# Quark colours?

1. Nov 30, 2004

### davidmerritt

Hello, trying to get my head around the (MANY) particles I'm learning about at the moment.

First question is about quark colours. I understand the different types of quark (up , down, strange, charmed, bottom and top) but I dont understand how the colours work, why is it necesary for the colours to neutralise, can someone outline its relevence to the exclusion principle in their answer too (please).

Second question is a really easy one I guess. I've seen that you can classify certain certain mediator particles in terms of their interaction. So a gloun is the mediator for a short range strong attraction, a gravition for long range gravitational interaction. But what are the purposes of leptons and hadrons etc.. are they just formed after collisions or are they too force mediators

Another question is it the pion that mediates nuclear force or muons?

Theres so god damn many of them! HELP APPRICIATED!

David

2. Nov 30, 2004

### dextercioby

I had written a big answer post,but for technical reasons which come from me being an idiot,it was lost,so now i can tell u only that in Yukawa's theory(1935) the neutral pion field mediates the interraction between fermionic fields which are taken as the protons.
The so-called Yukawa coupling involves a matrix which is usually selected as the Gamma 5 to compensate the fact that the field describing the boson pi 0 is a pseudoscalar.

Daniel.

3. Dec 1, 2004

### Major

Hi

i will try to answer, without entering into details (i will let detailled field theory explanation for someone else... i don't have the time now... sorry ... :p )

The color : The nature of a quark is called flavour. there are 6 different flavours (the ones you have cited). Each quark have also a quantum number of color. There are 3 different color : red, green and blue. In the Nature, the matter is colorless.

So, you cannot compose quarks as you wish to form hadrons (hadrons are particles composed by quarks). There are two types of hadrons, the baryons (composed by three quarks, one of each color, so the total is colorless) and the mesons (composed by one quark and one antiquark, the anitquark having the anticolor associated to the color of the quark, so the result is colorless).

Why do we need to have color ??? I will explain with baryons (composed of three quarks). Take the delta++ particle (3 u quarks) or the omega- (3 s quarks). The spin is 3/2. So you have three quarks of the same flavour with a spin projection equal to +1/2 and their orbital momentum is 0. So we have a symmetric wave function, and that's violating Pauli's principle. A solution is to introduce an additionnal quantum number : the color.

The color part of the wave function will be antisymmetric (so now, Pauli is ok) ==> And each quark will have a different color.

We imposed that the hadronic states are invariant under a color transformation, so we need color singlets. (i don't remember the real reason, sorry, maybe someone else will complete).

Quarks and leptons : they are simply needed to form all the matter of the universe. We need both to form atoms... (in fact i don't think i have really understood the question... :p )

Nuclear force : I will just add a word to dextercioby's answer : the charged pions can be used to modelise the interacitons between a proton and a neutron. There are also models based on 2 pions exchanges to modelize the baryon-baryon interaction.

Cya

Major

Last edited: Dec 1, 2004
4. Dec 1, 2004

### davidmerritt

Cheers for the help , I've been digging out some elementary particle physics notes. Particle physics is a very interesting topic, though the level of detail is higher than other topics in physics in my opinion. The main textbook I use (University Physics with Modern Physics by Young and Freedman, 11ed, which is generally excellent) only includes a brief chapter on particle physics and obviously they've tried to cram a little too much in their making it more use as a dictionary of particles, with only limited explainations of the role of quark properties and colours (to be fair its only aimed at 1st/2nd level students).

(if anyone else is interested the notes for the 3rd year course EPP are available for download from the University of Nottingham in the UK at www.nott.ac.uk/physics[/URL] and click on the current students --> module pages --> 3rd year they are very well written)

Last edited by a moderator: Apr 21, 2017
5. Dec 1, 2004

### davidmerritt

The colour can be seen as a type of charge it seems, with chronodynamics explaining the forces between quarks of different colours. Hence the colours and neutrality are important to explain how quarks interact, and hence how gluons and quarks interact.

Indeed Leptons are fundamental particles and in a way can act as force mediators. As leptons are charged they are involved in electromagnetic interaction.

I think thats correct... but if you know different please post!

6. Dec 1, 2004

### dextercioby

1.The colors of the quarks and gluons are red,yellow,blue.By mixing two colors (actually two different color quark fields) one gets the color of the antiquark.Antigluons are identical to gluons and hence have the same color.The analogy with optics is not random,since by combining all three colors,the result is "white",i.e.color number zero.The colors of the antquarks can be:green,indigo,orange.
2.The leptons are semiinteger spinor massive/massless fields which bear the name "matter fields".They may never intermediate a fundamental interaction,at least not in the Standard Model.That's the job for "vector bosons"/"gauge bosons",which are integer spin vector fields either massless (the photon,the gluon) or massive (the electroweak bosons:W plus,minus ans Z).Quarks are massive semiinteger spinor fields.Those are the bricks of the SM.The rest is merely speculation.So far...