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On Fri, 28 May 2004, Robert C. Helling wrote: > On Fri, 28 May 2004 02:01:34 $-0400,$ FrediFizzx wrote: > > I was wondering how string theory deals with $e+e-$ annihilation or the > > annihilation of any pair. If the electron and positron are represented by > > strings, do the strings just disappear forever when they annihilate? > > No. As in QED, they will send out at least to photons to satisfy > energy momentum conservation. Thus this process is at leading order a > pair of pants diagram (or better its open string analogue) with the > two incoming strings being in the electron mode and the two outgoing > string in the gauge boson mode. On the other hand, you certainly can have processes in which the string sort of "disappears", namely for instance in pure loop diagrams, if that's what Fredi is concerned about. Maybe the following general statement helps Fredi to think about this stuff: Giving any Feynman diagram of some field theory, one can imagine that it is the approximation to $a 2-d$ surface which would be obtained by blowing up the infinitesimally thin edges and vertices of the Feynman diagram, so that for instance, as Robert has mentioned, the trivalent vertex / / -----< \ \ becomes the "pair of pants" (for closed strings) or something like $/ // /$ ------/ / \ $------\ \\ \$ \ for open strings. See for instance figure 2 of van Proeyen Introduction to string theory http://itf.fys.kuleuven.ac.be/~toine/IITSStrings.pdf for better illustration. In particular, if we have Feynman diagrams like /\ $/ \/ \\ /\ /$ \/ where particles "appear and disappear" this corresponds to the torus (closed string) or the annulus=disc with a puncture (open string) worldsheet (in the oriented case) or even the Klein bottle and the Moebius strip (in the unoriented case). So in this sense I believe that it is fair to say that strings can "appear and disappear".