Polarization difference between hyperons and antihyperons induced by an external magnetic field

Abstract

We investigate the quantum correlated ΛΛ production in the reaction e+e-→J/ψ→ΛΛ. Since the Λ or Λ has a nonzero magnetic moment, its spin will undergo a Larmor precession in the magnetic field of the detector, such as the BESIII experiment. Because of the spin precession, the angular distribution of the Λ and Λ is slightly modified. Therefore, we obtain the corresponding term of the modified angular distribution due to the effect of the Larmor precession. We also estimate its potential effect on the measurements of CP violation, as well as the decay asymmetry parameter and polarization of Λ. The polarization of the Λ or Λ at the production vertex will rotate around the B-field axis, over an angle depending on the flight length, but it still could be measured by fit to the corrected angular distribution. Of important note, we conclude that a nonzero CP asymmetry of order 10-4 will be caused once neglecting spin precession of the Λ and Λ in the e+e-→J/ψ→ΛΛ process. The size of this CP asymmetry is several times that of predicted within Standard Model in the hyperon decay. Although this effect is small, it will play an important role in future high precision experiments, such as the super-tau-charm factory.