Pentaquarks

[Chowmein]

I read an article about how scientists have discovered evidence of pentaquarks. The article was a bit too "advanced" for me to understand, so I hope someone here can shed some light on the situation.

What exactly is a pentaquark and what types of particles does it make up? How does it differ from other forms of quarks?

[MC Master of Ceremonies]

Put very simply:

Quarks come in six different flavours, each flavour having an antiquark too baryons (protons, neutrons) are though to be composed of three quarks, while mesons (pions, kaons) are though to be composed of a quark-antiquark pair. A pentaquark merely means a particle composed up of 5 quarks there existance was thought a possibilty but up until now none had been observed (I think there not entirely sure if they saw a pentaquark or not).

[Lord Ashtar]

Link?

[MC Master of Ceremonies]

change 'though' into 'thought' and but a comma after 'too'.

[MC Master of Ceremonies]

also get rid of the 'up' after 'composed'. Godamnit I wish they had an edit funtion.

[Pasta]

A pentaquark is a composite particle made up of five regular quarks and antiquarks (just like, for example, a proton is made up of three quarks).

To elaborate...

Quarks are always locked up in bound states. That is, you won't find a free quark lying around; you'll only find composite particles made up of quarks. Composite particles made up of quarks are collectively called "hadrons".

Quarks have a quantum number called "color" which can take on 3 values (often labeled red, blue, and green for quarks and antired, antiblue, and antigreen for antiquarks). Color has these properties:

- a system with a red, a blue, and a green has no net color
- a system with a red and an antired has no net color
- same with blue/antiblue and green/antigreen

These properties are analogous to, "A system with a positively charged particle and a negatively charged particle has no net charge."

Colored hadrons are not allowed, and given the above properties of color, only specific color combinations of quarks and antiquarks can give you a color-neutral bound state:

(using q for quark and q* for antiquark...)
1) qq*, same color family for both
2) qqq or q*q*q*, all different colors
3) qq*qq*, various appropriate color combinations
4) qqqqq* or q*q*q*q*q, various appropriate color combinations
5) etc.

Type (1) composite particles (qq*) are called "mesons". Type (2), "baryons" (of which the proton is an example). Beyond that, there are no clever names: "multiquarks" generally; "tetra-", "penta-", and "hexaquarks" more specifically.

[Squink]

Pentaquark Discovery Confounds Skeptics (http://www.newscientist.com/news/news.jsp?id=ns99993903) (New Scientist)
Behold the Pentaquark (http://news.bbc.co.uk/2/hi/science/nature/3034754.stm) (BBC)

[bonzer]

Four quark exotics (the qq*qq* ones) are usually called baryonium in the literature, but they've never been experimentally observed.