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Quark Confinement and the discovery of quarks

  1. Nov 10, 2012 #1
    Okay, so I've just begun to have a grasp on the concepts of quantum chromodynamics. And what amazes me is that quarks never actually exist on their own as a single particle because of the strong interaction between them. I've read that when you try to pull apart a pair of quarks that is bonded together by a gluon, they act like a rubber band, and when they got into this "pulled limit", instead of breaking them apart, another pair of quarks is produced, and the old pair of quarks remain confined.

    1. I would appreciate if someone could explain and discuss more on this awesome process/phenomena. I just think this is pretty interesting.

    2. Do physicists work on trying to get around this quark confinement, in an attempt to find a way to break them apart?

    3. So if quarks remain confined as composite particles (hadrons), how did particle physicists experimentally observed, discovered, and identified each type of quarks (up, down, top, bottom, strange, and charm) as a single particle? It's like they "poked" into a hadron and observed that there are those quarks inside, and identified there different types. Or something like that.

    Thank you very in advance. I apologize if I said anything wrong in the post, being a beginner.
  2. jcsd
  3. Nov 10, 2012 #2


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    "Deep inelastic scattering" is what you're looking for.

  4. Nov 10, 2012 #3


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    You might be interested in Quark-gluon plasmas. The quarks are not free in the conventional way, but they are not bound in hadrons.

    Heavy quarks (charm, bottom) dominate the properties of the hadron they are in, so you can study the quarks via their hadrons. The top-quark even decays before it forms hadrons.
  5. Jan 4, 2013 #4
    Sorry for the late reply. I've been unable to log on to the forum for a while. Thanks for this! But can you provide some more explanation, in simple and layman's terms if possible? I've manage to read about it very briefly, but it's really hard to understand some of the technical terms as I still don't have even an undergraduate physics background. If it's no trouble, I would like to understand more of the process of the scattering. Thank you!
  6. Jan 4, 2013 #5
    Sorry for the late reply. I've been unable to log on to the forum for a while. Thank you very much. I've heard and read about quark-gluon plasma before, especially that it is actually considered as a new type of matter phase, is this right? More than that, I don't know anything else about it and that I'm not sure what it is - in terms of its properties and etc.

    How are the quarks "free" or not bound in hadrons in Quark-gluon plasmas? And what is exactly happening with the quarks and gluons inside these "plasmas"?
  7. Jan 5, 2013 #6
    In Deep Inelastic Scattering you basically 'fire' beams of electrons or neutrinos at targets containing nucleons and observe the scattering angles and outgoing energies of the particles that have interacted. From these observations some of the properties of the particles that scattered them can be deduced via appropriate mathematics.

    Electrons are assumed to interact by exchanging a virtual photon with a charged constituent of the nucleon - the term 'parton' is sometimes used for the latter. Neutrinos interact by the weak force (charged current), ie they exchange a virtual W boson with the target and in doing so become charged leptons which can then be easily detected.

    Repeating these experiments with different targets (ie different proportions of protons to neutrons) and comparing the resulting outgoing energy and scattering angle distributions allow the differences between these nucleons to be studied.

    It is found that, at lower energies, the beam particles behave as if scattered by the nucleon as a whole (Rutherford scattering) but at energies where the beam particle wavelength gets below the nucleon radius new behaviour emerges. This is consistent with the beam particles having been scattered by smaller individual partons. Assuming the proton to have two partons with charge +2/3 and one with -1/3, and the neutron to have one with +2/3 and two -1/3 fits very well with the observations.

    The maths of the interaction dynamics also allows the average proportion of the nucleon momentum carried by the 'struck' partons to be estimated, and the combined momenta of these charged partons is found to add up to only about 50% of that of the nucleon. The remainder therefore appears to be carried by the gluons that mediate the strong interactions between the charged partons (quarks).

    The other experiments we can do that involve quarks directly are high-energy particle/antiparticle collisions or similar, eg e+e-. The incoming particles annihilate into a virtual photon or Z boson which can then pair-produce a quark and an antiquark. The latter rapidly hadronise as described in an earlier post (unless the quark/antiquark are tops, in which case they decay first). By comparing the rate of production of hadrons to that of (eg) μ+μ- the charges of the different quark types can be further corroborated. In addition, resonance peaks (of higher than otherwise expected proportions of hadrons produced) are observed at specific energies. This is actually how many of the heavier mesons, and in particular those containing charm and bottom quarks, were discovered.

    A book I'd strongly recommend is 'The ideas of Particle Physics' by Coughlan, Dodd and Gripaios. This gives a great phenomological description of fundamental particles, interactions and experiments without going into too much maths - it assumes some knowledge of the relevant school maths and physics - but without dropping into the kind of woolly narratives often found in popular science books.
  8. Jan 5, 2013 #7


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    Depends on the definition of "matter phase", but in general... right.

    Compare it with a regular plasma and gas: In a gas, every molecule has some nuclei and electrons, bound to those nuclei. If you heat it up (and/or increase its density), electrons become unbound, and you have a plasma where electrons and nuclei (even those bound in molecules before) all move independently. They interact, but they are not bound any more.
  9. Jan 16, 2013 #8
    I see. But what exactly are "scattering angles"? Sorry for my ignorance. :)

    So it is by the values and numbers from the resulted data, that each value correspond to a specific property of a particle (i.e. spin, momentum, charge, etc?), that we can identify and deduce specific and unique particles, in particular on this context, specific quarks?

    So after the interaction with the target, when these outgoing neutrinos are detected with a certain charge, it then indicates that the neutrino have interacted with a certain specific particle? Did I understood this right?

    I see. By different proportions of protons to neutrons, you mean isotopes/nuclides? So really, it is by data/values of the results of the experiments that we are able to distinguish, differentiate, and identify different particles and their properties so that we could discover and study them, not by physically "seeing" them separately?

    What kind of new behavior is that? How is the interaction look like?

    Is the math the framework of Quantum Chromodynamics?

    Wow cool. I find that very interesting. How did they conclude that it was the Gluons that were carrying those remainding momenta? Was that how, in a way, gluons was discovered/observed experimentally?

    Ooooh yes I'm familiar with the basic electron-positron annihilation Feynman graph. I almost forgot they ultimately result to produce quark-antiquark pair. The flavor/the kind of quarks produce in the pair is random? And also, I don't get it in the first place, why and how a photon would "decay" and produce an quark-antiquark pair? And when the pair produce are tops, as you said would decay first before they could form into a hadron, would they decay still confined to each other? (I would like to clear out that quark confinement does not necessarily mean they are hadronized?)

    Ohh okay okay. This helps a lot! I've notice though that it's Leptons, like the e+ e- and μ+ μ- you mentioned above, that we use in high energy collisions to study quarks, hadrons, nucleons, and the strong interaction.

    Oh that's why we get more new particles discovered as we use more energies in particle accelerators and experiments?

    I'll check it out next time if they have it on our local stores. :) Thank you very much your post has been helpful! But I must admit, my working knowledge of physics needs more work, as it takes a lot of effort for me to really grasp them all. Haha I don't think I really fully still understand and appreciate the concepts and processes. I think visualizing the concepts can really help, but it's still hard for me to picture and imagine these particular ones. Plus I need to familiarize myself with the terms.

    Thank you and apologize if I may have asked or stated any wrong statements.
    Last edited: Jan 17, 2013
  10. Jan 17, 2013 #9
    Haha are there other definitions of "matter phase", besides the usual definition that we know of as the "solid-liquid-gas-plasma" phases of matter?

    Ooooh I see. Is this why atoms in this environment/conditions (i.e. hot dense plasmas) become ions because those electrons inside the atoms are "released" become "unbound" and "independent"?

    How do they interact, in what way(s)? It's hard for me to picture/visualize it.

    And if all those electrons and nuclei that make up the atoms and molecules become "unbound", wouldn't that "destroy" the chemical elements/materials that make up the gas/plasma altogether? Because all that gas/plasma, as matter, is composed of an element, like say Hydrogen in the core of the sun, right? And that Hydrogen molecules are composed of atoms, which then are composed of electrons and nuclei. Did I understand this correctly?

    Sorry for my ignorance and I thank you for your patience.
  11. Jan 17, 2013 #10


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    Hit a particle with a beam, observe the angles between scattering products and your incoming beam.

    Usually, you cannot do anything with a single collision - you need many collisions, and statistical methods to analyze them. The angular distribution can tell you something about the spin of particles, for example.

    AdrianTheRock described two different scattering processes here: Electron+nucleon to anything and neutrino + nucleon to anything. Neutrinos as scattering products are nearly impossible to detect, as their interaction is so weak. And they are uncharged.

    If a neutrino hits a proton and you get a neutron plus a positron afterwards (those can be detected), you know that the incoming particle was an anti-electronneutrino. In addition, it confirms that you can transform a proton into a neutron if one up-type quark (here: an up-quark) is converted to a down-type-quark (here: a down-quark).

    Correct. There are events where you can be quite sure that particle X was there, but this is never 100% certainty.

    [quoteWhat kind of new behavior is that? How is the interaction look like? [/quote]
    That is scattering at the individual components of protons - quarks or gluons.

  12. Jan 25, 2013 #11
    Go to Frank Wilczek's homepage - He oughta' know about this stuff! - and find his paper titled, "QCD Made Simple".

    Lotsa' nice info!

  13. Jan 27, 2013 #12
    Looking back at this again, a couple of quick follow-ups:
    The proton will not necessarily just be transformed into a neutron. At high energies a shower of mesons is likely to emerge; the one remaining baryon may be either a neutron or a proton (eg if one more [itex]\pi^-[/itex] than [itex]\pi^+[/itex]s is produced). But in these experiments the actual makeup of the hadronic debris is not observed.
    The maths I alluded to previously is just the kinematics of scattering processes. QCD wasn't known at the time these experiments were first conducted, and indeed the experiments provided a lot of useful information on which the forumlation of QCD was then based.

    Knowing the incoming beam energy and the energies and scattering angles of the outgoing particles, we can then calculate parameters such as [itex]Q^2[/itex], the square of the 4-momentum transferred from the incoming particle to the nucleon, and [itex]\nu[/itex], the energy transferred to the target in the lab frame. A further, dimensionless, quantity of considerable import is
    [tex]x = \frac{Q^2}{2\nu M_N}[/tex] This turns out to be the proportion of the nucleon momentum carried by the parton 'struck' by the incoming beam particle.

    The fact that basic scattering behaviour re-emerges at very high energies gave key evidence for 'asymptotic freedom' in QCD. Essentially, at these energies the quarks behave approximately as free particles, so for the actual scattering interactions we don't have to employ any model of the strong force. The nucleons can be modelled using a set of 'structure functions' [itex]F_i(x)[/itex] which describe the charge densities of partons carrying [itex]x[/itex] proportion of their momenta.
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