The carbon vacancy (V sub(C)) has been suggested by different studies to be involved in the Z sub(1)/Z sub(2) defect-a carrier lifetime killer in SiC. However, the correlation between the Z sub(1)/Z sub(2) deep level with V sub(C) is not possible since only the negative carbon vacancy (V super(-) sub(C)) at the hexagonal site, V super(-) sub(C)(h), with unclear negative-U behaviors was identified by electron paramagnetic resonance (EPR). Using freestanding n-type 4H -SiC epilayers irradiated with low energy (250 keV) electrons at room temperature to introduce mainly V sub(C) and defects in the C sublattice, we observed the strong EPR signals of V super(-) sub(C) (h) and another S = 1/2 center. Electron paramagnetic resonance experiments show a negative-U behavior of the two centers and their similar symmetry lowering from C sub(3v) to C sub(1h) at low temperatures. Comparing the Si and C ligand hyperfine constants observed by EPR and first principles calculations, the new center is identified as V super(-) sub(C) (k). The negative-U behavior is further confirmed by large scale density functional theory supercell calculations using different charge correction schemes. The results support the identification of the lifetime limiting Z sub(1)/Z sub(2) defect to be related to acceptor states of the carbon vacancy.