= Deuteron quadrupole evidence for a tensor force
An isotropic pure $L=0$ spatial state has zero <electric quadrupole moment>. The nonzero <deuteron> quadrupole moment consequently requires anisotropic structure. With ground-state $J^P=1^+$, a <nuclear tensor force> can mix $\,{}^3S_1$ and $\,{}^3D_1$ waves while preserving total <angular momentum> and <parity>. A purely central spin-independent potential cannot produce this mixing. The inference of an orientation-dependent force was already explicit in 1939.<The Electric Quadrupole Moment of the Deuteron and the Field Theory of Nuclear Forces by Christian Møller and Léon Rosenfeld>[Abstract and opening discussion, Nature 144, 476–477]{ref}
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