Neal Weiner from NYU was at Stanford this week talking about his ideas concerning dark matter. He has been pursuing a pretty simple idea for the past couple of years which has recently garnered a lot of attention thanks to some new data from a satellite called PAMELA. I've had trouble finding a decent picture, but the excitement is over a few nice data points:
The red points are the new ones, which were only recently released.What is this a graph of? The PAMELA satelite is measuring positrons in space. This is interesting, because it is very possible that there is some new physics which produces high energy positrons--specifically, high energy positrons are a generic signal for dark matter anihilations. Dark matter is the stuff that comprises about 25% of the universe. ("Normal matter", like protons and electrons, which doesn't interact with dark matter, makes up only about 4-5% of the universe.) The graph shows you (essentially) how many positrons they measure, and at what energy they measure them. The excitement is that PAMELA measures more positrons than people though there should be.
There are a few things about the graph that should jump out. First of all, the black line is what the expected background should be. That is, there are already tons of positrons shooting around space, which come from the sun, for example. The heavy black line is how many we expect to see if we understand everything completely. You shouldn't be worried that PAMELA sees too few of the lower energy positrons (energies less than 10 GeV). The experts assure me that this is due to natural solar fluctuations.
Second, there are a lot of other data points from older experiments. The difference between the old data points and the new data points is the size of the error bars. The best that we can ever do in physics is to get a bound on a measurement---for example, if you're buying meat, the butcher charges you for the nearest 1/100th of a pound. This is as accurately as he can measure things. You don't know EXACTLY how much ham you've bought, you can only confidently say that you have 1.03 pounds---in other words, you're limited by the accuracy of the measuring device. A similar story happens here---the older experiments were too inaccurate to be able to see the extra positrons with any definiteness because the error bars (mostly) overlapped the background. The new PAMELA data, however, has very small error bars, so we know for sure what we're dealing with.
The excitement is that Neal may have predicted this excess.
The thing that is interesting to me is that, if Neal is correct, we have to reconsider a guiding light which we use in physics, called minimalism. The idea behind minimalism is you use as few things as possible to explain as many things as possible. So, for example, if one were to talk about colors, we don't need to discuss the 16 million or so different shades that my computer can make. 16 million colors is too much to think about. Instead, we can think about three colors, and make all of the other colors by combining red, blue, and yellow in different ratios.
What Neal did was to depart from this approach. We HAVE been trying to explain dark matter as some single component thing. In general, people have worked very hard to get their models to fit the data, and they find a few things that are just not quite right. Of course, who's to say that Nature hasn't chosen something that looks "not quite right" to us---it certainly wouldn't be the first time that we've been surprised. But when you actually sit down to calculate things, they don't work out as nicely as we would like.
Neal proposed that the stuff that makes up dark matter is much more complex than what we've understood. Imagine that we only know about colors that can be made out of blue and yellow, and then one day someone discovers purple. Purple is an anomaly, and lots of people rush out to try and explain purple by mixing some ratio of blue and yellow. Maybe some people get close, but the colors they make aren't quite right. Then Neal Weiner comes along and says, "Well, what happens if you invent red?"
Now the technical part.
Neal's proposal involves having a hidden gauge group, usually just a U(1), but maybe something larger. The gauge group is higgsed at about a GeV, and the matter in the hidden sector (the dark matter) has MeV mass splittings induced by the spontaneously broken symmetry. The dark matter communicates with the visible sector through some operators linking the gauge kinetic terms of the hidden sector gauge group and the visible sector gauge group (called kinetic mixing). The typical process is DM + DM -> dark photon -> photon -> electron + positron.
Because the dark matter has it's own dark force, the scattering cross sections are enhanced via something called Sommerfield enhancement. This is a familiar effect, for example, in gravitational or electromagnetic scattering. Having particles which are charged under some force increases the effective impact parameter. This helps to explain why the cross section implied by the PAMELA data is so much larger than the cross section implied by the typical thermal relic calculations.
The thing that you'll notice is that the previous two paragraphs have been incredibly generic. Neal and his collaborators have only really drawn vague outlines about what the dark sector could possibly look like---it's up to others to actually fill these in.
I'll close with an example Neal used in his talk about "Mr. Dark Matter". Suppose that there are people made of dark matter, and that they measure the universe in the same way that we do. What they'd find is that 25% of the universe is made of "normal" stuff, 70% of the universe is made of dark energy (whatever THAT is), and 4-5% of the universe is made of some mysterious new things that aren't explained. Obviously, the Mr. Dark Matters will spend lots of time trying to understand what this last 5% of the universe could possibly be made of. Now suppose some dark scientist comes along and says "I propose that the mysterious stuff is actually SU(3)xSU(2)xU(1). There are three families of quarks and leptons, with approximately massless neutrinos. The quark masses will range from about 175 GeV to 4 MeV, and the leptons will range in mass from about 1 GeV to 0.5 MeV. The SU(2)xU(1) will be spontaneously broken at about 100 GeV, and the SU(3) will become strongly coupled at roughly the same scale, for no apparent reason, ..." The point is, nothing about physics as we know it is really minimal. In fact, the Standard Model is about the least minimal thing that we could have. (Ok, this is hyperbole, but still, the Standard Model is pretty complicated.)
We are at a point in physics where the typical approaches have gotten us close to the answer, but something is just not quite right. The minimal dark matter models that people have worked on in the past are very well-motivated, but they all suffer from the same tunings---it's like the pieces of the puzzle almost fit. Many times it takes a different perspective to see that you're trying to fit the wrong pieces of the puzzle together.
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Edit: A good friend who works in the field has asked me to clarify that Neil Weiner was not the first person to think about positrons as they pertain to dark matter anihilations/decays. The story started in the 1980's. But Neal is the first person to use the types of hidden sectors to model the data, which is the idea that I like. --bd
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