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Light Higgs channel of the resonant decay of magnon condensate in superfluid $^3$He-B

In superfluids the order parameter, which describes spontaneous symmetry breaking, is an analogue of the Higgs field in the Standard Model of particle physics. Oscillations of the field amplitude are massive Higgs bosons, while oscillations of the orientation are massless Nambu-Goldstone bosons. The 125~GeV Higgs boson, discovered at Large Hadron Collider, is light compared to electroweak energy scale, which led to a suggestion of the ``little Higgs'' extension of the Standard Model, in which the light Higgs appears as a NG mode acquiring mass due to violation of a hidden symmetry. Here we show that such light Higgs exists in superfluid $^3$He-B, where one of three Nambu-Goldstone spin-wave modes acquires small mass due to the spin-orbit interaction. Other modes become optical and acoustic magnons. We observe parametric decay of Bose-Einstein condensate of optical magnons to light Higgs modes and decay of optical to acoustic magnons. Formation of a light Higgs from a Nambu-Goldstone mode observed in $^3$He-B opens a possibility that such scenario can be realized in other systems, where violation of some hidden symmetry is possible, including the Standard Model.