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Pair-Production A QUESTION

  1. Feb 18, 2006 #1
    1) If an electron/positron pair can be produced by a gamma ray with kT=or>1MeV striking a nucleus, can heavier particle/anti-particle pairs be created by photons with even higher energies?

    2) Can a photon with kT>800MeV create a Up/Anit-Up quark pair for example?

    Thanks, I am dying to know.
     
  2. jcsd
  3. Feb 18, 2006 #2

    selfAdjoint

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    Yes, as long as conservation laws are satisfied.

    I believe so.


    Don't kick the bucket just yet!:surprised
     
  4. Feb 18, 2006 #3

    Astronuc

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    The threshold for pair production is 2x rest mass of electron - 1.022 MeV.

    In the low MeV range, gamma photons would likely eject protons or neutrons from a nucleus - photoneutron effect. Creation of proton-antiproton would require a photon energy of at least 2x rest mass or about 1876.544 MeV for a proton-anitproton pair. For lower photon energies, I imagine that nucleons or alpha particles would be ejected, rather than quark-antiquark pair production.

    However, it maybe more likely that lepton pairs (electron-positron, or muon-antimuon are likely). Nature tends to favor lower energy processes to higher energy processes.

    See -

    Photoproduction of large-mass lepton pairs at HERA as a probe of the small x structure of the proton
    http://www.citebase.org/cgi-bin/citations?id=oai:arXiv.org:hep-ph/9307340

    Prompt photon, Drell-Yan and Bethe-Heitler processes in hard photoproduction
    http://ppewww.ph.gla.ac.uk/preprints/1996/06/glapre.html8

    http://ppewww.ph.gla.ac.uk/preprints/1996/06/glapre/glapre.html
     
  5. Feb 27, 2006 #4
    Thanks Astronuc.

    So let me ask you this....

    Knowing that the 'Top Quark' has a rest mass of 174 GeV and its corresponding anti-particle the 'Anti-Top Quark' has the same rest mass of 174 GeV. Can we assume that a photon with energy in excess of 348 GeV could produce a 'Top Quark/ Anti-Top Quark' pair?
     
  6. Feb 28, 2006 #5

    Astronuc

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    Here are some tables of mesons, hadrons and quarks.

    I am not sure what a top quark and anti-top quark would form.

    I am also not sure how one would form a 348 GeV photon.

    It is not clear to me that photons could produce quarks, rather they more likely form leptons, e.g. electron-positron pairs.
     
  7. Mar 1, 2006 #6
    Thanks Astronuc, I appreciate you egoless honesty.

    As for the 348GeV photon, the 'Friedmann Equation' can be formatted to predict the time at which the average kT (photon) equalled 348GeV, the result of which is 2.3x10*-11 seconds.

    So in essence, the BigBang itself formed the extremely energetic 348GeV photon needed to (theoretically) produce a Top/ Anti-Top Quark pair.

    I seem to be at a bottleneck as I've not been able to determine from inquiring around if in fact, particles other than electon/ positrons can be pair produced from photons. I'd appreciate it if you could ask some colleagues to try and find out, since you are in the field and seem like a nice, intelligent fellow.

    Thanks again.
     
  8. Mar 1, 2006 #7

    arivero

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    Here the answer has a little trick. They can be produced (taking care of preserving linear and angular momentum, of course) but they are so heavy (and strong force so weak) that they will not bind themselves into a meson, as the rest of quarks do.
     
  9. Mar 1, 2006 #8

    selfAdjoint

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    I wondered about that, since the force already at the charm quark mass is as weak as electromagnetism (cf states of the J/psi meson, alias charmonium).
     
  10. Mar 2, 2006 #9
    arivero

    1) So you are saying that this type of pair production (with the obvious conservation of momentum laws) can occur for all quarks right?



    2) But, you are also saying that the Top/ anti-Top quark doesn't form a meson because the color force isn't strong enough to bind them right?



    3) Finally, tell me if this list of particles that can be pair-produced is complete:

    QUARKS: up, down, charm, strange, top bottom

    Leptons: electon & e neutrino; muon & muon neutrino; tau & tau neutrino

    ARE THERE ANY OTHER PARTICLES THAT CAN BE PAIR-PRODUCED?




    Thanks, great posts arivero.
     
  11. Mar 3, 2006 #10

    arivero

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    Yes, see 3 below.

    Exactly, both because the weakness of color and because of the high mass of the top, that causes it to disintegrate faster than the time to do an orbit, if you want to think semiclassically.


    Note that the top has 175 GeV, you need a > 300 GeV photon.

    Also note we are speaking of production from gamma, it could have sense to speak of producing pairs from W+ or from gluons, simply preserve charge in the produced particles.

    I am in doubt now if there is a W+W- gamma vertex (I think so). Then also W could be produced as a pair. same question about a Z0 Z0 gamma vertex (in principle are orthogonal objects but quantum life is strange), I should check.
     
  12. Mar 3, 2006 #11
    Note that the top has 175 GeV, you need a > 300 GeV photon

    How come it requires only 300 GeV as the lower limit if 175 x 2 = 350GeV? Simple typo, or an exotic concept that I am missing?




    it could have sense to speak of producing pairs from W+ or from gluons, simply preserve charge in the produced particles.

    You are saying that photons aren't the only ones that can pair-produce? You are saying that W+ vector bosons and gluons can also pair-produce?
    If so, basically you are saying 3 of the 4 known messenger particles can pair produce, so with this in mind do you think the theoretical graviton could also pair-produce? Hmmm this thread is getting real interesting, real fast!

    Awesome posts as usual arivero, thanks.
     
  13. Mar 4, 2006 #12

    arivero

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    Consider it a typo. I meant to say > 3E2, which is the sci way of saying "greater than 250-350". Of course 350 is a minumum for pair creation.

    Well the graviton is not inside the standard theory of elementary particles. Actually I do not know how one particle (spin 1/2) can emit one single graviton (spin 2) and preserve angular momentum. Virtual graviton, it must be, forcefully.

    As for the rest of the force messengers, yes, and indeer pair creation via gluons is usual mechanism in strong decay. Perhaps the foton one is the purest because it does not carry charge; W+ carries electric charge and thus creates differenty charged particles (electron and antineutrino). Gluons carry colour so they create differently coloured particles (say a red up quark and an antigreen up antiquark.). And then they can not create leptons.
     
  14. Mar 6, 2006 #13

    So, two Zo Intermediate Vector bosons can annihilate eachother and pair-produce?

    If so, what would they produce given that their combined Rest energies are so high 92Gev + 92GeV = 184 GeV!! The only particle I can think of that comes close to that huge energy is the Top Quark at 174 GeV.


    Also, is a gluon massive? I know it mediates the Strong nuclear force between quarks, but does it have mass, or is it like the photon?


    Thanks again arivero.
     
  15. Mar 7, 2006 #14

    arivero

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    Yeah, but we need to produce two. So nope. Z0 colliding to two tops is not kinematically allowed. Besides to be a second order process, each Z0 g giving one weak fermi coupling factor to the product.
    No, the gluon is massless. Its short range is due to colour confinement. The pion is massive, and manages the strong force between protons and neutrons.

    Thanks again arivero.[/QUOTE]
     
  16. Mar 10, 2006 #15
    arivero

    Can you give me an example of what particle(s) two Zo bosons could pair produce?

    Also, what could two Gluons pair produce?


    Thanks
     
  17. Mar 10, 2006 #16

    nrqed

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    You don't even need a *pair*. As single Zo could produced any lepton-antilepton pair or quark-antiquark pair. muon-antimuon, a charm-anti-charm, and on and on...even aney neutrino-antineutrino (whether it's electronic or muonic or tauonic). And since the Zo is massive, you don't even need aninteraction with a nucleus or something else to conserve four-momentum lik eyou need for the photon.

    As for a gluon, it can produce any quark-antiquark pair. To lowest order, it cannot produce a lepton-anti lepton pait because these have no color. However, if you go to higher order processes (with loops if you think in terms of Feynman diagrams) then anything is possible, as long as you don't if other stuff is produced. So you could get out of a gluon an electron-positron pair plus other stuff...or almost anything you want!


    Pat
     
  18. Mar 13, 2006 #17
    Pat


    Very interesting Pat, thank you.
    So in a nutshell you are saying that the vector bosons can spontaneously create any particle, period, without the need for interaction to do so. My question is how can a Zo boson (M=~92GeV) create a TOP-quark (M=~174GeV)? Is this possible due to the Zo's momentum energy combined with RestMass energy being high enough to produce the TOP? Or perhaps due to the fact that quark RestMass values are only guidelines and not absolute?

    Any experiments, references, websites supporting your post would be appreciated. Thanks again Pat.
     
  19. Mar 13, 2006 #18

    selfAdjoint

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    It's not that the Z0 is a vector boson that enables it to do this, but the fact that it is its own antiparticle, something that is also true of the photon. As to your other question, remember that it's not just the mass of the particle, but its total energy that goes into pair production, if it is moving fast enough its kinetic energy can be as large as you like.
     
  20. Mar 14, 2006 #19

    nrqed

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    Well, let's be careful. I mean without interaction with any other stuff (like a nearby nucleus). But the decay is due to an interaction (if you think in terms of Feynman diagrams) between the Z_0 and the particles it creates (example, the positron-electron, etc). So there is no need for an interaction with "extra" stuff (in the sense of something extra to the interaction between the Z0 and the particles created)...but there is of course an interaction between the Z0 and the particles created, otherwise the decay would not occur (to lowest order).


    Also, you have to be careful about the distinction between lower order processes and higher order processes (in the second case, the pair-anti pair is not directly created but is a byproduct of other processes).

    In the lowest order, it is not true that any vector boson can create anything. The Z0 is special in that respect. For example, a photon can't create (in lowest order) a neutrino-antineutrino pair (because they have no electric charge). Likewise, a gluon could not create an electron-positron pair (because they have no color charge). And a W+ could not create any particle-antiparticle pair because the charge would not be conserved. So the Z0 is a bit special because it has no electric charge and it couples to anything with a weak charge which includes all the leptons and quarks.

    You would need a table of the Standard Model and the vertices present in the Standard Model. I know many books with that but no good website (they usually don't provide the Feynman rules).

    Pat

    Your first answer is the correct one. This is just E=mc^2 at work....energy can be converted into mass and vice versa. If the z0 has enough kinetic energy, this, added to its rest mass energy, can be enough to create anything...
     
    Last edited: Mar 14, 2006
  21. Mar 15, 2006 #20
    Pat

    Wow Pat, you are most intelligent poster ever on PF.

    You didn't beat around the bush and you explained it so wonderfully that I could actually understand it. Thanks a million.

    Drawing on the fact that the Zo boson can create any lepton or quark because it is its own anti-particle, is it plausible to infer that it was the first particle ever created? By your description, it seems to me that the Zo can produce any two daughter particles known to man, is this a fair statement or not?


    P.S. You should be a moderator Pat, PF could use some more like you.
    Thanks again.
     
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