Protected nodes and the collapse of the Fermi arcs in high $T_{c}$ cuprates

Angle resolved photoemission studies on underdoped samples of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ reveal that the superconducting gap's magnitude and anisotropy remain unchanged up to $T_{c}$. Above $T_{c}$, the nodes of the d-wave gap abruptly expand into finite length Fermi arcs. As this change occurs within the resistive width of the transition, we argue that the Fermi arcs are not simply thermally broadened nodes, but rather a unique signature of the pseudogap phase. This is in contrast to BCS theory, which predicts a gap with fixed anisotropy that changes with temperature and disappears above $T_{c}$.