  We performed inelastic neutron scattering experiments on
Cu$_2$$^{114}$Cd$^{11}$B$_2$O$_6$ powder. The magnetic excitations at low
temperatures are similar to those of the interacting spin-1/2 tetramers in the
ordered state. The weak excitations existing above 3 meV suggest that the Higgs
mode appears in Cu$_2$CdB$_2$O$_6$ at ambient pressure and zero magnetic field.
We evaluated $J_1 = 27.3 \pm 1.0$ and $J_2 = -14.0 \pm 1.4$ meV for the
intra-tetramer interactions and $J_3 = -0.4 \pm 0.2$ and $J_4 = 1.4 \pm 0.2$
meV for the inter-tetramer interactions. The spin gap in the isolated spin
tetramer was calculated to be 1.6 meV, which is less than the effective
inter-tetramer interaction value ($3.6 \pm 0.8$ meV). Therefore,
antiferromagnetic long-range order is possible, although the ground state of
the isolated spin tetramer is the spin-singlet state. We discuss the
temperature dependence of the magnetic excitations.
