How was electroweak symmetry broken when the universe cooled below ~ 100 GeV? In the standard model, electroweak symmetry-breaking takes place through the Higgs mechanism in which the neutral component of an SU(2)_L complex scalar doublet acquires a non-zero vacuum expectation value (vev). This paradigm predicts the existence of a neutral scalar particle that characterizes quantum fluctuations of the Higgs field around the vev. Of all the predictions of the standard model, the Higgs mechanism is the only one that remains to be confirmed. Global analyses of radiative corrections to electroweak precision observables (EWPOs) imply that if the Higgs mechanism is correct, then the Higgs boson should be quite light. The best fit value for the Higgs mass is roughly 85 GeV, yet direct searches for the Higgs boson at LEP and the Tevatron have failed to find a neutral scalar with mass below 114 GeV.

It may be that discovery of the Higgs boson is just around the corner at the Tevatron or Large Hadron Collider. If so, confirmation of the Higgs mechanism will require detailed studies of the Higgs boson and its interactions at the LHC and possibly a future e+e- collider. If not, then perhaps the SM Higgs mechanism is not the correct description of electroweak symmetry-breaking. The scalar sector of whatever is to be the "new standard model" may be more complicated, containing additional scalars beyond those of the complex SU(2)_L field. Additional new physics at scales above the electroweak scale may also modify the lower-energy interactions of the Higgs boson. It is also possible that electroweak symmetry-breaking occurs, not through a Higgs-like mechanism at all, but some other mechanism involving new strong interactions or additional spacetime dimensions.

NPAC theorists and collaborators are studying possible modifications of the SM scalar sector and the corresponding implications for Higgs boson searches at the LHC, Higgs boson studies at a linear collider, and the characteristics of an electroweak phase transition if one occurred (see Baryogenesis). We have been pursuing both supersymmetric and non-supersymmetric extensions of the scalar sector. Recently, we showed how the presence of a relatively light SM singlet scalar that interacts with the Higgs doublet can lead to a strong first order electroweak phase transition as needed for successful baryogenesis; alleviate the tension between EWPO and direct search bounds on the Higgs mass; provide a candidate for cold dark matter; and modify the Higgs boson discovery potential at the LHC and the Higgs boson properties. We are currently studying other candidates for an augmented scalar sector and their implications for EWPOs, the electroweak phase transition, and Higgs searches at the LHC.