Higgs self-coupling and electroweak vacuum stability

4 problems

• Was the electroweak phase transition first order?

  Open

  In the SM, the Higgs field switched on smoothly as the early universe cooled. New particles could instead have made it an abrupt boiling transition, which could produce the matter excess and gravitational waves.

• How precisely does the measured top mass equal the pole mass?

  Open

  Colliders extract the top-quark mass by matching data to computer event simulations, and that parameter is not exactly the mass defined in theory calculations. The mismatch is now as large as the measurement errors.

• What is the Higgs trilinear self-coupling?

  Open

  The Higgs interacts with itself, and the SM predicts the exact strength from the Higgs mass. Measuring it requires seeing Higgs bosons produced in pairs, an extremely rare process.

• Is the electroweak vacuum absolutely stable if the SM holds to $M_{\mathrm{Pl}}$?

  Open

  If no new particles appear up to the Planck scale, the measured Higgs and top-quark masses seem to make our vacuum only long-lived instead of permanently stable. The verdict depends on the top mass to a fraction of a GeV.