  Simultaneous neutron-scattering and thermal expansion measurements on the
heavy-fermion superconductor URu2Si2 under hydrostatic pressure of 0.67 GPa
have been performed in order to detect the successive paramagnetic, hidden
order, and large moment antiferromagnetic phases on cooling. The temperature
dependence of the sharp low energy excitation at the wavevector Q_0=(100) shows
that this excitation is clearly a signature of the hidden order state. In the
antiferromagnetic phase, this collective mode disappears. The higher energy
excitation at the incommensurate wavevector Q_1=(1.4,0,0) persists in the
antiferromagnetic phase but increases in energy. The collapse of the inelastic
neutron scattering at Q_0 coincides with the previous observation of the
disappearance of superconductivity.
