Quenching of the π0p_{3/2}-π0p_{1/2} Spin-Orbit Splitting in ^{20}O and the Effect of the Tensor Force.

We present the first direct measurement of the Z=6 shell gap in the neutron-rich ^{20}O nucleus. The one-proton removal transfer reaction ^{2}H(^{20}O,^{3}He)^{19}N has been studied using the ACTAR TPC setup at GANIL. The use of ACTAR TPC enabled the measurement of low-cross section proton-removal reactions while preserving resolution. Eight p-hole states with ℓ=1 were identified in ^{19}N accounting for total strengths of 86% and 72% of the 0p_{3/2} and 0p_{1/2} single-particle orbitals, respectively. The energies and spectroscopic factors of the measured states allowed to determine the proton spin-orbit splitting π0p_{3/2}-π0p_{1/2} in ^{20}O. The Z=6 shell gap has been established to be 5.30(14) MeV. These findings indicate a reduction of the Z=6 shell gap while adding neutrons to the sd-valence orbitals, consistent with the effects of the tensor force predicted by state-of-the-art shell model interaction SFO-tls while at variance with the emergence of a large Z=6 gap observed in other studies.