cosmic rays than positrons. We live in a world of matter, so there all sorts of sources of electrons but fewer for positrons. So if you look at the fraction of positron with respect to the sum of electrons and positrons, this fraction is naturally below 50% since there are other sources for electrons. What is puzzling right now is precisely that it is hard to explain that we see so many positrons, especially that there is a rise in this fraction at high energy.

    I hope this helps.

  • Dale Berry

    Thank you for your interesting and highly readable posts, especially for the non-physicist. If dark matter feels only gravity, wouldn’t there be an increase in self-annihilation near a black hole? Especially if the dark matter is cold, I would imagine that the physics of dark matter would be pretty unusual near such an object, especially since those particles would not be affected by the extreme local electromagnetic environment. Thank you.

  • Dear Dale,

    thank you for this interesting question. I guess what you are asking is: given the strong gravitational field around a black hole, could one see an accumulation of dark matter in its surroundings? And would it behave in such a way that we could detect it? If this was the case, we would surely have heard about it. This is really a question for an astrophysicist (something I am not) but will try to find the answer for you.


  • CERN (Francais)

    Hello again Dale,

    sorry for the late reply. I was away the whole month of May. I finally had a chance to ask an expert. What he says is that indeed, since a black hole attracts matter through gravity, one would find more dark matter around it.

    For your second point,will the dark matter feel special effects near a black hole and behave differently. To answer this, according to relativity, remember that the laws of physics must be the same no matter in which frame of reference you are looking. One could imagine for example a reference frame moving free falling into the black hole.within that frame of reference, one would not know that it is moving and would not experience any change. Its mass would be the same as ever. So it means that it would be the same in all frames of reference.

    So there will be a higher density of dark matter around a black hole but it will behave as usual even though its speed as seen from outside will be greater.Its mass will be unchanged.

    I hope this helps, Pauline