Thermodynamic Identification of the Internal Superconducting Phase Boundary in UTe$_2$ for $H \parallel b$

The $H$--$T$ phase diagram of UTe$_2$ for magnetic field along the hard $b$ axis contains an unresolved internal boundary near $\mu_0H \sim 14$--15~T, previously inferred from ac susceptibility and transport experiments but lacking thermodynamic evidence. We report ultrasound results for several elastic modes in an ultraclean UTe$_2$ single crystal with $T_c>2$~K for $H \parallel b$ down to 0.33~K and up to 18~T. A pronounced anomaly in the longitudinal $C_{33}$ mode, with a weaker response in $C_{44}$ and no resolvable anomaly in $C_{55}$, establishes this feature as a bulk thermodynamic phase boundary and reveals a symmetry-selective coupling to lattice strain. The phase line remains nearly constant in field near 14~T and terminates near 13.5~T and 1.25~K at a tetracritical point, providing the thermodynamic evidence for the fourth phase boundary in the $H$--$T$ phase diagram. The results constrain the order-parameter structure of the high-field phase and support field-induced multicomponent superconductivity in UTe$_2$.