The deceleration of a fast charged particle penetrating through matter differs from that of its anti-particle. This difference is called the Barkas effect. According to Bethe, the stopping power of a fast charged particle penetrating through matter is proportional only to the square of its charge. Smith et al., however, found a small difference between ranges of positive and negative pions in emulsion. This was later attributed to small difference in the stopping power resulting from polarisation.
1. W.H. Barkas, J.N. Dyer and H.H. Heckman. Phys. Rev. Lett. 11 (1963), p. 26.
2. F.M. Smith, W. Birnbaum and W.H. Barkas. Phys. Rev. 91 (1953), p. 765. Full Text via CrossRef