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The Goldstone theorem mandates that a spontaneous symmetry breaking entailsthe emergence of gap(mass)less excitations. In the case where a rotationalinvariance of a system of spin magnetic moments is broken by anantiferromagnetic order, these are well-known transverse spin waves. Theinteraction of such Goldstone magnons with the Higgs amplitude mode of theorder parameter is usually discarded, even though glimpses of Higgs physicshave recently been reported in a quantum magnet, a topological insulator, andferroelectric and disordered superconductor systems. The Goldstone-Higgsinteractions could be expected to grow in importance near a quantum criticalpoint (QCP), where the symmetry-breaking order is weak, and its amplitudefluctuations are significant. Here we report an electron spin resonance (ESR)study of a nearly one-dimensional spin-1/2 chain system, Sr$_2$CuO$_3$, whichpresents exactly such a case. The ESR spectra at $T > T_N$, in the disorderedLuttinger-spin-liquid phase with unconfined spinons reveal idealHeisenberg-chain behavior with only very small, field-independent linewidth,$\sim 1/T$. In the ordered state, below $T_N$, we identify antiferromagneticresonance (AFMR) modes, which are well described by pseudo-Goldstone magnons inthe model of a collinear biaxial antiferromagnet with two gaps, $\Delta_1 =23.0$ GHz and $\Delta_2 = 13.3$ GHz. Additionally, we observe a major resonantresponse of special nature, which we attribute to magnon interaction with theHiggs amplitude mode in a weakly ordered antiferromagnet. Its unusual fielddependence indicates the presence of a quantum phase transition at $\mu_0 H\simeq 9.4$ T.

Magnons in Sr$_2$CuO$_3$: possible evidence for Goldstone-Higgs interaction in a weakly ordered spin-1/2 chain antiferromagnet