Monday, 5 August 2013

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.

There's a lot going on here.  Let's start with the W and Z bosons themselves.  I've mentioned them before on this blog, but I don't think I've ever gone into detail about the underlying symmetry breaking. Thankfully, I can link to Wikipedia for a quite reasonable summary.  The points needed here are:

  1. The simplest way to describe the observed (force-carrying) vector fields is in terms of a single hypercharge boson, B; three fields W1, W2, W3; and eight gluons, irrelevant for the rest of what I have to say.
  2. In the presence of the Higgs, the B and W3 fields mix to form the photon and the Z; while the W1 and W2 fields mix to form the two charged W fields.

This is all related to the fundamental reason we introduce the Higgs to begin with, the need to explain why some of the observed vector fields have mass.

In supersymmetric theories, all the Standard Model (SM) particles gain partners of opposite spin-statistics.  This means that the B and Wi fields gain partner fermions, the Bino and the Winos.  These properties of these particles are determined by their SM partners; in particular, this means that the Bino and one of the Winos are electrically neutral.  One of them can also be made stable, and so can be the observed dark matter in the Universe.

In practice, the same effect that mixes the B and Wi fields also mixes the Bino and neutral Wino.  It's actually worse here, because they also mix with two other fields, the neutral Higgsinos (which, surprise surprise, are the partners of the Higgs).  The difference is that all four of these particles can have masses even without the Higgs; for appropriate choices of these parameters, the mixing can be small.  In this limit, we can talk of "the Bino", "the Wino" and so on.

Now, the most common models of supersymmetry have always had the Bino as the lightest of these four particles, and indeed of all the new fields we need to add.  In these models, the Bino is expected to be the observed dark matter.  A notable exception are models of anomaly-mediated symmetry breaking (AMSB).  These models predict that the neutral Wino is probably lightest.  Historically this has been seen as a weakness in AMSB, since the expectation was that superpartners would have masses only two or three times the W and Z bosons, and a Wino of such mass can only make up about one-tenth the observed dark matter without unusual stuff going on in the early Universe.  A Wino would instead need to be about twenty times more massive than the Z to explain dark matter in the simplest fashion.

This is less of a problem in view of the current observational status, where supersymmetry has not been discovered and the limits on particle masses keep going up.  With one of the strongest arguments for supersymmetry -- the hierarchy problem -- coming under threat, people have instead considered alternatives: dark matter and gauge unification.  It was quickly realised that we could satisfy those two properties, and easily evade all experimental constraints, in Split Supersymmetry.  This is a family of models where most of the new particles are very heavy, a hundred to ten thousand times the mass of the Z.  Such particles would be impossible to produce at the LHC due to simple conservation of energy.  Only a few of the new particles need to be light, including whatever is the dark matter.  Finally, one of the easiest ways to generate a Split Supersymmetry model is using AMSB.

This leads us to consider a model where the lightest particle we haven't discovered is a Wino at around twenty times the W or Z boson masses, or ten times that of the top quark.  Such a particle is technically light enough to be produced at the LHC, but the production rate is just too small to ever hope to see it.  So we have argued ourselves to a well-motivated model that is just invisible at the LHC, or any feasible collider experiment in the near future.

Are there any other ways to probe these models?  Well, there is one big thing.  The Wino has to be the dark matter, and fills the Universe.  We have a number of searches out there looking for dark matter; can they shed any light on the matter?

This is where the two papers I mentioned come in.  Previously, the answer to the question of the previous paragraph was no.  Note that the density of dark matter in the Universe is fixed; so for more massive dark matter particles, the number of particles goes down.  This makes all searches hard; so much so that it looked like current and next-generation experiments could not make any definitive statements.

The exception, that gets past this, came from a surprising place: the Sommerfeld effect, which has more commonly been considered for very light dark matter particles.  The Sommerfeld effect is based on the simple idea that when there is an attractive force between two particles, they are more likely to interact because that force pulls them closer.  However, to get a significant effect you need the force-carrying particle to be many times lighter than the dark matter particles.

Hang on ... didn't I say that the Wino is twenty times heavier than the W?

Yes, that's the key observation.  The rest is details and calculation, but the end result is nicely shown in this plot from 1307.4082:

The blue line corresponds to rate for the Wino to annihilate to energetic photons; the yellow band is the region where a Wino can explain all the observed dark matter; and the blue shaded region is ruled out by the ground-based telescope HESS.  The region where the blue line and yellow band intersect lies in that excluded region.
Exclusion plot from 1307.4082; see paper for full details.

There are a number of caveats, of course, most notably some astrophysical uncertainties.  The authors of 1307.4082 do a good job of covering most exceptions; the main conclusion is that almost all remaining wiggle room will be removed by the successor to HESS, ACT.

Finally, I consider this result to be significant.  For some time, I've felt that these types of Split Supersymmetry models would be the last refuge, refusing to be ruled out no matter what.  Which shows superlative foresight, as they've now been dealt a nearly mortal blow while many alternative models, which can be seen at the LHC, remain.

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