We study a scale-invariant $SU(2)\times U(1{)}_Y\times U(1{)}_s$ model which has only dimensionless couplings. The shadow $U(1{)}_s$ is hidden, and it interacts with the standard model (SM) solely through mixing in the scalar sector and kinetic mixing of the $U(1)$ gauge bosons. The gauge symmetries are broken radiatively by the Coleman-Weinberg mechanism. Lifting of the flat direction in the scalar potential gives rise to a light scalar, the scalon, or the shadow Higgs, and a heavier scalar which we identify as the SM Higgs boson. The phenomenology of this model is discussed. In particular, the constraints on the shadow Higgs in different mass ranges, and the possibility of discovering a shadow Higgs with a mass a few tens of GeV in precision $t$-quark studies at the LHC, are investigated.

• Received 9 February 2007

DOI:https://doi.org/10.1103/PhysRevD.75.115016