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Why is the top quark so massive?

  1. May 7, 2005 #1
    The energy equivalence of the mass of a proton is 938.27 MeV;
    that of a top quark is published as being 173800 MeV.
    I thought that quarks were contained within nucleons and anti-nucleons.
    Can anyone explain this phenomenon? Cheers, Jim
  2. jcsd
  3. May 7, 2005 #2


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    Nope.According to Heisenberg-Ivanenko hypothesis (confirmed experimentally),the only nucleons are the protons & neutrons.It was then the quark model that told us that these nucleons were made up of 3 quarks each,but only 2 flavors in total...So judging by your own belief,we should have only 2 flavors of quarks (and the corresponding antiquarks),but in fact we have discovered 6 and have a theory that allows 16 possible...

    I'll wait for the master in chromodynamics Marlon would care to explain why the top quark is so massive and why discovering of a 7-th flavor of quarks would require higher (maybe ~TeV) energies...

  4. May 8, 2005 #3

    Meir Achuz

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    The top quark is not in the nucleon. It is a constituent of much heavier particles.
    Explanation of the masses of all the quarks and leptons is so far unexplained in the Standard Model.
    lt is unlikely that more than six quarks will be found. It is known that there are only three neutrinos (lighter than M(Z_0/2)), and thus only six leptons. The number of quarks and leptons should be equal for several theoretical reasons.
  5. May 8, 2005 #4
    There are no top quarks in a proton. A proton is made of two up quarks and a down quark.
  6. May 8, 2005 #5
    Why is the top quark so massive ? Well, that's a good question but actually we don't have an uniform and widely accepted answer. At least not to my knowledge. Check out :

    The clue really is (well one possible way out i mean) to study the coupling of a top quark to all of the gauge bosons. I mean, we study the interaction of a top quark with for example W bosons (ie weak interaction).The coupling of the top quark to the W is particularly interesting because it induces essentially all decays of the top quark. This decay is in principle an ordinary V-A decay like, for example, muon decay. The main difference is that the top quark is so heavy that the W is produced on-shell, i.e. as a real particle. This has a number of consequences for the kinematics of the decay, distributions of decay products etc.

    Firther reading here : http://wwwth.mppmu.mpg.de/webdocs/eng/top/topulaer.html

    Besides, the fact that the hadron mass is BIGGER then the sum of the constituent quarks already has been discussed on this forum. Check out : https://www.physicsforums.com/showthread.php?t=66384

  7. May 8, 2005 #6


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    Marlon,he's right.A proton is a 3-quark barion made up of 2 quarks UP and one quark DOWN.


    EDIT:Wow,u deleted the post... :surprised :tongue2: :biggrin:
  8. May 8, 2005 #7
    yeah, i misread that post...My mistake...i deleted my remark

    ps : I am readinf stuff on the top quark mass being linked to the Higgs boson mass, though i don't exactly now what that actual link is ? The Higgs boson must be heavier then this top quark, other wise when must have already observed it
    Last edited: May 8, 2005
  9. May 8, 2005 #8
    I am inclined to a belief that the experimental set-up, is where the resulting difference appears from. The fact that the increase in acceleration of particles (higher energies) will yeild incorrect "momentum" data, because of the interaction times?

    As an example, if one was to bring two opposing matters (Proton and Anti-Proton) together at a low velocity, instead of the high accelerated particle "smashers", then the yeild product of proton to anti-protons, should define the proton mass, equivilent to that of the anti-proton, without the high-output "momentum" products being observable?

    There is also evidence that 'low-momentum' interactions produce truer values to ElectroMagnetic and Charge products, resulting from a Phase-Shift(which can be viewed as a 'Momentum-Shift' in some models) of Quark and Anti-Quark scattering without Proton Decay.

    The early thoughts have been developed from this:http://www-personal.umich.edu/~jcv/imb/imb.html

    to more recent:http://hep.bu.edu/~superk/pdk.html

    and of course the final recent experiments have concluded that the Penta-Quark does not exist!

    Nature or us?..have been firing Blanks :blushing:

    Last edited: May 8, 2005
  10. May 9, 2005 #9
    Okay Gentlepeople,
    If the top-quark is not a constiuent of a nucleon, what the heck is it doing in QM besides confusing a lot of would be physicists?
    Perhaps you folks can extend your "thought experiments" to explain why the Einstein equivalence of the "c"- quark of 1400 Mev, or that of the "b"-quark of 4400 MeV, or of the 4th generation "b'"-quark of 128000 Mev.
    Isn't it possible that these apparent SR mass enhancements are hiding the true rest mass of these "free quarks"? Cheers, Jim
  11. May 9, 2005 #10
    To Marlon: The Higgs mass depends on the Top mass because of radiative corrections. Since the Higgs couplings are proportional to mass the effect of top loops is enourmous and completely swamps any contributions from all the other fermions.
  12. May 9, 2005 #11


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    Neo, your remark goes down to Isaac Rabi, who asked "Who ordered that?" when the muon was confirmed. We do not know why the heck do we need any of the two upper generations, this is muon, tau, charm, strange, bottom and top. Besides, the masses of these particles have a puzzling distribution.
  13. May 10, 2005 #12


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    The joy of string theory and its cousins is that if you look at a standard model table of fundamental particles it really looks like there should be some sort of simple theory that should explain the relationship between the numbers but no one has found it.

    The very name "Standard Model" shows the ambivalence the field itself has about it. I strongly suspect that most physicists really believe that there is a deeper structure of some kind.
  14. May 16, 2005 #13
    When a proton emits an electron it becomes a nuetron. This seems to imply that an up quark is the equivalent of a down quark plus an electron. I assume that that this also implies that the mass of a proton is greater than that of a nuetron. Is it possible that a nuecleon can contain three up quarks or would it immediately self destruct? From the nature of the foregoing you can probably tell I am atomically naive.
  15. May 16, 2005 #14
    Got your facts mixed up man.

    Last edited by a moderator: Apr 21, 2017
  16. May 16, 2005 #15


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    Impossible.Check the conservation of ELECTRIC charge,which is really fundamental even in GUT and SUSY extensions.

  17. May 17, 2005 #16
    Hi rowan, and welcome to physicsforums!

    Perhaps you're thinking of neutron beta-decay, in which a neutron emits an electron to become a proton? (Although there's an antineutrino involved as well, and I don't know if "emit" is really the right word).

    This process has what looks like a nice explanation at http://particleadventure.org/particleadventure/frameless/npe.html, a wonderful resource for all would-be nuclear physicists:
    "One of the the down quarks is transformed into an up quark. Since the down quark has a charge of -1/3 and and the up quark has a charge of 2/3, it follows that this process is mediated by a virtual W- particle, which carries away a (-1) charge."

    "An electron and antineutrino emerge from the virtual W- boson."

    Don't ask me to explain how any of this happens.
    Last edited by a moderator: Apr 21, 2017
  18. May 17, 2005 #17
    The net charge of a free neutron is = ZERO.
    That of the products of the reaction are a neg. 1eV of the electron and a plus 1 eV of the proton, so that the net charge of the resultants is also = ZERO. Therefore, the charge is, in fact, conserved. Perhaps it is the GUT and the SUSY that have misinformed us QM folks. Its just a thought - don't you think? Cheers, Jim
    PS The ejected electron always carries off 0.782 MeV of exothermic energy that proves, quite adequately, the spontaneity of the reaction.
  19. May 17, 2005 #18
    What the heck are you talking about? The beta decay of a neutron is a 3-body decay, and thus the proton and electron do NOT have monochromatic energies. It wouldn't be 1eV anyhow.

    That is why the neutrino was postulated. Because experiments did not show a monchromatic energy spectrum.

  20. May 19, 2005 #19


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    Yea I never quite understood this line of reason. Someone refresh my memory.

    1) There is plenty of finetuning already in the Standard Model, it wouldn't be drastic if there was even more (High Higgs mass = lots of finetuning). The Higgs mass is unprotected by any classical symmetry, ergo we're in trouble a fortiori.

    2) Extensions of the Standard model could output GUT or SUSY scale particles that could in principle swamp the top quarks radiative corrections, so why aren't we talking about the Higgs mass relative to them?

    I wish I knew a little more about the Higgs sector, its still a bit of a blackbox in my knowledge of physics beyond classical field theory.
  21. May 19, 2005 #20
    Is there? There are lots of free parameters which are chosen to fit experiments, but they aren't fine tuned- changing the electron mass slightly would change lots of other parameters due to quantum effects, but it won't have any drastic effects on the structure of the theory. A Higgs mass >1Tev spoils unitarity, for example, so its value would require fine tuning if there is no other mechanism to keep it under control.

    Which is one reason a lot of theorists don't like the minimal SM Higgs.

    I was only refering to the SM. You're right, when calculating quantum corrections in SUSY you have to include stop loops as well.
  22. May 19, 2005 #21


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    Well you are right, other than the smallness of the neutrino mass relative to the electron mass (it acquires a Higgs Vev at electroweak symmetry breaking scales, so naturally you would want the ratio to be of order ~ 1) I can think of no other unnatural finetuning in the minimal standard model.

    I am interested however in why it breaks unitarity bounds (partial wave unitarity I take it) at >1tev. I have never seen that calculation done before.
  23. May 19, 2005 #22
    There are no neutrino masses to fine-tune in the SM, technically... ;) But yes, neutrino masses need fine-tuning too. Let's hear it for the See-Saw mechanism!

    I'd never actually seen the unitarity calculation myself until your post inspired me to look! You're right, it's a partial wave analysis. This paper gives a pretty good exposition.

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