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I've been reading that if one places a Dp brane in a 10d space then it's possible to have a string with [11] Chan-Paton index i.e. a string that starts and ends on the same Dp brane. Since the separation of the brane with itself is 0, this string corresponds to a massless particle. Why is this massless particle the photon? Surely it could be a massless scalar or some other massless boson? Why does it even have to be a boson?

Secondly, if the above does in fact give us the photon then that is the U(1) part of the standard model. How do we get the SU(3) and SU(2) parts? Presumably by arranging other Dp-branes inside the space but I can't find any information on how this works - a schematic answer would suffice for now haha!

Lastly, suppose we live on a particular Dp-brane, and this [11] sector string does give us a photon, since 2 ends of the string touch the brane, does this mean that it really gives us 2 photons? If so, are these photons connected in some way? And is it true to say that as the photon moves through our universe, this corresponds to the endpoint of the brane moving around on the Dp brane worldvolume?

I'm fairly new to D-branes so a lot of the above might be nonsense but I'd appreciate it if anyone could put me on the right track!

Thanks.

LC

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# D-Branes and Standard Model

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