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There are said to be eight gluons

  1. Mar 30, 2005 #1
    There are said to be eight gluons, and I understand the reasoning, but I cannot determine for certain if this is a purely theoretical position or is it possible to observe gluons by experiment?
    My understanding is that the machines needed to separate and observe gluons are still under construction.
     
  2. jcsd
  3. Mar 31, 2005 #2
    There will never be any machine that can observe a single gluon for a simple reason : gluons are never "white" and only "white" particles can be observed in free state. By "white" I mean "invariant under color rotation".

    This is the postulate of confinement. It is not proven yet, but has never failed to be true.

    The way we "prove" gluons exist is thus inderect. You probably know the expression "bremsstrahlung". It usually refers to the loss of energy due to photon emission by an accelerated electromagnetic charge. There is a very similar phenomena in QCD, which is sometimes also called bremsstrahlung : only in this case, the charge is color and it is a gluon that is emmited. Bremsstrahlung is very friendly, because it can be computed to a high degree of accuracy using standard perturbation technics. The complication here, is that one cannot observe the gluon. The emmited gluon will hadronize, forming a jet of hadronic particles. By identifying such events of hadronization of gluon emmited in bremsstrahlung, one can easily reconstruct the energy and momentum of the initial gluon. And it occurs that this is in perfect agreement with the perturbative calculation of gluon-bremsstrahlung.
     
  4. Mar 31, 2005 #3
    very good question

    https://www.physicsforums.com/journal.php?s=&action=view&journalid=13790&perpage=10&page=5

    scroll down to the 'why eight gluons' entry

    regards
    marlon
     
  5. Apr 1, 2005 #4
    humanino

    Thanks for your reply, it is just what I expected.
    As far as I have discovered these findings are based on experiments using baryons. This means the gluons produced by up and down quarks are experimentally proven. Perhaps other experiments I am not aware of used the strange quark, but I would be suprised if there was experimental proof using top and bottom quarks.
    My point being that perhaps two or three gluons (four at the most) are found experimentally, the rest are purely theoretical. Can you confirm that this is the case?

    marlon

    Thanks for your reply, I understand why there are six colours, my difficulty is in getting a clear understanding of why there are eight gluons. This comes down to knowing which gluon have been experimentally proven and which are theoretically derived from the finding of experimentally proven gluons.
     
  6. Apr 1, 2005 #5

    jtbell

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    Staff: Mentor

    Do a Google search on "gluon jets" (with the quotes probably works better). Not quite the same as a "direct" observation (whatever "direct" means in this context), but pretty good evidence nevertheless.
     
  7. Apr 2, 2005 #6

    jtbell

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    Staff: Mentor

    OK, I should have read humanino's posting before firing off mine. Nevertheless, I think I can still contribute something...

    The different kinds of gluons are related to the color charge (red, green, blue) of the quarks, not their flavor (up, down, strange, charm, top, bottom). Each flavor of quark occurs in all three colors.
     
  8. Apr 2, 2005 #7
    Well that is exactly what is explained in the link that i gave you.

    marlon
     
  9. Apr 3, 2005 #8
    Thanks, I think I have enough to start making progress again. Its always easy for those who know the answers!

    elas
     
  10. Apr 4, 2005 #9
    If the wavefunction of some gluon is completely white, then there is no colour and no interaction can go on. Basically this state is a singulet which means that it can never change (invariant under colour-rotations). Now for a gluonstate to be white there is ONE condition: the particle cannot have any preference for any colour. This means that the gluon must be red-antired, green-antigreen, AND blue-antiblue.

    Now incorporating some normalization-constants we have that : the
    white gluon is (red-antired + blue-antiblue + green-antigreen)/sqrt(3).

    For example there are two kinds of wavefunctions that are not actually white: (red-antired -green-antigreen)/sqrt(6)
    and (red-antired + green-antigreen -2*blue-antiblue)/sqrt(6).

    These two gluons can interact without changing
    the color of a quark, but they are not completely white.Indeed, colourcharges interact via the exchange of colour. So the singlet state cannot interact because it cannot change its colours. that's why it is a singlet !!!Now, a red-antired gluon is indeed white BUT NO SINGLET because the colour can be changed. Indeed the total colour is white but so is the total colour of a blue-antiblue gluon so it can change into that for example. You see the difference between being white and being TRULY white ???That is the main point

    An analogous thing happens in the quantum information theory. Suppose you have a wavefunction that is a superposition of spin up and down. Suppose that the probability for measuring the spins along some axis is 1/2 then you really know nothing at all do you ???This same thing happens with the TRULY white wavefunction. For this reason all combinations that yield white must be included

    marlon
     
  11. Apr 4, 2005 #10
    Up to today, we are not doing so bad up to bottom mass !

    HEAVY-QUARK FRAGMENTATION IN ee ANNIHILATION

    Experimental review of b & c Hadronization

    You will have understood that the situation is far from simple, and wether for charm or bottom quarks, we still have progress to fully understand what is going on. I am not aware on data about top quark.

    Yet, I would have to point a subtelty : we cannot make differences between the eight gluons. We do never know of which kind of gluon was radiated. Color is not observable.


    There are also studies concerning the quark-gluon plasma :
    Gluon Radiation and Heavy Quark Radiative Energy Loss in Quark Gluon Plasma

    Lecture notes on evidences for gluons


    P.S.: I know of an excellent review :
    Historical and futuristic perturbative and non-perturbative aspects of QCD jets physics
    by the eminent profesor Dokschitzer.
     
  12. Apr 4, 2005 #11
    Once again my thanks to marlin and humanino. Now I have a clear picture.

    elas
     
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