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Stringy corrections to SM propagators

  1. Jan 12, 2009 #1
    First let me express my ignorance about this subject so please forgive me if these questions have well-known answers.

    The main objections I've heard voiced toward string theory are (1) it's incredible diversity of vacua caused by large number of possible Calabi-Yau compactifications, and (2) it's lack of background independence.

    I would like to question from my naive viewpoint, whether either of these are as serious as purported. As far as I know, ST naturally incorporates gauge theories on sets of coincident D-branes. As such, it should be be possible to embed the standard model (ad hoc) into string theory on a flat Minkowski background.

    With particle content and spacetime background set by hand, what is stopping people from computing stringy corrections to the standard model propagators. Has this already been achieved and is it unique?

    If this can be done, then even though corrections are manifest only at the Planck scale, the fault appears to lie more with experimental limitations than with string theory.
  2. jcsd
  3. Jan 12, 2009 #2
    The effective operators are characterized by the Planck mass, as you point out. So things are more or less completely decoupled from low energy physics---of course, this depends on what the "planck mass" means...if you have a large extra dimension, this scale can be pushed low enough to see it at LHC.

    About uniqueness...it depends. Because of the large number of vacua present in string theory, "uniqueness" loses most of it's meaning. In principle, because of the large freedom afforded you by the landscape, it looks like you can tune an arbitrary model to make it work. (This statement is not rigorous, and may be wrong :) ) This is what people typically mean when they say "you can embed the standard model into string theory"---as of yet, we lack an explicit stringy construction of the standard model, down to yukawa couplings, but it appears that we have enough degrees of freedom to "make it work". In particular, the mathematical structures needed to get the MSSM out of string theory definitely exist. In fact, they exist in more than one way---coincident d branes is one way, but there are also other ways to get Lie groups out, such as geometrically near singularities on a Calabi Yau, or from internal degrees of freedom in the heterotic string.

    Either way, the problem is that you have to compute the corrections within a specific string theory, and we're not sure which one we should be using. People have computed such corrections to some large classes of string models---typically the easiest thing to do is to figure out how the massive string states effect gauge coupling unification. These calculations are highly model dependent, and far from "unique" as different models may have very similar results.
  4. Jan 14, 2009 #3
    Thanks Ben, that clears it up quite nicely.
  5. Jan 18, 2009 #4


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    If so, then why do Denef et al ask "After all, we believe that string/M theory has a finite number of vacua, and thus can lead to a finite number of 4d low energy theories; could we imagine showing that the data is fit by none of these theories, thus falsifying the theory?"
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