Techni-Dilaton Signatures at LHC

We explore discovery signatures of techni-dilaton (TD) at LHC. The TD was predicted long ago as a composite pseudo Nambu-Goldstone boson (pNGB) associated with the spontaneous breaking of the approximate scale symmetry in the walking technicolor (WTC) (initially dubbed scale-invariant technicolor). Being pNGB, whose mass arises from the explicit scale-symmetry breaking due to the spontaneous breaking itself (dynamical mass generation), the TD as a composite scalar should have a mass M-TD lighter than other techni-hadrons, say M-TD similar or equal to 600 GeV for the typical WTC model, which is well in the discovery range of the ongoing LHC experiment. We develop a spurion method of nonlinear realization to calculate the TD couplings to the standard model (SM) particles and explicitly evaluate the TD LHC production cross sections at root s = 7 TeV times the branching ratios in terms of M-TD as an input parameter for the region 200 GeV < M-TD < 1000 GeV in the typical WTC models. It turns out that the TD signatures are quite different from those of the SM Higgs: In the one-doublet model (1DM) all the cross sections including the WW/ZZ mode are suppressed compared to those of the SM Higgs due to the suppressed TD couplings, while in the one-family model (IFM) all those cross sections get highly enhanced because of the presence of extra colored fermion (techni-quark) contributions. We compare the TD -> WW/ZZ signature with the recent ATLAS and CMS bounds and find that in the case of 1DM the signature is consistent over the whole mass range 200 GeV < M-TD < 1000 GeV due to the large suppression of TD couplings, and by the same token the signal is too tiny for the TD to be visible through this channel at LHC. As for the 1FMs, on the other hand, a severe constraint is given on the TD mass to exclude the TD with mass less than or similar to 600 GeV, which, however, would imply an emergence of somewhat dramatic excess as the TD signature at 600 GeV less than or similar to M-TD < 1000 GeV in the near future. We further find a characteristic signature coming from the gamma gamma mode in the 1FM. In sharp contrast to the SM Higgs case, it provides highly enhanced cross section similar to 0.10-1.0 fb at around the TD mass similar or equal to 600 GeV, which is large enough to be discovered during the first few year's run at LBC.