There was a thread about how to describe the proton in string theory that was deleted while I was writing an answer, so I will post my answer here in a new thread.(adsbygoogle = window.adsbygoogle || []).push({});

How the proton, and processes inside the proton, are described in string theory, is an open research question, part of the area of "holographic QCD", which is an attempt to describe hadrons using AdS/CFT holographic duality.

The basic answer may have been figured out by Witten in 1998, in "Baryons And Branes In Anti de Sitter Space". In holographic QCD, there are typically "color branes" and "flavor branes", and a "green up quark" is a string connecting a green color brane with an up flavor brane. For a baryon made of N quarks (N is 3 for real-world QCD, but mathematically you can describe a QCD-like interaction with any number of "colors"), you have a new brane which is connected by N strings to flavor branes. So on this model, a proton should look like a brane of its own, connected by 3 "quark strings" to the up and down flavor branes.

However, so far these string descriptions only exist for various "QCD-like" theories, and not yet for the QCD of the standard model. Typically, these models are supersymmetric and defined for large values of N, the number of "colors" in the QCD-like theory. An influential example is the Sakai-Sugimoto model, which was progress because it could reproduce the "chiral symmetry breaking" property of real QCD.

The deleted question was asking about creation and annihilation of virtual quarks inside the proton. Building on Witten's brane model, some such fluctuations might correspond to strings that start and end on the proton brane, rather than connecting to the flavor branes. But until the correct dual of QCD is identified, it might be hard to say. We have the basic idea of holographic QCD now, but there are lots of ways you can hook up strings and branes in a higher-dimensional space, and perhaps we won't know exactly what the various field-theoretic phenomena of QCD correspond to, until someone hits upon the exactly correct dual.

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# QCD from string theory

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