Antimatter. It's out there.
April 3, 2013 4:44 PM   Subscribe

A number of headlines have proclaimed the detection of "dark matter" today, but Science News has a more measured take. What we do know is that the Alpha Magnetic Spectrometer riding aboard the ISS has detected positrons at high energy. Some theorists suggests that dark matter collisions would generate these positrons, but dark matter annihilation should also produce antiprotons, gamma rays and radio waves, which have not yet been observed. Since dark matter is suspected to account for far more of the universe than ordinary matter, the AMS data is a tantalizing hint of what we might learn.
posted by CheeseDigestsAll (30 comments total) 12 users marked this as a favorite
I hate it when articles leave me with more questions than I started with.

I love it when articles leave me with more questions than I started with.
posted by Strass at 5:18 PM on April 3, 2013 [1 favorite]

The dark matter is where all the flavor positrons are!
posted by phaedon at 5:21 PM on April 3, 2013 [3 favorites]

Dark matter, wattsamatter with that?
posted by Fizz at 5:23 PM on April 3, 2013

I'm pretty sure dark matter and antimatter are different things.
posted by uosuaq at 5:24 PM on April 3, 2013

uosuaq: I think the article is saying that dark matter collisions decay into antimatter (aka positrons, the opposite of electrons and anti-protons, self-explanatory). The positrons are high energy (and presumably relatively unique, with the same mass as an electron and a charge of |e|, the opposite of an electron's charge), which is why the Alpha Magnetic Spectrometer is looking for them (and found them). However it hasn't detected other exotic decay particles like the anti-protons, so we don't know if these positrons are really from dark matter.
posted by Strass at 5:29 PM on April 3, 2013

I saw that, Strass, but the title of the post is "Antimatter. It's out there". I have a friend who works with antimatter, so I don't think that's news. Just a quibble.
posted by uosuaq at 5:35 PM on April 3, 2013 [1 favorite]

Phil Plait explains.
posted by stevis23 at 5:36 PM on April 3, 2013

Aah, well in that case then yeah, you're right. Positrons were discovered pretty early in the 20th century IIRC.
posted by Strass at 5:42 PM on April 3, 2013

Man, the things Google will do to promote their Ingress ARG.
posted by radwolf76 at 6:18 PM on April 3, 2013

The dark matter is where all the flavor positrons are!

Is that why robot brains are so delicious?
posted by RonButNotStupid at 6:48 PM on April 3, 2013

Let me be utterly clear: AMS did not find dark matter. AMS did not even get us a great deal closer to dark matter. AMS confirmed the positron fraction measured by the previous satellite experiment PAMELA. When the PAMELA results first came out, they were unexpected, and a great deal of work was done to try to explain the results in terms of dark matter. I'll get into the details in a minute. However, PAMELA results could be explained by nearby pulsars (again, I'll explain the details shortly). Unfortunately, both these explanations give extremely similar predictions (as they need to, since they're explaining the same set of data), and AMS certainly has not given any reason to suspect that dark matter is the source of the positron excess, and I'm somewhat at a loss at this point to imagine what data AMS will have in the next decade that will be able to distinguish between the two.

OK. So now I'll explain what AMS (and PAMELA before it) measured, then how that result can be explained by either dark matter or by pulsars.

AMS and PAMELA are space-based particle detectors that can measure both the energy of incoming particles and their charge. Both experiments are somewhat similar, in that they consist of layers of tracking calorimeters (basically "things that measure energy") inside a strong magnet. So, when a photon (called a gamma ray at these energies), or an electron, or a positron (the antimatter partner of an electron), or a proton, or an antiproton, etc from space enters the detector, it causes energy to be deposited inside the layers. The magnetic field causes the charged particles to bend, and the change in the track location as the particle moves through the layers of calorimeter allow us to determine the sign of the charge (positive and negative charges bend opposite directions in a magnetic field). The pattern of energy deposition gives the mass, so all together we get the mass, energy, and charge of the cosmic ray, and the combination of mass and charge tells us whether it's a proton, electron, positron, etc (there's only one thing in the Universe with mass 0.511 keV and charge +1, and that's a positron, for example). What AMS does better than PAMELA is be bigger and have a larger magnetic field. This allows more particles to be seen (larger detector = more cosmic rays), and their charges and energies to be better measured (deeper detector = more layers = better energy resolution, and bigger magnetic field = more bend in charged tracks = better charge discrimination).

The thing of interest today is the positron fraction: the ratio of the number of positrons at a given energy to the number of positrons + electrons. You can see the PAMELA and AMS results in Figure 3 here, along with the measurements from previous balloon-borne experiments at the South Pole (and Fermi, which is a whole nother thing). AMS can measure many other things, but this was the one we physicists were all interested in.

Why would you care about this? Well, the Universe is mostly made of matter, and not antimatter (a fact that we have yet to explain, but again, whole nother thing). Antimatter, positrons in this case, can be produced along with an electron in high energy collisions of other particles (this is called pair production). Locally (as in, the visible Universe), this sort of process should be the source of essentially 100% of the antimatter we detect. So, any excess of antimatter over predictions is a sign of some interesting physics that's creating electron-positron (e-/e+) pairs. At high energies, say, above 10 GeV (10 times the mass-energy of a proton, more or less) there are not a lot of known ways to produce large amounts of antimatter.

So, looking at the Figure I linked to previously, you see that from below 1 GeV up to 5 GeV, the positron fraction is dropping (look at the PAMELA and AMS data points in the Figure). The difference at very low energies is probably due to the fact that PAMELA collected data in ~2007, and AMS in 2012. Low energy positrons are primarily from the Sun, and the Sun has an ~11 solar cycle that significantly alters the local low-energy cosmic ray composition. That's my best guess as to that low energy difference between the two. Above ~5 Gev, both experiments see a significant rise in the positron fraction. This is not at all expected from our understanding of the "normal" processes that create cosmic rays, so it needs an explanation.

The explanation that people immediately jumped on in 2008 when the PAMELA results came out was dark matter. Dark matter could have been created in the early Universe due to "thermal freeze-out." Basically, right after the Big Bang, the Universe was so hot that all sorts of heavy particles were created in pairs, annihilated, and were recreated. Dark matter particles (whatever they end up being) would be among them. As the Universe cooled, the large mass of the dark matter would make it harder and harder to find two other particles with enough energy to smash together and create two new DM particles. But at the same time, the dropping number of DM particles would make it harder for two of them to hit each other and annihilate. Eventually, the rate of production and the rate of annihilation would both be low enough that essentially no dark matter particles were being created or destroyed: the dark matter had "frozen in" and we're stuck with the remainder.

Interestingly, if the dark matter interaction rate (measured by the "cross section" - think of it as the size of a dark matter particle) was larger, we'd have LESS dark matter today; the annihilation would have continued for longer. Normal particle-antiparticle pairs have such a large cross section with itself that protons (for example) cannot be a thermal relic like dark matter might be. For one thing, there are essentially no antiprotons going around so they can't have been pair produced, and for another, there'd be 10^10 fewer protons around if they were created thermally.

However, while this sort of "thermal" dark matter would be frozen in, annihilations never really stop. They're just very very rare. Sometimes, two dark matter particles in our galactic halo will find each other, hit, and annihilate into... something. If they annihilated into e-/e+ pairs, we'd see that as an increase in the positron fraction. If the dark matter were heavy (several 1000 GeV), it could explain the PAMELA (and now AMS) excess.

There are some issues with this from a theory perspective (my area of particle physics). For one, the rate needed to explain PAMELA/AMS is about 1000 times larger than the rate of annihilation in the early Universe that would give us the observed amount of dark matter today. Remember that as the cross section increases, the amount of frozen-in DM decreases. So if the cross section needed to explain PAMELA were the cross section of DM back in the day, we'd have only about 1/1000 of the DM we see see. There are ways around this, which lead to some interesting model-building ideas, and PAMELA kicked off a minor industry of theorists working out the consequences of these ideas (the first major paper on this was this one). One of the consequences of this work by theorists was a realization that we had missed a whole class of possible dark matter models, and that there were things we could do with existing machines to look for them. So regardless if PAMELA/AMS is dark matter, it revealed an important line of inquiry that's being pursued today.

However, there's another possibility. This is the boring option (from a particle physicist's standpoint). The positrons could be coming from pulsars. Pulsars are the neutron star cores left over from the supernova of massive stars (ones that aren't quite big enough to be black holes). Like a skater pulling in their arms, as the star collapses into an object a few 10's of miles across, it's angular momentum causes it to spin faster. Furthermore, the huge magnetic field of the star can be inherited by the pulsar; this magnetic field can accelerate charged particles in jets along magnetic north and south (which can be misaligned with the axis of rotation). If that jet happens to point to the Earth sometimes during the spin, we see the pulsar as "blinking" on and off like a lighthouse (thus, a pulsing star = pulsar).

The huge magnetic field of pulsars can create positrons through interactions with photons. The spectrum of positrons depends on the age and magnetic field strength of the pulsar, but for some of them, the positrons would be in the 10-100's of GeV range, and the positron fraction is remarkably similar to that seen by PAMELA/AMS. There'd have to be some nearby pulsars to give the observed rate (local here is 1-2 kpc, or a few thousand light years), but we know of a few that fit the bill: Geminga and Vela are two.

I just want stop and emphasize that the boring option is that antimatter is being produced by the rapidly spinning corpse of a dead star, accelerated to insane energies by magnetic fields so intense they'd kill you from AU away, and thrown across a thousand light years.

So that's where we are: we have two experiments seeing very interesting data that could be explained either by new physics (dark matter) or "boring" physics (pulsars). The spectra are not quite the same; depending on the pulsars, we'd expect the positron fraction to drop more rapidly at high energies in the dark matter hypothesis, but as you see from the results today, AMS doesn't see a turn-over in the spectrum, and in my personal opinion, even if it did, it wouldn't be 100% convincing to me that we're seeing new physics. There's just too much uncertainly in how these particular pulsars create positrons (As Anna Karenina said, pulsars are like unhappy families, they're each unique).

You might imagine that we'd see the positrons coming from the pulsars if that's where they're coming from. Both PAMELA and AMS have pointing information, after all. However, the Galaxy is full of magnetic fields stretching for kiloparsecs (I'm a bit unclear on how they're generated, but apparently, magnetic fields are a general consequence of plasmas, which the interstellar medium is composed of). Magnetic fields quickly bend electrons and positrons, even at these very high energies, nearly completely obscuring the origin (they also rob the particles of energy, so the source of these particles must be nearby - 1 or 2 kpc). So, for the known pulsars, we expect only a 0.1-1% anisotropy (that is, the intensity of positrons from the pulsar compared to another random direction would be up by on 0.1-1%). AMS ruled out anisotropies larger than 3.5%. Even if you did see an anisotropy, it'd be easy for me to come up with a dark matter model with a similar result.

So the bottom line: I don't see how AMS can say they've discovered dark matter, or even gotten us hints of dark matter. I'm actually very disappointed that they've pushed this narrative on the media: you guys who are interested in science deserve better. The experiment is a huge technical achievement, it's doing good science, and there's no reason they should be overplaying their results in an attempt to make it sound sexier or more important.
posted by physicsmatt at 7:05 PM on April 3, 2013 [156 favorites]

Wow, thanks physicsmatt for taking the time to write that great explanation.
posted by nixxon at 7:22 PM on April 3, 2013

*** Response from physicsmatt. Achievement unlocked. ***

I admit it, part of my rationale for posting this was to troll for an educated response to all the PR-y stuff in the papers. The Science News article did provide a negative counterpoint (both regarding the lack of anti-protons, and the quote from Falkowski), but it wasn't as clear as your response.

The thing I felt was most unclear about all the articles was why we believe that dark matter would generate particle/anti-particle pairs. From what you're saying, I gather that's based more on knowing what energies dark matter would have to generate than knowing anything about what dark matter is "made of."
posted by CheeseDigestsAll at 7:47 PM on April 3, 2013

Fantastic comment; I'm glad physicsmatt put in a few words about the "boring option" not being quite so boring.

A couple of minor additions to just the pulsar aspect:
* There are at least 106 pulsars with nominal distances less than a kiloparsec (3260 or so light years).
* They have magnetic fields ranging from a billion to a trillion Gauss (if a typical pulsar replaced the Moon, it would demagnetize the credit cards in your pocket, but that would be the least of your worries), in some cases going up to 100 trillion Gauss ("magnetars", but those are rare and much further away). One way pulsars might get these extraordinary magnetic field strengths is to retain the entire magnetic flux of their parent stars, but compress it down to their tiny surface areas. That works on a hand-waving level.

And the socio-political aspect: I understand that NASA has been under a LOT of pressure to justify the existence of the International Space Station, and it hasn't been a particularly good scientific platform so far. If I remember right, back when the AMS experiment was launched, there was a lot of grumbling about how it could have been done for cheaper and better using a custom satellite mission rather than a shuttle launch to take it to the ISS. So I think the first results from the AMS are inevitably going to be drowned in hype, whether the scientists want it or not...
posted by RedOrGreen at 7:58 PM on April 3, 2013 [2 favorites]

Yeah, you shouldn't think of it as dark matter "containing" e-/e+ pairs. If dark matter annihilates, it can annihilate into damn near anything: photons, neutrino pairs, electron/positron pairs, muon-antimuon, tau-antitau, quark-antiquark, W bosons, whatever. Most of these are unstable, and you'd end up with a cascade of stuff in the decays. For example, if you annihilate into W bosons, then you tend to get a shitload of electrons, positrons, photons, and quarks which hadronize into mesons and baryon-antibaryons. One of the other things we'd like to see from AMS actually is anti-deutrium and other anti-nuclei, since one of the only places you can get such heavy hadrons is through dark matter annihilation. If you create an electron however, you pretty much have to create an antipartner. This is because electrons carry a conserved quantum number called "lepton number" (leptons are non-strongly interacting matter fields: electrons, muons, taus, their partner neutrinos, and their antiparticles). Lepton number is not expected to be violated in dark matter interactions, and dark matter is not expected to possess "lepton number" (though of course, there are models where it could. We're theorists, we have models for everything). So you start with zero lepton number, and you must end with zero lepton number: which means making an electron and a positron (or a muon and an antimuon, etc).

The end products of dark matter annihilation are model-dependent (which means: we have no idea, as theorists, we try to guess what it could be). Constraints exist from many experiments, usually looking for gamma rays which are pretty ubiquitous in most charged particle final states. Actually forcing only e-/e+ pairs is tricky, and part of the model-building effort I talked about above. The one that Arkani-Hamed et al found ties in nicely with the proposed solution to make the cross section much larger, so it's nice when you solve two problems at once.

RedOrGreen, yes, that's true, and the personalities involved certainly also play a role. However, we're supposed to be better than that, and I think promising more than we can deliver will bite us in the ass in the end. Most people won't notice, but I'm sure the science fans who actually follow this sort of result would.
posted by physicsmatt at 8:08 PM on April 3, 2013 [2 favorites]

I had been under the impression too that attaching experiments to the outside of the ISS was a bit of an odd way to go about things, but I've talked to some people who are currently scoping out small missions and they are actually pretty optimistic about using the ISS. It basically saves the cost of a satellite bus, which can be a significant fraction for small missions. You loose pointing control and are stuck in LEO, but for some cases where you just want to be above the atmosphere it can be a good deal. Not sure what the current status of transporting hardware up there is though.
posted by kiltedtaco at 8:19 PM on April 3, 2013

Science headlines have gotten more than a little hyperbolic these days. Lots of "could be"s and "may"s and "promise"s. Lots of sell, not so much product.
posted by Twang at 8:50 PM on April 3, 2013

physicsmatt: can you explain to me exactly how dark matter is self-annihilating and what hypothesized particle actually explains this? Is it the bino and if so, is the bino its own anti-particle?
posted by empath at 9:33 PM on April 3, 2013

And also, if we're talking about something like the bino (or gravitino, etc) would having one of them necessitate there being the entire zoo of super-partners?
posted by empath at 9:34 PM on April 3, 2013

Lots of sell, not so much product.

That's crazy talk. We, right now, this very day, are living in the golden age of scientific discovery.
posted by amorphatist at 9:43 PM on April 3, 2013

What if the quantum-entangled galactic internet is built with high-energy positrons so intelligent civilizations will realize the particles are anomalous and, after ruling out all competing theories, figure out how to build a modem.
posted by crayz at 12:46 AM on April 4, 2013 [1 favorite]

I've still no idea what dark matter could be, and how it relates to the boring old undark world of particles, quarks and the rest of the menagerie, so retain the hope that the real answer to all the things it is the 'best' answer to, may be far weirder. But that doesn't bother me unduly; I'm happy to take the physicists' word for it that dark matter is the least bad answer by quite a long way and that, once we work out what the hell we're looking for, we'll find it. (Not that this seems to have worked for gravity waves, but.)

What bothers me is how pulsars work. I'm OK about where they come from, how those insane magnetic fields create the pulses, and some of the other amazing things they can do. But it only recently occurred to me, having seen lots of funky diagrams and animations over the years, that if neutron stars are made of neutrons, and magnetism comes from charged particles doing stuff (or having stuff done to them), and neutrons aren't charged particles... well, you see my confusion. Is it that there are enough electrons in a neutron star anyway? Is that scrotum-tightening density doing something mucky to normal rules?

This really bothers me, mostly because I love radio and pulsars are good at radio, and when I hit something that doesn't seem to make sense, I take it as a personal insult to hide my feelings of inadequacy.

As for science journalism being a bit pants at the moment: it so is. The reasons are manifold, but are all various colours of money. Or, perhaps, 'dark' money: we know it must be there, because it's distorting the cosmos, but by Jiminy I can't actually find any of it.
posted by Devonian at 3:43 AM on April 4, 2013 [1 favorite]

What bothers me is how pulsars work. I'm OK about where they come from, how those insane magnetic fields create the pulses, and some of the other amazing things they can do. But it only recently occurred to me, having seen lots of funky diagrams and animations over the years, that if neutron stars are made of neutrons, and magnetism comes from charged particles doing stuff (or having stuff done to them), and neutrons aren't charged particles

They also have stuff like accretion disks and polar jets from matter surrounding the star whirling around it, which can generate fields.
posted by empath at 4:40 AM on April 4, 2013

Devonian: Well neutron stars are for starters not simply balls of neutrons. There's a whole load of crazy stuff going on inside them. Stuff which is fairly poorly understood because we can't observe them easily and physics of nuclear matter is nontrivial. There's fairly mundane stuff at the outer parts made of more normal nuclei and electrons, regions with free moving neutrons further in, phases of 'spaghetti' strands and 'lasagne' sheets of nuclear matter, before it reaches more spongy nuclear structures, finally what you might have been thinking of as neutron star material and then beyond that... well, goodness knows what kind of weird particle plasmas might be possible in towards the core. Some of those regions have very high electrical conductivities too - ideal for supporting the currents to generate giant magnetic fields.
posted by edd at 5:17 AM on April 4, 2013

Actually brushing up on it, parts of the neutron star can be superconductive, not just highly conductive.
posted by edd at 5:26 AM on April 4, 2013

OK, last ridiculously long answer here, I swear... it's just too much fun to resist.

empath: Well, since we don't know what dark matter is (as in, we don't know the mass and interactions of whatever it is that is making dark matter), I have no idea *how* dark matter is annihilating. But I can tell you what we usually assume, and what you need to futz with to get PAMELA/AMS to work.

Let's start with something that we know exists, like e-/e+ annihilation. As I've said before, particles can be thought of as little vibrations of the particle field: so electrons and positrons are quantized vibrations of the "electron field." We say that the electron field is "coupled" to the photon (or electromagnetic) field (and the Z-boson field, and the gravitational field). What this means is that vibrations of the electron field (e- or e+ particles) can induce vibrations of the photon field (photons). Its like putting two strings near each other and plucking one: if there's a mechanical coupling between the two, the other will start vibrating.

Now, these are quantum fields, not strings, so there are some quirks. For one thing, the vibrations are quantized, so you either have an electron, or you don't. That means that the "vibration" in one field can't sort of bleed over slowly to the other. You don't see the electron and positron fade away to be replaced by something else, they're here, then they aren't and something else is. Another requirement is that there are all these "conserved quantum numbers" like angular momentum, lepton number, charge, color charge, etc, and they can't go away. So an e-/e+ will never create a quark pair (though they can create quark-antiquarks), because that would violate baryon number and color charge (and electric charge, and fermion number...). So the wave on a string analogy is a bit strained, and we pretty much never think about it that way when we're working on this stuff day-to-day (with the invention of Feynman diagrams, there's a really straightforward way to calculate this, but you asked how it happens, and I wanted to give a better answer than PHYSICS! - which isn't too helpful of a mental model).

So, when we throw a e-/e+ pair at each other, there is a chance that the combination of those two waves traveling in the e-field will induce transition into a photon (or a Z-boson, which is basically a photon's massive cousin who's into alternative lifestyles and generally being weird). It turns out that this photon (or Z) will have to decay away very rapidly (this is because typically - or always for the photon - this particle of "off its mass shell," but moving on...). So that intermediate state will have to be replaced by some other set of particles, via the same sort of field-coupling that I described earlier. The way we describe the chance of this whole process happening is in terms of "cross section," which you can think of as some physical area that the e-/e+ have to "hit" when they are thrown at each other in order to "collide" and create a new particle. So for a e-/e+ interaction, we have a cross section for e-/e+ -> photon pairs, quark-antiquarks, Z+W, tau-antitau, and many more. There's even e-/e+ -> e-/e+ where the two particles just exchange energy and momentum and deflect but don't annihilate. The largest cross sections are the most likely to happen, but ahead of time, we can't exactly predict what will occur in a particular interaction. That's the fun of quantum field theory.

So, for dark matter, we have no idea what is really going on, but we have some guesses. Basically, we postulate possible interactions (couplings between dark matter fields and other fields), and work out the conclusions. The possible couplings are constrained by the conserved quantum numbers of the final states, and by the assumed quantum numbers assigned to a particular model of dark matter. A popular model of dark matter is the one from supersymmetry: here dark matter is a combination of the superpartners of the photon, the z, and the higgs particles (supersymmetry has 2 higgs fields). You could call these particles the photino, the zino, and the higgsinos, but due to "electroweak symmetry breaking" which I've talked about on metafilter before, it's easier to work with the partner of the b gauge boson and the unbroken SU(2)_L gauge bosons, so the bino and the wino (that we-no, not wine-o, though I might prefer the latter as a name). These particles combine in various ways to form 4 massive particles, called neutralinos, the lightest of which is a dark matter candidate.

Because supersymmetry doesn't allow for much freedom of choice in the couplings (one of the reasons we like it actually), the neutralino annihilations can be worked out with some confidence (though as with anything in theoretical physics, if you're sufficiently clever you can do all sorts of fun stuff in your model). One of the important things to know is that neutralinos are their own anti-particle (like photons). So any two neutralinos could annihilate, unlike an electron, which can scatter off other electrons, but can only annihilate with positrons. Typically, neutralinos won't annihilate into electron/positron pairs, so they're a poor choice of dark matter candidate to explain PAMELA/AMS. Also, there's the issue that the cross section needed to explain PAMELA is too large compared to the cross section needed to make a thermal relic in the early Universe. So binos/neutralinos are a bad choice if you believe PAMELA and AMS are seeing dark matter.

So, you start from scratch. You just say: let there be dark matter. Call it X. Great name for something we don't know much about. X needs to do two things. X needs to annihilate with X into electrons and essentially nothing else (since there are constraints on anomalous production of "anything else" that are hard to reconcile with PAMELA/AMS if you assume X pairs go into stuff other than electrons). X also needs to annihilate more today than it used to. How to solve this?

What you do is say, well, why should X be boring? Matter isn't boring. Electrons have photons they talk to, and that lets us have chemistry and all sorts of wild things. One of the things that photons allow is long range forces (photons are the carrier of a long range force - electromagnetism). Long range forces have a weird property that particles that interact via them have larger interaction cross sections (chances of hitting each other) when they are slow moving than when they are fast moving. That's what we want!

There's a simple reason for this. Think of gravity (another long range force). Imagine you're in a spaceship, and you're zipping past the Earth. If you're moving at some ridiculous fraction of the speed of light, your interaction cross section with the Earth is just the physical size of the Earth. Either you hit that area, and crash, or you miss and you don't. Now imagine you're just puttering along. Earth's gravity will pull you in if you get too close, so the effective "size" of the Earth has grown (in the sense that if you pass through a larger area of space, you'll end up crashing into the Earth). Gravity will pull you if you're passing by at 0.9c, but it won't deflect you much, so it's less important. So gravitational cross section increases with decreasing velocity and kinetic energy.

Same thing could happen for dark matter X. Add another particle, call it phi, and assume that phi is much lighter than X. Then pairs of X can interact the exchange of phi particles, and that interaction can increase the chance of annihilation today, when the X particles are low energy and moving slowly, but are less important back in the day when the X are moving fast. Ta-da, we've got a solution (in particle physics this effect is called a Sommerfeld enhancement).

So, we've solve half the problem, now just postulate that phi is electrically neutral and heavier than an e-/e+ pair (so heavier than 2*511 keV), but lighter than anything else (electrons are the lightest particles other than neutrinos and photons). Then by the rules of "how things can couple to each other" and conservation of energy the only thing phi can decay into is e-/e+ pairs. So, with a single particle, we have solved the cross section problem, and assuming that X pairs annihilate into phi particles (which they would, under the rules we've set up), and phi decays into e-/e+ and nothing else, then the only thing we'd expect to see if electrons and positrons from dark matter decay.

Boom. Problems solved.

Now, this has consequences elsewhere. We have plenty of low energy electron beams in the world. They should be creating phi particles all the time. We could go look for them. It's damn hard, because the cross section of e-e+ -> phi can be very very small, and then the only thing you'd see is phi decaying back to e-e+ shortly thereafter. But you can do it (basically, you look for e-e+ pairs appearing on the other side of barriers; phi particles would pass through solid material like it wasn't there). We didn't think of this possibility before the PAMELA result, and it's a pretty general result - after all, the visible matter isn't boring, why should dark matter just be a single simple particle? It was motivated by a particular experimental result, but it's interesting on its own. This falls into something called "the intensity frontier." The LHC pushed "the energy frontier," looking for hard to find particles that are hard to find because they're heavy. You can use lower energy particle accelerators to look for lighter particles that are hard to find because they just don't interact strongly with normal stuff. They're complementary areas of research.

Now, you might be reading this saying: fucking theorists, all they do is come up with bullshit reasons to explain away why their original idea didn't work. If you are saying this, consider a career in experimental physics, you will fit right in. Most of a theorist's job is coming up with models to try to explain unusual results, or to make existing models more satisfying for various technical reasons. Roughly 99% of what we come up with will be wrong for various degrees of wrong. Being wrong is ok, as long as you were wrong for the right reasons. Our job to take confusing and unclear results from experiment and ask "how can I explain this, and then how can I prove my explanation wrong?" Normally, your explanation will be proven wrong. Sometimes it turns out we can't prove it wrong yet. We're still waiting for the explanation that is proven right. Sadly, AMS did not do that last bit for many dark matter models.

(Also, in writing this I just discovered that field is one of those words that just starts looking wrong if you type it 30 times in a row.)
posted by physicsmatt at 7:06 AM on April 4, 2013 [13 favorites]

Devonian: What bothers me is how pulsars work.

As a professional, I say to you: Join the club! No, seriously - the key issue is that the radio emission from a pulsar accounts for a tiny (tiny) fraction of the total energy output from a neutron star. Most of it is emitted as electromagnetic radiation but converted (how?) into a particle outflow "wind" - that wind in rare cases produces beautiful bow shock nebulae (link item 3). So trying to figure out the radio emission mechanism is like trying to understand how a tail wags when we don't know much about the dog. Sorry.

if neutron stars are made of neutrons, and magnetism comes from charged particles doing stuff (or having stuff done to them), and neutrons aren't charged particles...

Well, it's really just the interior of the neutron star that is all a superfluid superconducting soup of nucleons. (For example ... And if some theorists have their way, there's actually naked quark matter in there too.) We know about the superfluid/superconducting part because some pulsars are observed to glitch, where they suddenly jump in period and period derivative because of (maybe) a starquake where the crust slips, or (maybe) because of vortex unpinning and re-pinning inside the superfluid (sort of like a quantized amount of angular momentum shifting position after stresses build up).

I know this is hopelessly hand-waving in nature, but I'm mostly an observer, and the explanations I tried to dig up ran quickly into dense equations. But the bottom line is, while gravitational collapse is being halted by neutron degeneracy pressure, there's stuff in layers above the core that certainly supports electrodynamics, and pins magnetic fields so that the external magnetosphere can accelerate particles and create radio wave emission.
posted by RedOrGreen at 7:08 AM on April 4, 2013 [1 favorite] The AMS-02 detector on the International Space Station has not detected dark matter. It hasn’t found “indications” of dark matter, or even “hints.” It certainly is not providing the “best evidence yet” of dark matter’s existence.
posted by RedOrGreen at 1:21 PM on April 5, 2013

The comment by physicsmatt was nodded in Scientific American's Physics Week in Review.
posted by Wordshore at 9:04 AM on April 6, 2013

I just want to state for the record that physicsmatt's comments led me down a long rabbithole of astrophysics links, which terminated in me reading about magnetars and starquakes. I then spent most of the next night cowering sleeplessly under the duvet pondering the collapse of Western Civilization via EMP. So, thanks for that! I kid... mostly...
posted by lonefrontranger at 2:03 PM on April 12, 2013 [1 favorite]

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