The tensor force in the Deuteron
The deuteron is a proton-neutron bound state and it is the only stable particle which contains two nucleons. The deuteron has spin-1 and one of the forces that holds these two nucleons together is the tensor force [1,2]. Calculations show that the magnetic dipole of the nucleons cannot explain the tensor force, because it is too weak and because its sign is opposed to the measured sign of the deuteron’s tensor force.
A Yukawa force cannot explain the effect. Indeed, the is a spinless particle. Therefore, the laws of angular momentum addition prove that it cannot couple two spin-1/2 particles with the deuteron’s D-component. Furthermore, the is made of a quark-antiquark pair, whose radius is not much smaller than that of the proton, whereas a Yukawa particle is an elementary pointlike particle.
Research topic 1: Explain the tensor force in the deuteron.
 J. Schwinger, On the neutron-proton interaction, Phys. Rev. 55, 235 (1939).
 S. S. M. Wong, Introductory Nuclear Physics (Wiley, New York, 1998). Pp. 68-71.