In these days there have been some rumors in the blogosphere about string theorists and SUSY (see Motl, Woit and Dorigo) due to a recent preprint appeared on arxiv. Indeed SUSY is a relevant ingredient of string theory and the latter was the vehicle for the uncovering of this concept that obtained such a fortune in the community. I have listened a talk of Sergio Ferrara 
at Accademia dei Lincei in Rome a few months ago. Ferrara is one of the discoverers of supergravity and he gave a nice talk on the argument of supersymmetry. He was confident that supersymmetric particles will be seen at LHC. I would like to say that this was also the expectation for LEP and Tevatron but nothing has been seen so far. So the paper above seems like an attempt by a string theorist to be pessimistic and save the day.
Supersymmetry has some problems that still are in need for a satisfactory answer. One is philosophical as one can say that the number of particles simply doubles and so why should we expect such an anti-economical behavior by Nature? Ferrara argued against this question by saying that also with antimatter Nature doubled the number of particles so this would not be the first time that, in order to keep a symmetry, one needs such a doubling. One can say anyhow that for antimatter one has a discrete symmetry on a single field while for supersymmetry is the number of fields that doubles. The other point is about breaking of supersymmetry. There is no satisfactory model so far and such symmetry is not seen at low energies as we know. But this could be just a matter of time before someone finds a way out.
My view is that even if there is no supersymmetry at large, one can save supergravity. Indeed, all one needs is to observe a gravitino, that is a spin 3/2 particle, and we will have a theory of quantum gravity while, at large, no supersymmetry can exist. But this would not be enough for string theory. As a theoretical physicist I would like to see the discover of a gravitino and the failure of supersymmetry at large as this would imply a lot of interesting work to do and an incredible new scenario to understand.
The Gauge Connection 
the “doubling” of particles (actually, of degrees of freedom) has a mysterious economy principle: the number of particles (sleptons and squarks) you need to add is exactly, charge by charge, the number of mesons and diquarks we have already discovered. Check it yourself, remembering that the top quark does not bind into such pairs.
This peculiar coincidence between the states of a composite QCD string and the predicted states of a fundamental string, only happens for five light quarks and three total generations. It remains unexplained, but at least it hints that supersymmetry is not as antieconomical as it seems.
It seems rather a coincidence than a deep physical truth.
Marco
Well at least it is a physical truth, just not a deep one as it goes. Probably the same coincidence that happened with SU(3): it appeared first as global SU(3), flavour, and it entered the standard model as local gauged SU(3), colour, completely unrelated as far as physics goes, but still sharing some of the mathematics. And thus very illustrative.
My view is that supersymmtery is a beatiful approach and my hope to not see it at LHC is just that of any theoretical physicist about the “it has all been solved” trauma. This is the reason why I have described such an uncoventional scenario.
Anyhow, what are diquark states you are referring to besides mesons? Just curious.
Marco
If course, no free diquarks have been observed… so I was speaking of the expected combinations. Point is, for a meson of charge +1 there are six possible combinations: uD, uS, uB, cD, cS, cB and then the rest of the game is spectroscopy. In the same spirit, even if the spectroscopy (see Jaffe, Wilczek, etc) is more complicated, you can work out the number of possible flavour combinations for a +2/3 quark and for a -1/3 quark. Surprisingly, there are also six of each. For instance, for a +2/3 quark you have DD, SS, BB, DS, DB, SB.
So one would say that for the specific case of 3 generations and five light quarks (or SU(5) approximate global flavour group) there is some extra mathematical economy in SUSY, as their number of QCD pairs (bosons, at all) and their number of fermion degrees of freedom coincide, charge by charge. Is is not that the number of particles “simply doubles”; it is more that it “peculiarly doubles”.
Of course, for any other number of generations or for a massless top-quark or for a huge bottom quark, I agree that the doubling is anti-economical. But for the standard model, as we know it today, the case is different.
I am aware that there is a lot of literature about quark molecules but there is no evidence whatsoever for their existence. Of course, this does not mean that this kind of QCD objects will never be observed. Rather, this idea is creating some confusion about our understanding of the lower part of meson spectrum. There are several physicists that are convinced that some of the observed states are indeed tetraquarks (notably also ‘t Hooft and Maiani). At QCD 08 there has been a lively discussion between Achasov and Narison about this point. My personal view is that until we will not be able to manage low energy QCD (even if lattice computations can help here), a definite answer to the very existence of quark molecules cannot be given.
With this view in mind, I think that your point gives some clue that such states should exist using a nice argument with SUSY. Time will say and we have to wait yet. Meanwhile, a lot of experimental and theoretical activity is performed around the world and it is possible that in a near future, evidence for quark molecules will emerge.
Marco
[…] dei Lincei in Rome. That was when Sergio Ferrara come there to talk about supersymmetry (see here). Cabibbo awarded Ferrara with a medal of the Accademia and it was a very nice […]