Electroweak sphaleron revisited. I. Static solutions, energy barrier, and unstable modes

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msub> <a:mi>m</a:mi> <a:mi>H</a:mi> </a:msub> <a:mo>=</a:mo> <a:mn>125.1</a:mn> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mi>GeV</a:mi> </a:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msub> <c:mi>m</c:mi> <c:mi>W</c:mi> </c:msub> <c:mo>=</c:mo> <c:mn>80.4</c:mn> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi>GeV</c:mi> </c:math> , rather than the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msub> <e:mi>m</e:mi> <e:mi>H</e:mi> </e:msub> <e:mo>=</e:mo> <e:msub> <e:mi>m</e:mi> <e:mi>W</e:mi> </e:msub> </e:math> approximation common in earlier studies. The study includes: (i) a complete derivation of the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>S</g:mi> <g:mi>U</g:mi> <g:mo stretchy="false">(</g:mo> <g:mn>2</g:mn> <g:mo stretchy="false">)</g:mo> <g:mo>×</g:mo> <g:mi>U</g:mi> <g:mo stretchy="false">(</g:mo> <g:mn>1</g:mn> <g:mo stretchy="false">)</g:mo> </g:math> electroweak Lagrangian and field equations without gauge fixing constraints, (ii) high-precision numerical solutions for the static sphaleron configuration yielding a sphaleron energy <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:msub> <m:mi>E</m:mi> <m:mi>sph</m:mi> </m:msub> <m:mo>≃</m:mo> <m:mn>9.1</m:mn> <m:mtext> </m:mtext> <m:mtext> </m:mtext> <m:mi>TeV</m:mi> </m:math> , (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 <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:msubsup> <o:mi>ω</o:mi> <o:mo>−</o:mo> <o:mn>2</o:mn> </o:msubsup> <o:mo>=</o:mo> <o:mo>−</o:mo> <o:mn>2.7</o:mn> <o:msubsup> <o:mi>m</o:mi> <o:mi>W</o:mi> <o:mn>2</o:mn> </o:msubsup> </o:math> , 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.