  The theory of the strong force, quantum chromodynamics, describes the proton
in terms of quarks and gluons. The proton is a state of two up quarks and one
down quark bound by gluons, but quantum theory predicts that in addition there
is an infinite number of quark-antiquark pairs. Both light and heavy quarks,
whose mass is respectively smaller or bigger than the mass of the proton, are
revealed inside the proton in high-energy collisions. However, it is unclear
whether heavy quarks also exist as a part of the proton wavefunction, which is
determined by non-perturbative dynamics and accordingly unknown: so-called
intrinsic heavy quarks. It has been argued for a long time that the proton
could have a sizable intrinsic component of the lightest heavy quark, the charm
quark. Innumerable efforts to establish intrinsic charm in the proton have
remained inconclusive. Here we provide evidence for intrinsic charm by
exploiting a high-precision determination of the quark-gluon content of the
nucleon based on machine learning and a large experimental dataset. We
disentangle the intrinsic charm component from charm-anticharm pairs arising
from high-energy radiation. We establish the existence of intrinsic charm at
the 3-standard-deviation level, with a momentum distribution in remarkable
agreement with model predictions. We confirm these findings by comparing to
very recent data on Z-boson production with charm jets from the LHCb
experiment.
