  We report low temperature electron spin resonance experimental and
theoretical studies of an archetype $S=1/2$ strong-rung spin ladder material
(C$_{5}$H$_{12}$N)$_{2}$CuBr$_{4}$. Unexpected dynamics is detected deep in the
Tomonaga-Luttinger spin liquid regime. Close to the point where the system is
half-magnetized (and believed to be equivalent to a gapless easy plane chain in
zero field) we observed orientation-dependent spin gap and anomalous $g$-factor
values. Field theoretical analysis demonstrates that the observed low-energy
excitation modes in magnetized (C$_{5}$H$_{12}$N)$_{2}$CuBr$_{4}$ are solitonic
excitations caused by Dzyaloshinskii-Moriya interaction presence.

Strongly Correlated Electrons