  The superconductivity of cuprates, which has been a mystery ever since its
discovery decades ago, is created through doping electrons or holes into a Mott
insulator. There, however, exists an inherent electron-hole asymmetry in
cuprates. The layered crystal structures of cuprates enable collective charge
excitations fundamentally different from those of three-dimensional metals,
i.e., acoustic plasmons. Acoustic plasmons have been recently observed in
electron-doped cuprates by resonant inelastic X-ray scattering (RIXS); in
contrast, there is no evidence for acoustic plasmons in hole-doped cuprates,
despite extensive measurements. This contrast led us to investigate whether the
doped holes in cuprates La$_{2-x}$Sr$_x$CuO$_4$ are conducting carriers or are
too incoherent to induce collective charge excitation. Here we present
momentum-resolved RIXS measurements and calculations of collective charge
response via the loss function to reconcile the aforementioned issues. Our
results provide unprecedented spectroscopic evidence for the acoustic plasmons
and long sought conducting p holes in hole-doped cuprates.
