The second Kaluza-Klein neutral Higgs bosons in the minimal Universal Extra Dimension model

Loop-induced decay of a neutral Higgs boson into a pair of gluons or photons has great implications for the Higgs discovery at the LHC. If the Higgs boson is heavy with mass above $\sim 500\gev$, however, these radiative branching ratios are very suppressed in the standard model (SM), as the new decay channels are kinematically open. We note that these radiative decays can be sizable for the heavy CP-odd second Kaluza-Klein (KK) mode of the Higgs boson, $\chi^\tw$, in the minimal universal extra dimension model: highly degenerate mass spectrum of the theory prohibits kinematically the dominant KK-number-conserving decays into the first KK modes of the $W$, $Z$ and top quark. We find that the CP-even decay of $h^{(2)} \to g g$ is absent at one-loop level since $h^{(2)}$ couples with different mass eigenstates of $t_{1,2}^\on$ while a gluon does with the same mass eigenstates. The $h^\tw$ production at the LHC is very suppressed. On the contrary, the process $gg \to \chi^\tw \to \gamma\gamma$ in an optimal scenario can be observed with manageable SM backgrounds at the LHC.