How can pentaquarks and tetraquarks be colour neutral?

[Symbreak]

Has the discovery of four and five quark systems rendered the principle of 'colour neutrality' invalid?
i.e all the different colour charges of baryons and mesons must cancel out... but I don't see how this is possible in particles with four or five quarks.

 

[juvenal]

I haven't heard about the 4-quark systems. But the 5-quark systems (pentaquarks) don't violate color neutrality, and I don't think a 4-quark system would either. Look at it this way: a meson (2 quarks) is colorless, and so is a baryon (3 quarks). So obviously, 2+2, or 2+3 can still be colorless, since a combination of colorless systems is still colorless.

 

[dextercioby]

Yeah,antpiarticles have opposite color numbers to the particles (namely flavors of quarks)...

Daniel.

 

[humanino]

and by the way, not only 4 quarks irreductible systems have not been observed, but the pentaquark as well might not be confirmed. I am very biased, since the whole idea looks ugly to me from the beginning, and I do not know among you who is an expert about this stuff. Please just try to make sens out of those :

• None of the high energy experiments has seen any beginning of candidate for a pentaquark. Only a few intermediate-energy experiments have been able to display plots in which a very small spike gets out of the background.
• The alleged pentaquark should be very narrow : a few MeV wide. Note that this fact should make it a very long-lived system. But the discrepancies between the positions of the different "peaks" as observed by the different collaborations are larger than this width ! That means, people disagree on the mass of the pentaquark at a level which is 10 times the intrisic width of the particle. Yet, as you probably know, it is very easy to fit a gaussian so that one obtains a precision for the mean value of say one tenth of the gaussian width. These discrepancies are badly dissatisfactory.
• If the alleged particle existed, then according to QCD it should have two other isospin partners. None of the experiments has been able to display positive results concerning the two other pentaquarks. Worse : all the experiments, at high or intermediate energy, aiming to look at the partners obtained negative results (keeping in mind that they can only furnish upper bound for the production, none of them could ever say the production is strictly zero).

It appears to me that this is a theoretician playground. "Let's make wild hypothesis !". Except that Nature does not care how wild a theoretician can get. The pentaquarks have been suggested for decades by theoreticians without success. Now at some point, one of them is lucky enough to propose something that does indeed give results. But those results might as well be coincidental, due to some much more complicated mechanism (see for instance the so-called "kinematical reflections", which in that case appear not to solve the puzzle).

JLab should display results of dedicated experiments very soon at the APS meeting. Those results might unfortunately be very preliminary.