RIGHT-HANDED FERMION motion spin aligned → substrate (CW) core (CW) vinner vouter Δv small — low shear Both sides flow same way Relaxed boundary — invisible to W LEFT-HANDED FERMION motion spin ← opposed → substrate (CW) core (CCW) vinner ← → vouter Δv large — maximum shear Both sides flow opposite ways Strained boundary = weak charge W coupling: none · Z coupling: −Q sin²θW only W coupling: full · Z coupling: T₃ − Q sin²θW Co-rotating flow Counter-rotating boundary Substrate background Shear stress

Figure 12.4. Why the weak force is left-handed. Both panels show the same fermion structure — co-rotating core (amber), counter-rotating boundary (teal), ordered substrate (gold) — but with opposite chirality. Left: Right-handed fermion. Core and substrate rotate the same way, so the boundary between them is under minimal shear. The W boson cannot couple to a relaxed boundary. Right: Left-handed fermion. Core and substrate rotate opposite ways, placing the boundary under maximum shear stress. This stored strain energy is the weak charge — the physical quantity the W boson couples to. SU(2)L acts only on left-handed particles because it is the symmetry group of strained boundary states.