Universal extra dimensions

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Universal extra dimensions (UED) are theories in which extra spatial dimensions exist and all particles can travel in them. This differs from the ADD model, where the standard-model particles stay on a four-dimensional surface and only gravity extends into the extra dimensions. In UED, the extra dimensions are compact and relatively large compared to the Planck length, around 10^−18 meters. The presence of these dimensions would produce Kaluza–Klein excitations of standard-model particles at energies set by the inverse size of the extra dimensions, M_KK ≈ 1/R. Experimental data from the LHC bound the compactification scale for one or two universal extra dimensions to about 1 TeV, with other limits from precision electroweak measurements, the muon magnetic moment, and flavor-changing processes reaching several hundred GeV. If UED is used to explain dark matter, the compactification scale would be limited to a few TeV.