smaller than subatomic particles

Discussion in 'Physics & Math' started by Lexi, Jan 30, 2010.

  1. Lexi Registered Member

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    are there such things? if so, are there smaller particles than those too? is there no end to the fragmentation that physical particles can undergo?
     
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  3. Read-Only Valued Senior Member

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    I can see why you might think that way. But just like Absolute Zero temperature, there is a "bottom" to everything.
     
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  5. Creeping Death Out of darkness came light Registered Senior Member

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    The hypothesis is infinity!
     
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  7. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Lexi.
    This is a good question, and you may be surprised how many answers it gets.
    But it is physics not chemistry.
     
  8. James R Just this guy, you know? Staff Member

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    Neutrons and protons and electrons are all subatomic particles, since atoms are made of those.

    While electrons have no substructure that we are aware of, neutrons and protons are composed of "smaller" particles called quarks.

    It is worth mentioning, however, that at such small scales, the concept of the "size" of a particle itself becomes fuzzy. At these kinds of scales particles often act like waves rather than like little billiard balls, and waves are always spread out to some extent in space.
     
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  9. Fraggle Rocker Staff Member

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    I thought that in this model, all of the subatomic particles including electrons, neutrinos and whatever else there is, are composed of bosons or something like that?
     
  10. kurros Registered Senior Member

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    Well, the fundamental particles can be classified by their spin into two groups, fermions and bosons. They aren't composed of these though, they are them (please forgive my horrible overuse of they/them/these in that sentance

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    ). The fermions are the matter-like particles; electrons, quarks, neutrinos etc, while the bosons are the force-carrying particles (photons,gluons, W/Z bosons)
     
  11. James R Just this guy, you know? Staff Member

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    No. The fundamental building blocks of all particles appear to be three "families" of particles, each of which contains a pair of quarks, another fundamental particle and a neutrino.

    For example, one of those families (the most familiar one), consists of the up quark, the down quark, the electron and the electron neutrino. All 4 of these particles are considered fundamental in the standard model. Protons and neutrons are each made of a (different) combination of 3 up and down quarks.
     
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  12. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    I think that this discussion is limited by the definition of elementary particles.
    ie, particles which do not seem to consist of anything more basic, according to current theory.

    This is the "periodic table" of fundamental particles. There are only 16 of them.
    6 Quarks (blue), 6 Leptons (green), and 4 Bosons (red).

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    Graphic From Wiki

    They are also grouped into 3 generations.
    Can anyone explain to me what the "generations" are?
    Generation to me indicates succession. Does one generation produce the next?
    And what does it mean that electrons, electron neutrinos, and up and down quarks are all generation I?
    What characteristics do they share?
    And why aren't Bosons a generation?

    I could ask lots more questions, but I'll stop there

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    Last edited: Feb 2, 2010
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  13. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    The generations are grouped by mass and interaction. This is a bit technical, so ask questions, please!

    The generations are grouped by their interactions with W bosons. So, a W boson can decay into an electron and a neutrino, a muon and a muon neutrino, or a tau and a tau neutrino all with equal probability (about 11% of the time, or so).

    The story with quarks is a bit more subtle, as the quarks can mix with each other. (Actually, the leptons can mix with each other, too, but that's another story.)

    By interacting with a W boson, an up quark may change into a down quark, a strange quark, or a bottom quark. The process of an up quark going to a bottom quark is VERY rare (though not impossible), while the process of an up quark going to a strange quark is only kind of rare (if that makes any sense). Most of the time (I'd say 97% or so) an up quark turns into a down quark---this can turn (for example) a proton into a neutron in the nucleus and lead to the emission of a beta particle (nuclear beta decay).

    As far as characteristics, the easiest one to explain is mass: the up and down quark are both about 5 MeV (their masses are NOT the same, and this is very important---it makes the proton lighter than the neutron, and means (basically) that life can exist!). The charm quark is about 1000 MeV, and the bottom quark is about 100 MeV. The top quark is around 175 GeV and the bottom quark is around 5 GeV. (1 MeV is like 10^{-33} grams, and 1 GeV is 1000 MeV, for comparison). The electron is about 1/2 MeV, the muon is about 100 MeV, and the tau lepton is about 2 GeV. The neutrinos are all essentially massless.

    Again, this is a bit complicated, so ask questions if you don't understand something I said. I've been doing this over breakfast, so I may have missed something.
     
    Last edited: Feb 2, 2010
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  14. Walter L. Wagner Cosmic Truth Seeker Valued Senior Member

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    Ben:

    Recent work suggests that the neutrino has a very small mass.

    Also, the chart above shows the mass of the up quark as 2.4 MeV and the down quark as 4.8 Mev; I believe the chart is in error? What values do you have for the masses of those two quarks.
     
  15. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Sure---I didn't suggest otherwise. For completeness I should have talked about neutrino/lepton mixing in the same light as I talked about quark mixing, but that's another can of worms.

    I dunno. Check here: http://pdg.lbl.gov/

    The light quark masses aren't known so well---because QCD is strongly coupled, you can't observe single quarks. They're both about 5 MeV, and the up quark is lighter. What you really want to know is the mass parameters of the quarks in the lagrangian, but this (of course) is scheme dependent. Anyway, the question of light quark masses is definitely not experimental: quarks are the wrong degrees of freedom to describe the system below 300 MeV. You should rather talk about mesons and baryons and chiral effective theory.
     
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  16. Walter L. Wagner Cosmic Truth Seeker Valued Senior Member

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    Well, you said they were all "massless" ("The neutrinos are all essentially massless."); I guess your language was a little loose, being early in the AM and all, which I completely understand as that happens with me too, at times.
     
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  17. AlphaNumeric Fully ionized Moderator

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    It's clear he didn't say they were massless, but that they are essentially massless. That's a common style of speaking which means "Though its not precisely true its close enough for most purposes". The neutrinos can be regarded as massless in a lot of electroweak theory as you aren't interested in their mixing. It's only when you do precision neutrino experiments that you need to include their mass.
     
  18. rpenner Fully Wired Staff Member

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    The masses of the u and d quarks are not directly or even straightforwardly indirectly measured. What is measured is the breaking of SU(3) by the more massive s quark, which leads to an estimate of s quark mass and some rough bounds on effective masses of u and d quarks at high energy when then QCD interaction is weak. A typical energy scale for this asymptotic freedom is 2000 MeV, whereby one can come to estimates of
    u at 1.5 to 3.3 MeV and
    d at 3.5 to 6.0 MeV

    The actual details of how we get these mass estimates has been refined from a 1977 paper by Weinberg. Regretfully, as incomplete as our understanding of QCD is, our ability to put a limit on quark mass ratios is one of the strongest tools we have to estimate masses. As such, the errors in the estimates of quark masses are not independent.

    Advances in modeling QCD may be that someday we can calculate hadron masses ab initio with such precision where we might read off the quark masses to 10% accuracy from the measurement of proton and neutron masses alone. But that is not the situation today.

    http://pdg.lbl.gov/2009/tables/rpp2009-sum-quarks.pdf
    http://pdg.lbl.gov/2009/reviews/rpp2009-rev-quark-masses.pdf (Especially figure 2)
    http://books.google.com/books?id=YYulAXywIM4C&pg=PA267&lpg=PA267#v=onepage&q=&f=false (Reprint)
    http://pdg.lbl.gov/2009/reviews/rpp2009-rev-qcd.pdf
     
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  19. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Regarding the Generations. I've looked it up myself.


    Generation I is what is generally found in nature, , so for planet Earth it is a very simple "Periodic table", consisting of only eight elements.
    There are 4 Generation I particles, 2 Leptons and 2 Quarks. Plus there are 4 Force particles, the Bosons.
    The fundamental particles in Generations II and III are exited particles found only in cosmic rays and accelerators.

    The word Generation does imply a relationship, as the higher generations decay into the first generation.

    Going from Generation I to III, there is increased mass.


    I won't ask lots of questions this time, but why is the weak force W, so different in mass to the other forces?
     
    Last edited: Feb 3, 2010
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  20. Walter L. Wagner Cosmic Truth Seeker Valued Senior Member

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    I believe rpenner's statement regarding the masses of the up and down quarks is the best:

    "The masses of the u and d quarks are not directly or even straightforwardly indirectly measured. What is measured is the breaking of SU(3) by the more massive s quark, which leads to an estimate of s quark mass and some rough bounds on effective masses of u and d quarks at high energy when then QCD interaction is weak. A typical energy scale for this asymptotic freedom is 2000MeV, whereby one can come to estimates of
    u at 1.5 to 3.3 MeV and
    d at 3.5 to 6.0 MeV"

    But how do we reconcile that statement with the chart which shows:

    u at 2.4 MeV
    d at 4.8 Mev

    That chart implies that the quark masses are fairly precisely known, or at least to two decimal points. Error bars are not given, though it appears that the values are the mid-range of the estimates cited by rpenner. Is that just the author's assumption that he should use the mid-ranges as the actual mass?

    So, how do we reconcile the above with Ben The Man's statement:

    "As far as characteristics, the easiest one to explain is mass: the up and down quark are both about 5 MeV (their masses are NOT the same ..."

    So, it appears that we truly don't know the masses of the up and down quarks with any degree of precision, and only have broad ranges as noted by rpenner.

    Or does someone else have a 'take' on this?
     
    Last edited: Feb 3, 2010
  21. AlphaNumeric Fully ionized Moderator

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    6,697
    Walter, you seem surprised about a fact which is known to anyone who spends any time doing Standard model related quantum field theory. Infact, anyone familiar with any field theory which involves strong coupling will have come across systems which 'blur' together.

    The top quark mass is easy to measure, provided you have a big enough collidor to make them easily. Its so huge it decays via electroweak processes before it can form hardonic bound states and thus QCD processes have very little relevance to top quark physics. Thus the mass can easily be extracted from top quark processes. Up and down quarks are sufficiently light to be made in abundance but despite having known about their bound states for about a century we don't know their masses because you have to do strongly coupled gauge theory to work out how much of a proton's mass is due to gluons and how much is due to quarks. Even making the statement mean something concrete is the stuff of many a textbook on QCD.

    It arises in string theory too. At weak coupling, ie strings don't interact very much, you can describe physical systems as branes and strings. An oscillation in the brane can be viewed as a string stuck to the brane. Turn up the string coupling (via an S duality transformations) and you have a system which is not described via branes and strings. String theory only involves strings when interactions are weak! Just like QCD only involves quarks when energies are high.

    Using Lattice QCD and chiral perturbation theory its possible to extrapolate down to quark masses but Lattice methods aren't powerful enough yet to not require chiral perturbation methods.

    Once again Walter you show just how 'in over your head' you are when it comes to even basic qualitative knowledge expected of the people you're trying to take to court.

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    Don't you perhaps think you've made an arse of yourself?
     
  22. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Well, the first question is: does that REALLY matter?

    Again, if you're asking for the masses of the light quarks, I'd say you're asking the wrong question. The light quarks can never be measured by themselves, they always come in mesons and baryons because of the strong force. The light quarks are simply the wrong degrees of freedom to describe nature with. We know they exist, of course, because no one has a better alternative to QCD. We know their interactions, of course, because we can write down the standard model lagrangian.

    But at the end of the day, their masses are pretty unimportant. We know that the up is lighter than the down, and that makes the proton stable against decay into a neutron. Other than that, I don't think figuring out quark masses will tell you anything you couldn't have discovered by some other means.
     
  23. Walter L. Wagner Cosmic Truth Seeker Valued Senior Member

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    Actually, no, I'm not surprised. What I was surprised at is that the 'mass' for the up and down quarks was given to two decimal places in the chart; and that Ben asserted they were both about 5 MeV, when we really have no idea what their separate masses are, rather what their bound masses are in the form of protons or neutrons, with the mass-defect being the binding energy.
     

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