The Neutron’s Mean Square Charge Radius
The well-known value of the neutrons magnetic moment [1] indicates that it is not an electrically neutral elementary particle. It is now recognized that the neutron is composed of electrically charged particles (quarks). According to relevant measurements, the mean square charge radius of the neutron is a negative quantity [1]
It turns out that the present literature does not contain an adequate theoretical explanation for this effect.
Research topic: Provide a theoretical explanation for the negative value of the neutron’s mean square charge radius.
Remark
The explanation must depend on quark (and antiquark) distribution of the neutron. (For the antiquark existence in the neutron, see, e.g. [2, 3, 4] and apply the isospin symmetry). Explanations like the Cloudy Bag Model [5] are unacceptable, because they are based on pions as elementary pointlike particles that are represented by a function 
, where 
denotes a single set of four space-time coordinates. This treatment is inconsistent with the data, where the pion is not a pointlike particle [1]. Moreover, the pion comprises a 
pair and its function should take the form 
. Like the nucleon, pions may include additional pairs.
References
[1] M. Tanabashi et al. (Particle Data Group), Phys. Rev. D 98, 010001 (2018).
http://pdg.lbl.gov/2018/listings/contents_listings.html
[2] M. Alberg, Prog. Part. Nucl. Phys. 61 140 (2008).
[3] D. H. Perkins, Introduction to High Energy Physics (Addison-Wesley, Menlo Park CA, 1987). (p. 281).
[4] M. Thomson, Modern Particle Physics (Cambridge University Press, Cambridge, 2013). (p. 202).
[5] A. W. Thomas and S. Theberge, Phys. Rev. D24, 216 (1981).