  The theoretical prediction of Q-balls in relativistic quantum fields is
realized here experimentally in superfluid 3He-B. The condensed-matter analogs
of relativistic Q-balls are responsible for an extremely long lived signal of
magnetic induction -- the so-called Persistent Signal -- observed in NMR at the
lowest temperatures. This Q-ball is another representative of a state with
phase coherent precession of nuclear spins in 3He-B, similar to the well known
Homogeneously Precessing Domain which we interpret as Bose condensation of spin
waves -- magnons. At large Q the effect of self-localization is observed. In
the language of relativistic quantum fields it is caused by interaction between
the charged and neutral fields, where the neutral field provides the potential
for the charged one. In the process of self-localization the charged field
modifies locally the neutral field so that the potential well is formed in
which the charge is condensed.
