We have formulated the scattering theory on Majorana fermions emerging in the surface bound state of the superfluid $^3$He B-phase ($^3$He-B) by an impurity. By applying the theory to the electron bubble, which is regarded as the impurity, trapped below a free surface of $^3$He-B, the observed mobility of the electron bubble [Ikegami et al., J. Phys. Soc. Jpn. 82, 124607 (2013)] is quantitatively reproduced. The mobility is suppressed in low temperatures from the expected value in the bulk $^3$He-B by the contribution from the surface Majorana fermions. By contrast, the mobility does not depend on trapped depth of the electron bubble in spite of the spatial variation of the wave function of the surface Majorana fermions. Our formulated theory demonstrates the depth independent mobility by considering intermediate states in the scattering process. Therefore, we conclude that the experiment has succeeded in observing Majorana fermions in the surface bound state.
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