  Levitated nanoparticles are being investigated as ultrasensitive sensors of
forces and accelerations, with applications ranging from fundamental physics
phenomena such as dark matter or quantum gravity to real world applications.
Quantum cooling in up to 2 motional modes was achieved recently, and attention
is now turning to multimode and multiparticle regimes. We investigate here the
spectral characteristics of collective motion of $\Ntot= 2-20 $ nanoparticle
modes interacting via the optical light of a cavity. We find output spectra of
collective motion typically exhibit two dominant generic features: a broad
spectral feature, the collective bright mode (CBM) as expected; but we
introduce also the mechanical mode comb (MC) a -- not previously studied --
spectral feature. We investigate numerically, and can also precisely reproduce
all the intricate details of the MC spectra, including ponderomotive squeezing,
with a simple closed-form expression. We show the mechanical comb can
autonomously repair loss of teeth due to nanoparticle loss, a feature that may
offer robustness to a levitated sensor of external forces.
