The ab oscillations in absorbing matter are considered. The standard model based on optical potential does not describe the total ab transition probability as well as the channel corresponding to absorption of the b-particle. We calculate directly the off-diagonal matrix element in the framework of field-theoretical approach. Contrary to one-particle model, the final state absorption does not tend to suppress the channels mentioned above or, similarly, calculation with hermitian Hamiltonian leads to increase the corresponding values. The model reproduces all the results on the particle oscillations, however it is oriented to the description of the above-mentioned channels. Also we touch on the problem of infrared singularities. The approach under study is infrared-free.

It is shown that there is a double counting in the standard model of nn mixing in the medium, resulting in full cancellation of leading terms. The direct calculation of nn transition followed by annihilation is performed. The lower limit for the free-space nn oscillation time is nn ϳT nn Ͼ10 16 yr, where T nn is the lifetime of neutron bound in a nucleus. This limit exceeds the previous one by 16 orders of magnitude.

The role of strong absorption of particles in intermidiate and final states has been considered. The range of applicability of phenomenological model of absorption has been studied. This model is nonuniversal. Its applicability depends on the type of interaction Hamiltonian and matrix element used. We also demonstrate that the violation of the unitarity condition can produce a qualitative error in the results. The absorption (decay) in the final state does not tend to suppress the total process probability as well as the probability of the channel corresponding to absorption. This is true for the reactions, decays and nn conversion in the medium.

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