Quasiparticles of the Heisenberg spin-1/2 chain - spinons - represent the best experimentally accessible example of fractionalized excitations known to date. Dynamic spin response of the spin chain is typically dominated by the broad multi-spinon continuum that often masks subtle features, such as edge singularities, induced by the interaction between spinons. This, however, is not the case in the small momentum region of the magnetized spin chain where strong interaction between spinons leads to {em qualitative} changes to the response. Here we report experimental verification of the recently predicted collective modes of spinons in a model material K$_2$CuSO$_4$Br$_2$ by means of the electron spin resonance (ESR). We exploit the unique feature of the material - the uniform Dzyaloshinskii-Moriya interaction between chains spins - in order to access small momentum regime of the dynamic spin susceptibility. By measuring interaction-induced splitting between the two components of the ESR doublet we directly determine the magnitude of the marginally irrelevant backscattering interaction between spinons for the first time. We find it to be in an excellent agreement with the predictions of the effective field theory. Our results point out an intriguing similarity between the one-dimensional interacting liquid of neutral spinons and the Landau Fermi liquid of electrons.
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