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sessing different m . The quasi-interceptions, – i.e. the would-be interception of dispersion curves of two states taken as independent within a certain approximation,– would result in the mutual repulsion of the dispersion curves leading to formation of mixed states, polari- tons, an example of which is given by a photo-positronium - a mixed state between a photon and the electron-positron bound state created by it in a strong magnetic field [5, 17–19]. This situation is illustrated by Fig.1. 

<img src='content_image/365700.jpg'>

The refraction index squared n 2 a is defined for photons of mode a on the mass shell (23) as 

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It follows from (26) with m = f 1 / $^{2}$(0) = 0 that the refraction index is greater than unity - the statement common in standard optics of media (this is not, certainly, true for (massive) plasmon branches). Consequently, the modulus of the phase velocity in each mode v ph a = ( k$_{0}$/ | k | $^{2}$) k equal to 1 /n$_{a}$ is, for the photon proper , also smaller than the velocity of light in 

