A new two-body relativistic potential model for pionic hydrogen

The new potential model for pionic hydrogen, constructed with the employment of the two-body relativistic equation, is offered. The relativistic equation, based on the extension of the $SL(2,C)$ group to the $Sp(4,C)$ one, describes the effect of the proton spin and anomalous magnetic moment in accordance with the results of the quantum electrodynamics. Within this approach, using the experimental data on the strong energy level shift and width of the $1s$ state in pionic hydrogen as input, the pion-nucleon scattering lengths have been evaluated to be $a_{\pi^{-}p}=0.0860(6)m_{\pi}^{-1}$ and $a_{\pi^{0}n}=-0.1223(19)m_{\pi}^{-1}$.