QCD dynamics under a strong magnetic field is of great interest to the field of relativistic heavy-ion collisions and magnetars.
In this talk, I will discuss a new effect we recently found in Ref., 'Hadronic Paschen-Back effect (HPBE)', which is analogous to the Paschen-Back effect observed in atomic physics.
This effect is induced by the interplay between a strong magnetic field and finite orbital angular momenta in hadronic systems.
It allows the wave functions to drastically deform and leads to anisotropic decays.
Such a decay gives a possibility to measure the strength of the magnetic field in heavy-ion collision at LHC, RHIC and SPS, which has not experimentally been measured.
As an example of HPBE, I will report our results  of the mass spectra, wave functions, and mixing rates of P-wave charmonia in a wide range of magnetic fields by using the potential model and a numerical few-body technique.
Furthermore, I will talk about a systematic study for the radiative decays of P-wave quarkonia by HPBE based on potential non-relativistic QCD in Ref..
 arXiv:1802.04971  arXiv:1805.09787