feat: Add informal_definition and informal_lemma

HepLean/StandardModel/HiggsBoson/GaugeAction.lean

@@ -190,6 +190,13 @@ theorem rotate_fst_real_snd_zero (φ : HiggsVec) :
   · simp only [Fin.mk_one, Fin.isValue, Pi.smul_apply, Function.comp_apply, cons_val_one, head_cons,
       tail_cons, smul_zero]
 
+informal_lemma stablity_group where
+  physics := "The Higgs boson breaks electroweak symmetry down to the electromagnetic force."
+  math := "The stablity group of the action of `rep` on `![0, Complex.ofReal ‖φ‖]`,
+    for non-zero `‖φ‖`  is the `SU(3) x U(1)` subgroup of
+    `gaugeGroup := SU(3) x SU(2) x U(1)` with the embedding given by
+    `(g, e^{i θ}) ↦ (g, diag (e ^ {3 * i θ}, e ^ {- 3 * i θ}), e^{i θ})`."
+
 end HiggsVec
 
 /-! TODO: Define the global gauge action on HiggsField. -/