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.

Higgs Inflation Flexes its BICEPs

A couple of weeks ago we had the BICEP2 announcement, a new and exciting physics result that was perfectly timed with my parents visiting.  As such I rather missed my chance to comment at the time, and with inflation being somewhat beyond my area of expertise I wasn't sure I really had much to say that was better than, for example, Resonaances.

However, one thing that did strike me from that post was the following line:

Speaking about model building, Higgs inflation is ruled out, at least in the current version. A robust prediction of Higgs inflation is no tensor modes at an observable level. In other words, we have a new evidence for new physics beyond the Standard Model. 

If I've learnt anything in my time as a postdoc, it's that whenever you make this kind of statement it's just a matter of time before someone argues that it's not true.  In this case, it took a week.

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.

Higgs ... Numerology?

The Christmas season is generally a quiet time in terms of new research papers.  The reason's pretty obvious; with so many people taking time off, anything you put out won't be read as much.  And that's assuming you haven't dashed off somewhere yourself.  Still, things don't drop to a complete halt, and you get odd little papers like this one on the Higgs decay.

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.

Revisiting Pluto

Planetary astronomy is usually outside my sphere of interest, but I came across a paper last week that defied that trend.  As the title suggests, it returns to the decision in 2006 to reclassify Pluto as not being a planet.  To summarise for those who don't remember, the traditional set of nine planets was threatened by two consequences of modern telescopes.  First was the discovery of increasing numbers of planet-like bodies, including at least one as large as Pluto itself; second the realisation that Pluto was smaller than once thought, indeed likely smaller than Earth's moon.

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.

LED Higgs

Time to talk about another paper.

The Science of the Apollo Missions

On the subject of research papers, I recently came across a paper by Ian Crawford on the science of the Apollo missions.  It's a review written at a non-technical level; it's from the journal Astronomy and Geophysics, but the only link I've found is at the arXiv.  I encourage anyone curious about the scientific merits of the whole affair to read it.  One interesting quote compares what Prof. Crawford achieved on a field trip on Earth to the moon missions, concluding

I do not think that we were inefficient, and we were in fact well-pleased with what we accomplished (which will result  in several peer-reviewed publications), but clearly what we achieved in 42 man-days at one site in Iceland pales into insignificance to what the Apollo astronauts achieved in 25 man-days at six sites on the Moon under far more difficult operating conditions. Based on my own experience I find the field efficiency of the Apollo astronauts to  be simply staggering.

Three is a Large Number

One thing I sometimes like to joke is that in  physics, there are only three numbers: zero, one and infinity.  By that I mean that you can get a decent rough estimate in many cases by treating the relevant parameters as one of those three values.  The entire field of dimensional analysis involves setting numbers to be one in the appropriate units; for example, consider atomic physics.  We are in the quantum regime, so we need Planck's constant h; the dominant force is electromagnetism, so we'll need the vacuum permittivity ε₀; and the electrons form the "outside" of an atom, so let's also consider the electron charge e and mass m_e.  There's only one way to combine these objects to have the dimensions of energy:

$\frac{m_e e^4}{\epsilon_0^2 h^2}$

Up to an overall constant, this is the Rydberg, which indeed characterises the energies of atomic physics, and which is normally derived after several weeks of quantum mechanics.

Setting things to be zero is fairly intuitive.  Small things normally have small effects, and can be ignored at first.  Correcting for them being non-zero is then precisely a perturbation series.  Interestingly, setting numbers to infinity is pretty similar; there are plenty of situations where the mathematics can be exactly solved when a coupling g goes to infinty, and then corrections come as a series in inverse powers of g.  A somewhat different example is in the strong interaction, which has three colours (analogous to the single electric charge).  Before I was born, Dutch physicist Gerard 't Hooft was able to successfully analyse the strong interaction by setting the number of colours to be infinity.  Despite three not being very big, the approximation was successful.

In a similar vein, we have the paper I want to discuss today.  Like 't Hooft, Bai and Torroba are approximating a number that equals three by infinity.  Instead of gauge interactions and colour, they have chosen to look at flavour and the number of generations.

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.

How Constrained is Constrained SUSY?

It has become standard lore in the theoretical physics community that the fact that the LHC is already on the edge of ruling out Supersymmetry (SUSY).  The reason is quite simple: the standard argument for SUSY, the hierarchy problem, would suggest that the supersymmetric partners (superpartners) of the Standard Model should have masses less than about one thousand GeV (where the proton has mass of about one GeV).  The LHC has not found those partners, and has published exclusion plots like this one:

ATLAS LHC limits on Supersymmetry; stolen from Michael Kobel's talk at Planck 2012.

The different coloured lines correspond to the limits from different types of signals that could have been seen.  The areas below the lines are ruled out.  The coloured regions were either ruled out from earlier direct searches or theoretically.  The grey dashed lines correspond to superpartner masses in GeV; horizontally for the gluon superpartner, vertically for the quark superpartners.  Note that the regions for masses less than one thousand are almost entirely within the excluded region.

Now, there are a number of caveats, and a lot of work has been done in the last year to eighteen months exploring ways to get around these restrictions.  However, a recent paper by Balazs and his collaborators went back and examined the simplest situation more rigorously, and suggested that the LHC results have not actually had that much effect on the allowed parameter space.  How did they conclude this?  Join me below the fold!