Order-Parameter Collective Modes inHe3−A

The collective modes of the gap parameter for the axial (Anderson-Brinkman-Morel) state and their coupling to the quasiparticles are studied in the collisionless regime within a microscopic theory. In addition to the modes connected with the spontaneous breakdown of symmetries (sound, spin waves, and orbit waves), three weakly damped modes with a typical dispersion law ${\omega }^2\mathrm{}\left(q\right)={\omega }^2\mathrm{}\left(0\right)+\alpha {\left(v_{\mathrm{F}}q\right)}^2$ are found. The feasibility of detecting these modes in the sound absorption is discussed.