Anti-B --> X(s) gamma in two universal extra dimensions

We calculate the leading order corrections to the anti-B --> X(s) gamma decay in the standard model with two large flat universal extra dimensions. We find that the contributions involving the exchange of Kaluza-Klein modes of the physical scalar field a^+-_(kl) depend logarithmically on the ultraviolet cut-off scale Lambda. We emphasize that all flavor-changing neutral current transitions suffer from this problem. Although the ultraviolet sensitivity weakens the lower bound on the inverse compactification radius 1/R that follows from anti-B --> X(s) gamma, the constraint remains stronger than any other available direct measurement. After performing a careful study of the potential impact of cut-off and higher-order effects, we find 1/R > 650 GeV at 95% confidence level if errors are combined in quadrature. Our limit is at variance with the parameter region 1/R <~ 600 GeV preferred by dark matter constraints.