pentaquarks

[taylordnz]

why do models show pentaquark structures have 4 quarks and an anti-quark, why cant it be just five quarks?

[humanino]

it very simply because of the quantum numbers they carry ! For example, the first observed (to be confirmed!) pentaquark is the Theta+. This particle has positive strangeness, therefore one is forced to introduce an anti-strange quark (strange antiquark) (you know what I mean, right?) in the valence structure.

This in turn implies that this pentaquark is NOT just a regular 3-quark structure plus a vacuum fluctuation of meson orbiting around, but rather a resonant structure involving 5 quarks.

[Haelfix]

Look up the model called the chiral soliton model of the pentaquark. I think thats the current favored (though contentious) theoretical prescription.

[kurious]

5 quarks equals colour neutral - that's all there is to it.

[humanino]

Well !
It is certainly not that simple. I just began studying QCD about 1 year and a half ago. At this time, I was taught that hadrons are either qqq or q\bar{q}. In this respect, SU(3) is so natural, in terms of color confinement. On the contrary, a 5q state is not obviously a color singlet.

I find the qqqq\bar{q} very unnatural.

[elas]

Note how the answers to your question refer either to numerical sequences or to concept that are not real but merely match an observation (ie color force). They are mathematically correct but fail to give a non-mathematical reason for being correct.
A simple 'words only' explanation is to consider all particles as natures attempt to balance force and anti-force fields. In a field obeying the inverse square law, the quantities of force and anti force cannot be equal. In order to maintain near equality in a multi-particle field it is occassionally necessary to add a particle with "more anti-force than force" to balance the shortage of anti-force in an assembly of "more force than anti-force" particles (and vice versa).
This is the role of neutrons in an atomic nucleus and the role of electrons in an atomic field. The limits of construction being determined by the elasticity of the field.

[garytse86]

is there an antiforce in reality or is it just used to explain the concept?

[elas]

At the particle level it is necessary to have attractive and repulsive electromagnetism. On the cosmic scale anti-gravity is sometimes used to explain universal expansion. On the strong force scale plus and minus quantities are needed to explain quark behaviour. That said no one knows what causes these forces to exist or what they are in reality, for that you need the nutcases on theory developement.

[humanino]

Does someone knows the current status !?
I believe the theta+ was a spurious peak. I am not being provocative. I think somebody around could be analyzing some recent data from a dedicated experiment.