In differential geometry and especially Yang–Mills theory, a (weakly) stable Yang–Mills–Higgs (YMH) pair is a Yang–Mills–Higgs pair around which the Yang–Mills–Higgs action functional is positively or even strictly positively curved. Yang–Mills–Higgs pairs are solutions of the Yang–Mills–Higgs equations following from them being local extrema of the curvature of both fields, hence critical points of the Yang–Mills-Higgs action functional, which are determined by a vanishing first derivative of a variation. (Weakly) stable Yang–Mills-Higgs pairs furthermore have a positive or even strictly positive curved neighborhood and hence are determined by a positive or even strictly positive second derivative of a variation. (Weakly) stable Yang–Mills–Higgs pairs are named after Yang Chen-Ning, Robert Mills and Peter Higgs.
Definition Let G {\displaystyle G} be a compact Lie group with Lie algebra g {\displaystyle {\mathfrak {g}}} and E ↠ B {\displaystyle E\twoheadrightarrow B} be a principal G {\displaystyle G} -bundle with a compact orientable Riemannian manifold B {\displaystyle B} having a metric g {\displaystyle g} and a volume form vol g {\displaystyle \operatorname {vol} _{g}} . Let Ad ( E ) := E × G g {\displaystyle \operatorname {Ad} (E):=E\times _{G}{\mathfrak {g}}} be its adjoint bundle. A = Ω Ad 1 ( E , g ) {\displaystyle {\mathcal {A}}=\Omega _{\operatorname {Ad} }^{1}(E,{\mathfrak {g}})} , an affine vector space (not canonically) isomorphic to Ω 1 ( B , Ad ( E ) ) {\displaystyle \Omega ^{1}(B,\operatorname {Ad} (E))} , is the space of connections. These are under the adjoint representation Ad {\displaystyle \operatorname {Ad} } invariant g {\displaystyle {\mathfrak {g}}} -valued (Lie algebra–valued) differential forms on E {\displaystyle E} and through pullback along smooth sections B ↪ E {\displaystyle B\hookrightarrow E} differ by Ad ( E ) {\displaystyle \operatorname {Ad} (E)} -valued (vector bundle–valued) differential forms on B {\displaystyle B} . The Yang–Mills–Higgs action functional is given by:
YMH : Ω Ad 1 ( E , g ) × Γ ∞ ( B , Ad ( E ) ) → R , YMH ( A , Φ ) := ∫ B ‖ F A ‖ 2 + ‖ d A Φ ‖ 2 d vol g . {\displaystyle \operatorname {YMH} \colon \Omega _{\operatorname {Ad} }^{1}(E,{\mathfrak {g}})\times \Gamma ^{\infty }(B,\operatorname {Ad} (E))\rightarrow \mathbb {R} ,\operatorname {YMH} (A,\Phi ):=\int _{B}\|F_{A}\|^{2}+\|\mathrm {d} _{A}\Phi \|^{2}\mathrm {d} \operatorname {vol} _{g}.}
A Yang–Mills–Higgs pair A ∈ A = Ω Ad 1 ( E , g ) {\displaystyle A\in {\mathcal {A}}=\Omega _{\operatorname {Ad} }^{1}(E,{\mathfrak {g}})} and Φ ∈ Γ ∞ ( B , Ad ( E ) ) {\displaystyle \Phi \in \Gamma ^{\infty }(B,\operatorname {Ad} (E))} , hence which fulfill the Yang–Mills–Higgs equations, is called stable if:
d 2 d t 2 YMH ( α ( t ) , φ ( t ) ) | t = 0 > 0 {\displaystyle {\frac {\mathrm {d} ^{2}}{\mathrm {d} t^{2}}}\operatorname {YMH} (\alpha (t),\varphi (t))\vert _{t=0}>0}
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