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The proton asymmetry between the u antiquark and the
d antiquark is known for more than a decade [1,2].
The Regular Charge-Monopole Theory (RCMT) provides an obvious
and straighforward interpretation
for this effect. Indeed, quarks are Dirac particles obeying
the Pauli exclusion principle. Now, the proton has uud
valence quarks. Hence, more states of the u quark
are occupied than states of the d quark. Thus,
an addition of a quark-antiquark pair
of the d flavor is energetically easier
that an addition of a u counterpart pair.
For this reason, the
probability of finding a d antiquark in the proton is
larger than that of finding a u antiquark.
It turns out that people adhering to QCD have not predicted
this effect [1].
References:
[1] E. A. Hawker et al., Phys. Rev. Lett., 80, 3715 (1998).
[2] M. Alberg, Prog. Part. Nuc. Phys. 61, 140 (2008).
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