A sensitive outsider, watching and reading the press and the physics blogs carefully, might detect an unusual level of irrationality surrounding the topic of supersymmetry, both in terms of what the press is reporting and in terms of what scientists are saying. Not that this is intrinsically abnormal. Although science is in the long run a largely rational process, scientists themselves are no more rational than anyone else. Just because you are well-trained to employ rational thinking doesn’t mean you are trained (or could be trained!) to use it exclusively. Indeed the best scientists, in my experience, tend to have a very individual (and often strikingly unusual) brew of abilities and quirks, with the abilities to think rationally and do mathematics joined to far less stereotypical talents and characteristics. Some scientists are always passionately certain that their ideas are correct. Occasionally they are right, though it is rare. Others are by nature highly skeptical, dismissive of fancy ideas, or fatalistic. All this is to say that there is plenty of rationally-unjustifiable exuberance and scorn in science. But the amount of it surrounding supersymmetry is a bit higher than usual. What’s behind it?
The reason for the current irrational discussion over supersymmetry is that a substantial minority of particle physicists — mostly theorists but including a significant number of experimentalists too — were long ago fully persuaded by circumstantial evidence that supersymmetry would show up at the Large Hadron Collider (LHC). These scientists view each of the circumstantial threads as strong, and the case made from weaving them together as nearly airtight.
Meanwhile, there have been others in the field who have been staunch opponents of supersymmetry, for various reasons. Their view is, typically, that the case for supersymmetry is woven out of little threads that are individually weak, and that the case is threadbare. And they’ve been waiting for years for their opportunity to pounce on the supersymmetry advocates, who’ve been dominant politically in some circles.
[In case you're wondering, I, along with many of my colleagues, neither strongly advocate for nor denigrate supersymmetry. As someone who had nothing to do with the invention of the theory, but who has often worked on it over the years, I view the case in its favor as moderately plausible but hardly convincing. I don't myself think this is a place where theoretical arguments will get you anywhere; to resolve the issue requires data...which is why that's where I keep my focus.]
While there are many ideas for what might show up at the LHC, supersymmetry is the only one that has a significant number of supporters who basically viewed the case in favor as already closed, and its appearance at the LHC as virtually certain. That’s why the failure to find supersymmetry as of yet has received so much attention, whereas the failure to find signs of other speculative ideas (which I will cover on this site in coming weeks) has not.
In some previous articles I have argued that the standard search strategy for supersymmetry (to look for high-energy quarks, antiquarks or gluons [which make ``jets'' of hadrons] along with signs of new undetectable particles [which are inferred when observed particles appear to recoil against nothing, a phenomenon (mis-)named ``missing energy'']) is based on three assumptions:
- in any process, the number of superpartners can only change by an even number;
- the lightest superpartner [which is stable, by assumption 1] is a superpartner of a particle we know (and therefore, to avoid conflict with other data, an undetectable neutralino or sneutrino);
- the superpartners that are affected by the strong nuclear force are significantly heavier than the other superpartners of known particles.
These assumptions are not random. They are linked to the circumstantial evidence that is cited in the case for supersymmetry. If you’re a strong supersymmetry advocate, you will probably argue that the elegant simplicity and predictability of the minimal version of supersymmetry, the fact that it provides a possible dark matter candidate particle [a stable neutralino], and its consistency with the idea of grand unification (the speculation that the strong nuclear, weak nuclear and electromagnetic forces are all manifestations of a single force, something that would be obvious at a collider with collision energies 10,000,000,000,000 times larger than those of the LHC) form a significant part of your case. So you won’t easily give up any of these three assumptions.
- For instance, if you relax assumption 1, you lose the possibility that supersymmetry provides the particle that makes up dark matter, because there are no longer any stable supersymmetric particles.
- If you relax assumption 3, you are potentially giving up a prediction of the simplest form of grand unification: superpartners that feel the strong nuclear force are heavier than those that only feel the weak nuclear and electromagnetic forces by a ratio that is related to the ratio of the strengths of the various forces.
- If you relax assumption 2, you’re not necessarily losing anything, but dark matter might not work out so well anymore, and you’re certainly making life a lot more complicated. In particular it often becomes much harder to make definite predictions.
So over the 20 years preceding the LHC, those who were strongest defenders of supersymmetry made these assumptions and pushed hard for the particular search strategy that was pretty sure to find supersymmetry consistent with these assumptions. And that’s the search that — although it is not yet complete — has so far turned up empty.
A number of those who detest supersymmetry, and some of those who have not spent a lot of time understanding supersymmetry’s many variants, have
responded with the scornful claim scornfully responded with the claim that supersymmetry is now largely ruled out by the LHC, generating a lot of blog commentary and press articles. This is understandable as a reaction against those who said it would certainly be found easily at the LHC. But though understandable, I feel it is a baseless claim. It’s one thing to say that the particular version of supersymmetry most loved by the faithful is in trouble; it’s quite another to say that the general idea of supersymmetry at the LHC is out the door. There’s a lot more work to do, and it’s far too early to declare the case against supersymmetry closed, just as it was far too early before the LHC started to declare the case in favor of supersymmetry closed. The data still needs to speak, and we still need to listen.
Fortunately, many at the ATLAS and CMS experiments pay little or no attention to this controversy, and have been listening closely to the data. A number of very interesting searches have been done in the past few months looking for supersymmetry without one or more of the above assumptions. And so the task of excluding many more variants of supersymmetry is well underway.
I’ll be detailing some of these searches this week. The first one should be available later today. Keep an eye out for updates.