Can the electroweak vacuum be unstable?

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Abstract
The electroweak vacuum need not be absolutely stable. For certain top-quark and Higgs-boson masses in the minimal standard model, our vacuum is instead metastable with a lifetime exceeding the present age of the Universe. It has been suggested that a metastable vacuum is generally ruled out because high-energy cosmic-ray collisions would have long ago induced its decay. I argue that the reasoning for this conclusion is erroneous. As a consequence, upper bounds on the top-quark mass derived from stability arguments are relaxed. Also presented is an analytic method for accurately approximating the lifetime of the vacuum from the effective potential without solving for the 0(4) bounce solution numerically.