Unsolved puzzles with quark model of neutron
Electric dipole moment
The Standard Model of particle physics predicts a tiny separation of positive and negative charge within the neutron leading to a permanent electric dipole moment.[8] The predicted value is, however, well below the current sensitivity of experiments. From several unsolved puzzles in particle physics, it is clear that the Standard Model is not the final and full description of all particles and their interactions.
http://en.wikipedia.org/wiki/Neutron
Although the Standard Model is not the final and full description of all particles and their interactions,
however here the problem can be not with the Standard Model, but with the current quark model of neutron.
Neutron decay
According to Maxwell theory, when an electric particle (as the electron) is submitted to acceleration, it emits energy (photons).
So, consider the beta decay of a free neutron: n -> p + e + v
At once the neutron becomes a proton, electron, and antineutrino, the electron moves away the proton. As there is Coulombic attraction between the proton and the electron, this one has attraction with the proton, and so it is submitted to deceleration.
Therefore, according to Maxwell theory, the electron would have to emit photons, in any neutron beta decay.
But the emission of photons is not observed experimentally. And we realize that something is wrong with the neutron model composed by quarks.
Magnetic Moment
The neutron magnetic moment is the magnetic moment of the neutron. It was of particular interest, as magnetic moments are created by the movement of electric charges. Since the neutron is a neutral particle, the magnetic moment is an indication of substructure, i.e. that the neutron is made of other, electrically charged particles (quarks).
http://en.wikipedia.org/wiki/Neutron_magnetic_moment
Possibly the neutron's structure is not (d,u,d), as current theories consider.
From such structure the magnetic moment of neutron is not satisfactorily obtained theoretically.
Look at the calculation of magnetic moment of neutron, compared with the magnetic moment of proton, made by Gonzalez-Martin, I. Taboada:
http://prof.usb.ve/ggonzalm/invstg/pblc/MagMoment.pdf
In equation 13 they get the value M = 15 for the correction of the magnetic moment of proton, by considering 6 fermion boost lines.
In equation 18 they get the value M = 28 for the correction of the magnetic moment of neutron, by considering 8 fermion boost lines.
As the proton structure is (u,d,u), and the neutron structure is (d,u,d), there is no reasonable argument to explain why in the proton quark structure it's considered 6 fermion boost lines, and in the neutron quark structure it's considered 8 fermin boost lines.
Certainly Gonzalez-Martin and Taboada made an adjustment in the theoretical calculation, so that to get the magnetic moment of neutron agree to experimental data.
So, there are several evidences suggeting that neutron's structure is not (d,u,d), as supposed in current Particle Physics.
Perhaps neutron's structure is n=p+s , where p is a proton and s is the selectron. From such structure many puzzles of neutron can be eliminated.