  In superconductors, electrons bound into Cooper pairs conduct a
dissipationless current. The strength of the Cooper pairs scales with the value
of the critical transition temperature (Tc). In cuprate high-Tc
superconductors, however, the pairing mechanism is still unexplained. Here we
unveil why in the cuprates the Cooper pairs are so strongly bound to work out
the extraordinary high Tc. From one-to-one correspondence between numerical
simulation on a microscopic cuprate model and a simple two-component fermion
model, we show that hidden fermions emerge from the strong electron correlation
and give birth to the strongly bound Cooper pairs. This mechanism is distinct
from a conventional pairing mediated by some bosonic glue, such as phonons in
conventional superconductors. The hidden fermions survive even above Tc and
generate the strange-metal pseudogap phase. This reveals an unprecedented
direct relationship between the pseudogap phase and superconductivity in the
cuprates.
