Massive Scalar Quasinormal Modes, Greybody Factors, and Absorption Cross Section of a Parity-Symmetric Beyond-Horndeski Black Hole

We study quasinormal modes, greybody factors, and the absorption cross section of a massive scalar field in the asymptotically flat parity-symmetric beyond-Horndeski black-hole background. The scalar mass raises the asymptotic level of the effective potential and can eliminate its barrier peak, thereby changing both the ringing spectrum and the scattering characteristics relative to the massless case. Using Pad\'e-improved high-order WKB calculations together with time-domain evolution, we find that the damping rate decreases strongly as the field mass increases, indicating the approach to long-lived quasi-resonant states for representative parameter families. At the same time, in the large-mass regime these weakly damped modes become progressively harder to isolate in the time domain, because the oscillatory massive tails are expected to dominate on the Koyama--Tomimatsu scale $\mu_s t\gg \mu_s M$, which is comparatively early when $M=1$ and $\mu_s$ is not small. The time-domain profiles also exhibit the transition from quasinormal ringing to an oscillatory late-time tail. Interpreting the same effective potential semiclassically, we show that increasing the scalar mass suppresses low-frequency transmission and shifts the onset of efficient absorption to higher frequencies, while larger deviations from the Schwarzschild limit enhance the absorption cross section. These results show that the competition between long-lived modes and rapidly dominant massive tails makes the massive sector an especially subtle and sensitive probe of the interplay between field mass and geometric deformation in this class of black holes.