Dark Matter Interactions and Galaxy Structures

Another thing I rather fell behind on with my move to Korea is my academic reading.  So I only recently got around to a short and interesting paper from August on interacting dark matter (DM).¹  This is one of those papers that takes a shockingly simple idea, the kind that you look at afterwards and wonder why you didn't think about it.  But it should make it easier to address a long-standing and fairly perplexing puzzle in dark matter physics.

Annual Modulation at CoGeNT

When talking about dark matter (DM), there's a standard line that gets used from popular talks through to journal papers: we only know about its gravitational interactions.  That is, we've measured its existence and abundance from how it affects galaxy rotation curves, or the structure of the cosmic microwave background; but we have no direct information about any other types of coupling it might have to the ordinary stuff we are made of¹.

Of course, there are a lot of searches of various types looking for those interactions.  One of the most basic is direct detection, building a very sensitive and low-background experiment and looking for dark matter scattering off the atoms in your apparatus.  It is here that one of the more enticing, puzzling and long-standing mysteries of dark matter is to be found; the fact that several experiments claim signals, that seem to be ruled out by other searches that found nothing².

My attention was drawn by the publication on the arXiv today of another paper in the signal column.

Axion Dark Matter in Tabletop Experiments

A paper from last week offered a very interesting suggestion for a new type of search for axion dark matter, as well as the possibility that it may already have been seen.

I've talked about dark matter a few times on this blog, but I don't think I've yet mentioned axions.  Part of the reason for this is that axions are somewhat outside my area of expertise.  Still, the main points that are needed here are that axions are very light, very weakly interacting particles.  By light, we are talking at least a billion times lighter than the electron.  Like WIMPs, axions are introduced for unrelated reasons yet can serve as natural dark matter candidates.

Little is Cute

I've previously talked on this blog about supersymmetry and extra dimensions.  Another class of models for new physics is the Little Higgs (LH) family.  While these models have faded out of primary focus in recent years, even before the LHC turned on, I always have a soft spot for them as they were the subject of my early research as a grad student.  So I tend to read, browse, or at least skim new work in this area, and that's the subject of this post: a paper from the end of July on new dark matter constraints in LH models.

No Winos

Oh yes, I have a blog.

Yes, it's been a while since I've written anything here.  There's a number of reasons, but the main one is simply lack of time.  Not exactly dramatic, but sometimes that's the way it is.  I'm now hopeful that I can make at least semi-regular updates again.

I'll start with an overview of a couple of recent research papers, that came out in the same week with very similar results.  They relate to the Winos of the title, which is pronounced "weeno", not "weye-no".  Winos are particles that show up in supersymmetric theories (hence the suffix -ino) and are partners of the W and Z bosons.  They are also one of the possible dark matter particles in these theories.  The thrust of the two papers I'm considering, "Wino Dark Matter Under Siege" and "In Wino Veritas", is that these are ruled out in that role.

Collider Searches for Dark Matter

The quest to find non-gravitational evidence for dark matter proceeds along several different fronts.  The most direct approach is to look for dark matter particles scattering in detectors here on Earth.  Indirect searches, looking for cosmological signals of dark matter annihilation or decay, has provided a number of tantalising hints, of which the "line" signal at Fermi is the most recent.  However, directly producing dark matter at experiments like the LHC offers us the most control over the initial conditions and thus the least ambiguity in interpretation.

The problem with dark matter at collider experiments is that it is dark, i.e. it doesn't show up in the detectors.  To get around this problem we look for the production of other stuff as well as the dark matter itself.  We can then tell if the dark matter is there by seeing an apparent violation of conservation of momentum; the missing momentum is carried away by the unobserved dark matter particles.

The traditional approach to these type of searches is to take a complete model of new physics (such as supersymmetry) and use that to model the production process.  So in SUSY, we produce gluinos or squarks, which then go through a several-step decay producing dark matter and multiple Standard Model (SM) particles.  Indeed, even today the signal "jets and missing transverse momentum" is considered a characteristic SUSY search.

However, a couple of years ago an alternative and somewhat opposite approach began to become popular.

SUSY Mass Upper Limits

Supersymmetry remains the most popular theoretical extension of the Standard Model of particle physics.  It's not hard to see why; in addition to its structural appeal, many people have spent years working on it.  With all that time invested, a minor detail like it not showing up at the LHC is hardly going to dissuade us.

What has changed is the perspective we take, especially as far as the motivation for supersymmetry is concerned.  The traditional arguments about avoiding regions of theoretical fine-tuning have taken a battering from the combination of the observed Higgs mass, and the high exclusion limits on superpartner masses.  These already force most models to be tuned to at least one part in a thousand, often worse.

But once we abandon fine tuning as a motivation for supersymmetry, we also remove one of the main arguments for electroweak supersymmetry; that is, for the superpartners to be light enough to show up at the  LHC.  And this is reflected in recent model building, with theorists increasingly willing to consider models where some or all of the superpartners are heavy; for example, mini-Split SUSY models put most of the new scalar particles at a hundred to a thousand TeV, with the new fermion masses around one to ten TeV.

Against this, a recent paper develops some interesting arguments for upper limits on superpartner masses.

Lines and Boxes in the Sky

I've talked recently about the potential signal of dark matter found from gamma ray photons with an energy of 130 GeV.  One paper from a several weeks ago that I've wanted to discuss made a simple but interesting point.  You see, the most obvious interpretation of this signal is from dark matter self annihilating directly to a pair of photons.  This direct production gives us a mono-energetic spectrum.  But instead of a line, we might have a really narrow box, coming from a two-step process.  And this is actually quite natural in certain types of models.

To go into more detail, we'll need to cover some basic kinematics.

Shiny Dark Matter

A couple of months ago, I discussed a recent paper by Christoph Weniger that claimed strong evidence for dark matter from the Fermi Satellite Large Area Telescope.  In short, the Fermi LAT detects gamma rays, light of very short wavelength/very high energy.  Looking at gamma rays from the centre of the galaxy, Weniger claimed to see a feature in the spectrum:

Weniger's main result: black points are data with error bars; the green line is a featureless background; and the red line the combination of that and the proposed signal, shown in blue at the bottom.

Such a feature is difficult to produce by conventional astrophysics (stars, pulsars, etc).  It can be easily produced from dark matter.  For example if two dark matter particles annihilate into two photons, then conservation of energy and momentum forces those two photons to have the same energy, equal to the mass-energy of the dark matter.  Throw in the effect of experimental resolution and we get something that looks like the above, a small peak in the spectrum.

Since Weniger's original paper there has been a lot of work done.  In summary, this feature stands up to reanalysis but is not statistically strong enough to claim a true discovery.  Non-dark matter explanations have been offered, but are not compelling.  However, the dark matter explanation has problems of its own; the signal seems to be too big.

Brought to You by the Letter S

Ah, Supersymmetry.  I've been meaning to talk about Supersymmetry, or SUSY for short, for some time.  I was obviously setting things up in this post from almost a month ago, back when I was in England, but I've been planning this post for longer than that.  SUSY is the most popular theoretical framework for new particle physics, and as much as I'd like it not to be true, I need to worry about it all the same.¹

Dark Matter Found (or not)

So earlier this week I offered a brief overview of the Dark Matter problem.  (See also the Font of All Knowledge for more.)  Today I want to talk about a paper from two weeks ago relating to a possible discovery (or more accurately, hint of a signal).

Now, this is far from the first time such a hint has been found.  The DAMA experiment is perhaps the longest-standing claim of discovery; that question, and why it's not widely accepted, is a whole blog post in itself.  I want to start with this one because it is recent, and also most closely related to the work I have done in dark matter detection.

The Dark Matter Problem

One big and as-yet unanswered problem in modern physics is the dark matter problem.  The problem is astrophysical: a number of observations, from galactic to universal scales, show a difference between the mass distributions observed directly (in visible stars, galaxies etc) and indirectly (through its gravitational effects).  As the name of the problem suggests, it looks as though there is a lot of extra matter that we can't see (because it's dark).

The canonical example of such an observation is also the first one made (by the brilliant but prickly Zwicky), that of galactic rotation curves.  In particular, let us focus on objects (stars, globular clusters) orbiting a galaxy but not really part of it.  We are in the limit of weak gravitational fields and small speeds, so Newtonian mechanics is adequate.  The gravitational force due to the galaxy drops off with the standard inverse-squared law:
$F = \frac{G M m}{R^2}$
Here, G is Newton's constant; M and m are the masses of the galaxy and the object orbiting it, respectively; R is the distance between them and F the force.  Using Newton's second law of motion gives us the acceleration:
$a = \frac{G M}{R^2}$
Lastly, we use the relation between acceleration and velocity for objects moving in a circle:
$a = \frac{V^2}{R} ; \therefore V = \sqrt{\frac{G M}{R}}$
The main point is that we expect the speeds of objects orbiting a galaxy to decrease as they get further away from it.  We can extend this for objects within the galaxy itself, but then we need to take the finite size of the galaxy into account.  The result is that we expect the orbital speed to increase with distance within the galaxy, then decrease with distance outside it.

What we see looks like this: