The Search for Supersymmetry Continues

Some of you may remember that a few months back there were widespread reports in the media — and various blog posts, including even ones by particle physicists such as University of California-Davis’s John Conway on Cosmic Variance — giving the impression that the 2011 summer’s results from the ATLAS and CMS experiments at the Large Hadron Collider (LHC) had all but ruled out a particular speculative idea called “supersymmetry”. (Here are links to my description of supersymmetry and the classic strategy people use to look for it, as well as the assumptions that underlie that strategy and what happens when you relax them.) I complained loudly back then that this was a terribly premature conclusion — that only the most popular version of supersymmetry had been excluded by the data at that time, and that there were still many types of searches that would need to be carried out before such a broad conclusion could be drawn. And I wasn’t the theoretical particle physicist to say so.

Well, a few months on, I’m glad to say that the experimentalists at ATLAS and CMS have collectively come to the conclusion that indeed there is a great deal of work left to do. This was reflected in several talks on supersymmetry searches at the HCP conference in Paris two weeks ago, including the final talk, given by Giacomo Polesello of ATLAS. Speaking for both ATLAS and CMS, he summarized the ongoing search for supersymmetry with these conclusions (shown in bold face, accompanied in italics by my personal interpretations of what they mean, which — Nota Bene — Polesello has not seen, much less approved!)

Null results of searches are eroding the number of SUSY breaking scheme candidates for describing our world

 “eroding” — i.e., the achievements so far are in the process of eliminating or restricting certain variants of supersymmetry (SUSY for short), though the process is not complete and many variants remain…

• Early generation searches based on simplifying assumptions and on very constrained models yield limits on squarks and gluinos in the TeV range

  and by implication: when you drop any of those simplifying assumptions and/or those very strong constraints, which I have written about here, the limits are, in general, much weaker.  (Squarks and gluinos are the superpartner particles of quarks and gluons.)

• Complete exploration of SUSY requires:
   i.e. it’s not complete yet
– Extending the mass coverage in ‘basic’ scenarios
  i.e. keep doing what we’re doing and go further, to exclude the most popular scenarios more fully
– Searching for squarks and gluinos in more complex/general scenarios
  i.e. use a broader array of strategies for scenarios that are rather different from the most popular
– Addressing exotic signatures
  i.e. don’t forget about particularly weird scenarios that require specialized techniques
– Look for low cross-section direct production of sparticles which should be light in SUSY
  i.e. address especially the possibility that the most commonly produced superpartner particles (“sparticles”) are relatively lightweight but difficult-to-produce, perhaps top squarks and/or charginos and/or neutralinos.
• For all of the above points both experiments have active analysis groups, and in many cases results are available already with 2010/early 2011 data
  i.e. the experimentalists are aware of these issues and in some cases have gotten started in these directions already
• By the time of 2012 winter conferences results based on this approach with the full 5 fb^-1 will be available
  i.e. we will see major steps forward by March 2012, when analyses using the full 2011 data set will become available.

Clearly these are not the sort of conclusions you would expect Polesello to reach if supersymmetry had, in fact, been mostly ruled out back in July!

What classes of supersymmetry variants have not been ruled out so far?  When I spoke at a small CMS and theorists  meeting in London this year, I grouped them into three categories (not that this is the complete list, but it covers a lot):

1. Squeezed: The popular models’ big gap between the masses of the heavy squarks or gluinos and the masses of the light charginos and neutralinos are somewhat reduced.  (Some people are under the mistaken impression that tight squeezing is necessary to escape this summer’s results; this is not the case.  Only moderate squeezing is required.)
2. Stretched: The above-mentioned big gap is increased, and so the superpartners most commonly (but not very commonly) produced are lightweight charginos and neutralinos, and possibly top and/or bottom squarks.
3. Busy: The decays of the superpartner particles are complicated by many-stage cascades [where one particle decays to several which decay to even more], either because the Standard Model superpartners have masses that allow for such decays, or because there are extra particles that the minimal version of supersymmetry does not require but that are certainly allowed.  Some of these scenarios can become quite exotic.

All of these variants can evade the standard search strategy, which assumes the existence of large numbers of collisions with a few high-energy jets and strong signs of invisible particles through “missing energy”.  Each of them can reduce the missing energy and make the jets lower-energy and/or larger in number, while the stretched case always reduces the production rate as well.

I do expect, given Polesello’s remarks, that we will see a major dent put in the remaining possibilities for supersymmetry by March. It won’t be the final word yet, but many variants of supersymmetry will move from the “unexplored” or “barely explored” column to the “significantly constrained” or “largely excluded” column, at least for particles with mass-energy (E=mc² energy, that is) up to about a TeV. This will not be true of all of the interesting variants, however — you can calculate that the 2011 data set simply isn’t large enough. We’ll need the data from 2012, at a bare minimum.  But I suspect that if nothing surprising has shown up by the time the 2012 data is fully analyzed (somewhere in 2013) we will see a widespread opinion developing that supersymmetry at the TeV energy scale is profoundly constrained.

Two more remarks:

First, one of the reasons I feel so strongly about this is not that I’m a supersymmetry zealot but that I want to make sure we don’t have large gaps in our strategies for searching through LHC data.  It’s not just supersymmetry that is at stake, but a variety of other speculative ideas that produce effects that largely or partially mimic signals of supersymmetry.  Gaps in the search for supersymmetry imply gaps more generally; and closing those gaps increases the potential for discovery of new phenomena that may have nothing to do with supersymmetry at all.

Second, many variants of supersymmetry predict a Standard Model-like Higgs (along with four others Higgs particles that are harder to detect) and so they will have to be discarded if the search for a Standard Model Higgs particle fails to turn one up with mass-energy between 115 and 130 GeV.  Unfortunately this won’t be definitive either, because other variants of supersymmetry can make the Higgses a little or a lot harder to find.