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We present a combined study from angle-resolved photoemission anddensity-functional theory calculations of the temperature-dependent electronicstructure in the excitonic insulator candidate Ta$_2$NiSe$_5$. Our experimentalmeasurements unambiguously establish the normal state as a semimetal with asignificant band overlap of $>$100~meV. Our temperature-dependent measurementsindicate how these low-energy states hybridise when cooling through thewell-known 327~K phase transition in this system. From our calculations andpolarisation-dependent photoemission measurements, we demonstrate theimportance of a loss of mirror symmetry in enabling the band hybridisation,driven by a shear-like structural distortion which reduces the crystal symmetryfrom orthorhombic to monoclinic. Our results thus point to the key role of thelattice distortion in enabling the phase transition of Ta$_2$NiSe$_5$.

Band hybridisation at the semimetal-semiconductor transition of Ta$_2$NiSe$_5$ enabled by mirror-symmetry breaking