What are quarks?

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Online chris

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What are quarks?
« on: 23/03/2012 07:51:53 »
In relation to an atom, what is a quark?
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Offline imatfaal

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Re: What are quarks?
« Reply #1 on: 23/03/2012 12:12:58 »
Atoms are composed of 3 particles -
protons (+ve charge - simplistically the number of protons determines which element)
neutrons (no charge, hydrogen doesn't have one, about same mass as proton)
electrons (-ve charge but of same magnitude as proton, same number as proton in atom, small in mass compared to proton and neutrons)

The protons and neutrons hold together (via the strong nuclear force) in the nucleus - the electron are dispersed to a greater or less extent around the nucleus.  Electrons are thought to be fundamental - that is to say that we do not believe they can be broken down to simpler constituents.  This used to be thought about protons and neutrons.

It is now known that protons and neutrons are not fundemental - they are in fact constructed from quarks; three quarks make up the proton and neutron.

There are 6 quarks that can be viewed as 3 pairs (generations) or alternatively as two families of three:
The 1st generation quarks are called Up and Down - ignore the meaning of the name, they are just names not descriptions! 
The Up and Down are the lightest ( and thus stablest of the quarks) Up has a +2/3 charge, and down has a -1/3 charge.  If we have two Ups and a Down - we have 2/3 + 2/3 - 1/3 = +1  ie a proton.  If we have and Up and two Downs  - we have 2/3 -1/3 -1/3 = 0 ie a neutron.

The 2nd and 3rd Generations also have one quark with +2/3 charge and one with -1/3.  These are the Charm (+2/3) and the Strange (-1/3) - and the 3rd generation Top (+2/3) and Bottom (-1/3).  They are very heavy, unstable and rare - in very broad and dodgy terms they tend to give out some of their mass as energy and become something(s) lighter and more stable.

Quarks (and electrons) are what are called Fermions - these are the particle of matter, they are fundamental, they obey very special laws, and have in that strange concept called Spin (the name is  a bit misleading - but does have to do with angular momentum) they are spin 1/2.  Quarks also have a slightly bonkers property called colour (again ignore the name - it's just a name).  The combination and manipulation of colour and flavour is encompassed in Quantum Chromodynamics - this study explains how the strong interaction which holds quarks together to make protons and neutrons AND protons and neutrons together to make nuclei works.  QCD works on the transference of messenger virtual particles (gauge bosons) called gluons (this is very similar to Quantum Electrodynamics which understands electromagnetism by the transfer of virtual gauge bosons called photons).

in summary quarks are the building block of the building blocks  - they are as low down the ladder of the standard model of particle physics that we can go (at the moment!)

wikipedia has a very good introfduction here  as does Hyperphysics here
 
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Offline Soul Surfer

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Re: What are quarks?
« Reply #2 on: 23/03/2012 12:35:22 »
A good description but there is an important detail missing.    The quarks and gluons are never seen on their own and all these properties are measured by their interaction to produce other particles that can be seen on their own and measured.  The reason for this is that the attractive force between quarks actually increases with separation and knocking one of the quarks out of a proton requires so much energy that it creates more quarks which bind together in particle anti particle pairs to form a meson.
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Offline imatfaal

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Re: What are quarks?
« Reply #3 on: 23/03/2012 13:09:28 »
I was desperately trying to cover everything at a simple level but forgot colour confinement - good catch
Thereís no sense in being precise when you donít even know what youíre talking about.  John Von Neumann

At the surface, we may appear as intellects, helpful people, friendly staff or protectors of the interwebs. Deep down inside, we're all trolls. CaptainPanic @ sf.n

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Offline graham.d

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Re: What are quarks?
« Reply #4 on: 23/03/2012 14:08:56 »
The whole explanation is very good. Well done you guys. It's often hard to express all the salient points of a subject concisely and in simple terms like that.

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Offline syhprum

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Re: What are quarks?
« Reply #5 on: 23/03/2012 19:31:47 »
One interesting point the masses of the constituent quarks in a nucleus is very much smaller than the masses of the protons and nuetrons that they form the difference is due to the energy of the gluons.
syhprum

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Offline Bill S

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Re: What are quarks?
« Reply #6 on: 29/03/2012 19:51:03 »
It's well worth following threads on NSF for explanations like that.  Thanks folks, keep it up and I might eventually understand some physics.  :)