Universal extra dimensions and the Higgs boson mass

We study the combined constraints on the compactification scale $1/R$ and the Higgs boson mass $m_H$ in the standard model with one or two universal extra dimensions. Focusing on precision measurements and employing the Peskin-Takeuchi S and T parameters, we analyze the allowed region in the ${(m}_H,1/R)$ parameter space consistent with current experiments. For this purpose, we calculate complete one-loop KK mode contributions to S, T, and U, and also estimate the contributions from physics above the cutoff of the higher-dimensional standard model. A compactification scale $1/R$ as low as 250 GeV and significantly extended regions of $m_H$ are found to be consistent with current precision data.