  After showing the characteristic features of the chiral transition in QCD at
finite temperature $T$, we note the significance of the sigma-mesonic mode in
the chiral transition : Sigma meson may be regarded as the "Higgs particle" in
QCD, though the existence of such a mode in the real world is still unclear. We
show the properties of the sigma meson in hot and/or hadronic matter, from
which we propose experiments to reveal the existence the sigma meson clearly;
the experiments include relativistic heavy-ion collisions to create hot
hadronic matter, and electro-production in heavy nuclei.
