The Electroweak Sphaleron Revisited: I. Static Solutions, Energy Barrier, and Unstable Modes

TL;DR

This paper provides an updated, high-precision analysis of the electroweak sphaleron, including static solutions, energy barrier, and unstable modes, using current Standard Model parameters to improve understanding of baryon number violation.

Contribution

It offers a comprehensive derivation, numerical solutions, and analysis of the sphaleron with realistic Higgs mass, enhancing previous approximations and informing baryon violation studies.

Findings

Sphaleron energy approximately 9.1 TeV

Unstable mode eigenvalue $oxed{-2.7m_W^2}$

Modified unstable mode frequency with measured Higgs mass

Abstract

The electroweak sphaleron is a static, unstable solution of the Standard Model classical field equations, representing the energy barrier between topologically distinct vacua. In this work, we present a comprehensive updated analysis of the sphaleron using current Standard Model parameters with the physical Higgs boson mass of $m_H = 125.1$ GeV and $m_W = 80.4$ GeV, rather than the $m_H = m_W$ approximation common in earlier studies. The study includes: (i) a complete derivation of the $SU(2)\times U(1)$ electroweak Lagrangian and field equations without gauge fixing constraints, (ii) high-precision numerical solutions for the static sphaleron configuration yielding a sphaleron energy $E_{\rm{sph}} \simeq 9.1$ TeV, (iii) an analysis of the minimum energy path in field space connecting the sphaleron to the vacuum (a 1D potential barrier as a function of Chern-Simons number), and (iv) calculation of the sphaleron single unstable mode with negative eigenvalue $\omega^2_{-} = -2.7m^2_W$, providing analytical fits for its eigenfunction. We find that using the measured Higgs mass modifies the unstable mode frequency, with important implications for baryon number violation rates in both early universe cosmology and potential high-energy collider signatures. These results provide essential input for accurate lattice simulations of sphaleron transitions and precision calculations of baryon number violation processes.