When The Standard Assumptions About Supersymmetry are Relaxed

I am by no means alone in having written papers and given many talks pointing out the assumptions on which standard search strategies for supersymmetry are based, namely

  1.  in any process, the number of superpartners can only change by an even number;
  2. 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);
  3. the superpartners that are affected by the strong nuclear force are significantly heavier than the other superpartners of known particles,

and emphasizing how easily they can be evaded in very wide classes of supersymmetric models.  There’s surely much earlier work by many other people, but to give you an example, in Fig. 1 I’ve grabbed a plot from a talk I gave in August 2007 at the CERN laboratory (based on this paper) showing what happens if you relax assumption 2, which I feel is by far the weakest of the three. [If anyone reads this who has a similar plot from the 1990’s, please send it to me!]  The reduction in the “missing energy” signal is enough to make this variant of supersymmetry much harder to see.

Fig. 1: Relaxing assumption 2 often reduces the amount of "missing energy" by an amount sufficient to make discovery much more difficult with standard methods. At right, the neutralino, the lightest superpartner of a known particle, decays to lighter, unknown, "hidden" particles, which then decay back to known particles that we detect. (In this talk I referred to the hidden particles as part of a "hidden valley sector", or "v-sector" for short.)

And in Figure 2 is a similar but more revealing plot from a paper that I just completed a couple of weeks back (with Mariangela Lisanti, Natalia Toro and Philip Schuster), which shows what happens to a particular variant of supersymmetry (in red) if you relax assumption 2 (in blue) or 1 (in green.)  The background (as measured by CMS in 2010 data) is given by the black points.  You can see how much more difficult it would be to find the blue or green variants compared to the red, standard variant.

Fig. 2: How a variant of supersymmetry (red curve) that is relatively easy to discover can become much more difficult to find using standard techniques if assumption 2 (blue curve) or assumption 1 (green curve) are false. Black dots are CMS data from 2010, showing backgrounds far exceed the signals if the assumptions are relaxed. Other experimental techniques must be used to find these variants.

Meanwhile, the research group of Professor Jay Wacker at what used to be called  the Stanford Linear Accelerator Center [where I was a grad student] has been especially vocal and active in pointing out the effect of relaxing assumption 3; I haven’t yet found a killer plot illustrating his points, but here’s a talk of his from spring 2010, though the work started quite a bit earlier.

Both they and we are not merely pointing out the problem, but have also suggested specific solutions.    The message is that although there are substantial and well-known gaps in the current LHC search strategies for supersymmetry, it appears they can be largely filled with relatively simple techniques. [I say “appears” because, given the extreme difficulty and complexity of the LHC experiments, only the experimentalists themselves can confirm such a statement definitively.]

There are smaller but more difficult gaps in coverage that will be trickier and take longer to close. But first things first; let’s fill the big holes.

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