  The fundamental building blocks of the proton, quarks and gluons, have been
known for decades. However, we still have an incomplete theoretical and
experimental understanding of how these particles and their dynamics give rise
to the quantum bound state of the proton and its physical properties, such as
for example its spin. The two up and the single down quarks that comprise the
proton in the simplest picture account only for a few percent of the proton
mass, the bulk of which is in the form of quark kinetic and potential energy
and gluon energy from the strong force. An essential feature of this force, as
described by quantum chromodynamics, is its ability to create matter-antimatter
quark pairs inside the proton that exist only for a very short time. Their
fleeting existence makes the antimatter quarks within protons difficult to
study, but their existence is discernible in reactions where a
matter-antimatter quark pair annihilates. In this picture of quark-antiquark
creation by the strong force, the probability distributions as a function of
momentum for the presence of up and down antimatter quarks should be nearly
identical, since their masses are quite similar and small compared to the mass
of the proton. In the present manuscript, we show evidence from muon pair
production measurements that these distributions are significantly different,
with more abundant down antimatter quarks than up antimatter quarks over a wide
range of momentum. These results revive interest in several proposed mechanisms
as the origin of this antimatter asymmetry in the proton that had been
disfavored by the previous results and point to the future measurements that
can distinguish between these mechanisms.
