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Effective Field Theory with Dibaryon Fields: Nucleon-Nucleon amplitudes at NNLO

We calculate the nucleon-nucleon scattering amplitudes in the 1S0 and 3S1-3D1 channels at next-to-next to leading order starting from a recently proposed non-relativistic chiral effective theory, which includes dibaryon fields as fundamental degrees of freedom. We restrict ourselves to center of mass energies (E) smaller than the pion mass (m_pi), and further divide the calculation into two relative momentum (p) regions, a high energy one p ~ m_pi >> delta m_i, δm_i being the dibaryon residual masses, and low energy one p ~ delta m_i. We first match to a lower energy effective theory in which we calculate the amplitudes in the high energy region. We further match this effective theory to the so called pionless effective theory in the low energy region, and carry out the calculations in the latter. Dimensional regularization and minimal subtraction scheme are used throughout. For 1S0 channel a good description of the phase shift data is obtained for E < 50 MeV. For the 3S1-3D1 channel, the 3S1 phase shift data is only well described up to E < 20 MeV.