Ok, today I’ve posted the article you’ve been waiting for: What does the LHC have to say, so far, about supersymmetry? [Here's an article about supersymmetry and what it predicts, and another about standard ways to look for signs of it at the Large Hadron Collider, under certain assumptions; if you want to review the known particles and forces first, you can read about them here.]
If you’ve been reading the press, you may have seen statements such as “the air is getting thin for supersymmetry” or “we’re painting supersymmetry into a corner”. And recently on Cosmic Variance some broad statements about supersymmetry being in serious trouble were made by experimentalist John Conway (before being somewhat watered down after I raised an objection.)
[Update: in response to a comment posted below, I reread this post, and I see that indeed I did not, in the original version, give enough words attesting to the tremendous progress that has been made. So let me state clearly: what the LHC and the ATLAS and CMS experiments have achieved, in the supersymmetry-aimed searches made public so far, is fantastic. They have wiped not only many variants of supersymmetry, but also variants of many other speculative ideas, off the map. And I don't mean in any way to downplay this... just to try to put it in proper perspective. To do so better, I also added a sentence after the list of assumptions below.]
A number of experimentalists seem to have (and, in their public statements, give) the impression that most supersymmetry theorists, faced with an apparent disaster, are rushing around in circles desperately trying to think of new ways that supersymmetry might have escaped notice. This view also seems to be showing up on various blogs. This is a profound misconception, one that history clearly contradicts. Rather than a picture of universally panicked theorists trying to figure out how to save their favorite theory, the image should be of a number of theorists sitting calmly in their chairs, saying, “we knew this situation was a reasonable possibility — and we’ve been trying to alert you to it for a very long time.” The statements have been in the scientific literature for years.
Many theorists, including myself [and let me emphasize that I am not a huge proponent of supersymmetry, though I have worked on it and do view it as a serious contender as a theory of nature] have been saying for years that the main search strategies planned at the LHC would be insufficient to cover large regions of the supersymmetric continent. Important papers on this subject go back into the 1990s and a good chunk of the last decade. Here is the point. The bulk of the search strategy used at the LHC to find supersymmetry rests upon 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 unreasonable, and there are strong arguments in their favor, especially for assumption 1 and to a degree assumption 3 (described here). But we must keep in mind that if we relax any one of those assumptions, the limits on supersymmetry from current LHC data become much weaker. If you want to read more details about how this works, you can click here, though you will find it useful to read today’s article first unless you already know a lot about the subject. Or click on the figure below (taken from this post, which also explains why these assumptions are often made) to understand how these assumptions lead to the prediction of “jets and missing energy” on which the most powerful search strategies — including the ones we’ll hear about at this week’s conference in Mumbai — are all based.
So until you hear a consensus building, you should be cautious in making too much of statements that supersymmetry, as a part of nature and a solution to the hierarchy problem, is in serious trouble. The truth is that certain variants of supersymmetry — ones based on certain assumptions that might be wrong — are indeed strongly constrained by current data. Others, simply put, are not. In short, we have a long way to go.