Extra Dimensions and the Muon Anomalous Magnetic Moment

Abstract

It has been proposed recently that the scale of strong gravity can be very close to the weak scale. Dimensions of sizes anywhere from $\sim $mm to $\sim $TeV$^{-1}$ can be populated by bulk gravitons, vector bosons and fermions. In this paper the one-loop correction of these bulk particles to the muon magnetic moment (MMM) are investigated. In all the scenarios considered here it is found that the natural value for the MMM is $O(10^{-8}-10^{-9})$. One main result is that the contribution of each Kaluza$-$Klein graviton to the MMM is remarkably finite. The bulk graviton loop implies a limit of $\sim 400$ GeV on the scale of strong gravity. This could be pushed up to $\sim 1-2$ TeV, even in the case of six extra dimensions, if the BNL E821 experiment reaches an expected sensitivity of $\sim 10^{-9}$. Limits on a bulk $B-L$ gauge boson are interesting, but still allow for forces $10^6-10^7$ times stronger than gravity at mm$^{-1}$ distances. The correction of a bulk right-handed neutrino to the MMM in one recent proposal for generating small Dirac neutrino masses is considered in the context of a two Higgs doublet model, and is found to be close to $10^{-9}$. The contributions of all these bulk particles to the MMM are (roughly) independent of both the total number of extra dimensions and the dimension of the subspace occupied by the bulk states. Finally, limits on the size of ``small'' compact dimensions gotten from the MMM and atomic parity violation are determined and compared.