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I was attempting to read this paper after watching a show with Brian Greene. As I understand it, sparticles are a prediction of supersymetry, so I was wondering:

  • Wouldn't the discovery of supersymmetry lend validity to string theory? Isn't this a testable result, or is this type of supersymmetry predicted by most models?

  • As I understand it, sparticles require very high energies to detect. Are they candidates for non-baryonic particles?

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migrated from Dec 5 '11 at 3:52

The answer to you first point can be summarized very briefly as: String Theory $\Rightarrow$ Supersymmetry Supersymmetry $\nRightarrow$ String Theory where by $\nRightarrow$ I mean does not necessarily imply. I'm sure that an expert will answer this question much better than I can. – Kyle Dec 5 '11 at 3:20
@Larian, this is a site for professional physicists to ask questions related to their research. I will therefore direct your question to where it can be sharpened and get a good answer. – user566 Dec 5 '11 at 3:50
up vote 6 down vote accepted

Superstring theory requires supersymmetry, but not the other way around. So I wouldn't say that the discovery of supersymmetric partner particles would lend validity to string theory. But the alternative (i.e. proving they don't exist, if we had a way to do that) would rule out superstring theory.

There is such a thing as non-supersymmetric string theory, but it doesn't really work - for one thing, it doesn't include any fermions. So that's not really a viable candidate for a theory. Given that, it is fair to say that string theory would be invalid if we could rule out supersymmetry.

And as Bowler said, superpartner particles are potential theoretical candidates for dark matter.

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"There is such a thing as non-supersymmetric string theory, but [..] it doesn't include any fermions." Any ref on this pls? can't easily see why fermions inclusion is problematic. – xi45 Jul 9 at 17:17
@xi45 I can't remember anything offhand, but if I find one I'll edit it in. – David Z Jul 9 at 17:18

I'm not a string theorist but I believe that supersymmetry is indeed a pre-requisite for string theory. Although, whether this is a concrete requirement in that superstring theory were dead if supersymmetry was found ot not exist I am not sure.

Your second question I can be a little more help on. The lightest supersymmetric partner of the neutrino, the neutralino, is predicted to be stable. If supersymmetry were confirmed this would become a prime candidate for being a major source of cold dark matter in the universe. I have assumed that by non-baryonic matter you are referring to dark matter. Free electrons, muons, neutrinos are also non-baryonic matter. The mass of the neutralino is poorly constrained by theory but would be in the region of tens to thousnds of GeV.

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High energy supersymmetry is required, at the Planck scale. Low energy supersymmetry is natural to assume, but not strictly necessary. The studied phenomenology models in string theory are all supersymmetric, but that doesn't mean there isn't a subtle breaking mechanism which leaves nonsupersymmetric low energy field theory. – Ron Maimon Dec 5 '11 at 19:21

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