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.

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.

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!