  Our goal is to discover possible new 4-dimensional euclidean solutions
(instantons) in fundamental SU(2) Yang-Mills-Higgs theory, with a constraint
added to prevent collapse of the scale. We show that, most likely, there exists
one particular new constrained instanton (\Istar) with vanishing Pontryagin
index. This is based on a topological argument that involves the construction
of a non-contractible loop of 4-dimensional configurations with a certain
upperbound on the action, which we establish numerically. We expect \Istar to
be the lowest action non-trivial solution in the vacuum sector of the theory.
There also exists a related static, but unstable, solution, the new sphaleron
\Sstar. Possible applications of \Istar to the electroweak interactions include
the asymptotics of perturbation theory and the high-energy behaviour of the
total cross-section.

High Energy Physics - Phenomenology