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  1. H

    Hadron physics

    Thanks for the link - interesting stuff. But I do not feel competent enough to comment on this.
  2. H

    What happens when an electron and positron collide

    Of course, there is also the possibility of elastic scattering: Elastic scattering of electrons and positrons
  3. H

    What is Beta+ Decay

    You are right: a free proton cannot beta decay. This can only happen in a nucleus where the binding energy of the child nucleus may compensate the loss of energy through the decay.
  4. H

    Hadron physics

    I find it difficult to get a clue what you are asking about. Vertex functions appear in renormalizable field theories in higher order pertubation theory as corrections to vertices in Feynman diagrams. However, there is no such field theory dealing mesons as fundamental particles - the standard...
  5. H

    Gluons, where do they come from and where do they go?

    For instance effective field theories like chiral pertubation theory could be a candidate: http://en.wikipedia.org/wiki/Chiral_perturbation_theory These are promising attempts to describe the low energy region of strong interactions, where QCD perturbation theory is not applicable. BTW, this...
  6. H

    Gluons, where do they come from and where do they go?

    Getting curious: what exactly do you think is wrong in my post?
  7. H

    Gluons, where do they come from and where do they go?

    Particles which carry the quantum number "colour" interact with each other by exchanging gluons ("strong force"). Proton and neutron however are neutral with respect to the colour quantum number. As already mentioned in other replys, the quarks do have colour - they are glued to each other by...
  8. H

    Why is the strength of weak nuclear force important ?

    Well, the weak force is responsible for the beta decays. Thus, a larger coupling constant would result in reduced lifetimes of the decaying particles - e.g. the neutron. The beta decay also plays a role in the fusion process H + H -> He. In the 1st step, a deuterium nucleus will be formed p + p...
  9. H

    How can photons arise out of pure energy?

    Of course, you are right when saying that it is no scientific concept. However, I guess that people using the term "pure energy" will usually emphasize that no matter is involved, i.e. photons do not carry any matter quantum numbers like "lepton number", "baryon number", "electric charge" etc..
  10. H

    CP Violation & Antimatter

    The relevance of CP for flipping particles ino their antiparticles is dictated by the neutrinos: http://en.wikipedia.org/wiki/Antineutrino#Antineutrinos Helicity is the projection of spin on the direction of momentum. Thus, in order to create an antineutrino out of its neutrino, you woul...
  11. H

    Will SUSY soon be ruled out ?

    Hi, accounting for the new LHC results, e.g. Search for squarks and gluinos ... Search for supersymmetry... Search for Supersymmetry in pp Collisions... it seems that SUSY is becoming problems. I guess that there are reasons why the masses of the supersymmetric particles cannot be...
  12. H

    Is antimatter a theory or does it exist?

    Yes, you have to invert all matter quantum numbers to change from a particle to its anti-particle. In addition to the already mentioned electric charge, there are things like lepton number and baryon number.
  13. H

    There exist only quarks and leptons being elementary particles?

    It depends on what and how you count. The 6 quarks come in 3 colors; so one may be inclined to count in fact 18 quarks. In addition to these spin 1/2 particles which you have mentioned there are also spin 1 fields, which are denoted as "particles" too - the so-called gauge bosons: 8 gluons, 2...
  14. H

    Leptonic lifetimes

    You are completely right - thanks for your remark.
  15. H

    Leptonic lifetimes

    That's a nontrivial task. In principle, you have to calculate the probability amplitude for each channel separately up to the desired order of accuracy. You have to add them up, square the sum and make the usual integration over the phase space of the various final states. You will get something...
  16. H

    GIM mechanism and Z0 suppression?

    Agreed: the sum of these box graphs (and still higher orders) should be regarded as an effective fcnc vertex. BTW, I was wondering about your last statement referring to the "modern" interpretation of the GIM-paper. In fact, I had learned this stuff 30 years ago and I am not aware of any change...
  17. H

    Leptonic lifetimes

    Usually, "lifetime" uniquely denotes a measurable quantity: you have a large number of samples and measure the time until a certain fraction of the original size remains. Theoretically, you calculate this quantity by taking into account all possible decay channels because each of them...
  18. H

    GIM mechanism and Z0 suppression?

    Yes, that's in fact, what GIM is referring to: "flavor changing neutral currents". Its implication is that the quarks have to come in doubletts: for each quark of the down type, there must be a corresponding partner quark of the up type and vice versa.
  19. H

    GIM mechanism and Z0 suppression?

    I am referring to the statement: This is what I meant with "destructive interference". Another important thing is that consideration of all diagrams is required to obtain a finite result. During calculation, you have to regularize each diagram dimensionally or by a cut-off. When summing...
  20. H

    GIM mechanism and Z0 suppression?

    In the standard model, flavor changing neutral currents are mediated in higher orders perurbation theory by the so-called box diagrams exchanging virtual W bosons and quarks. Glashow, Iliopoulos and Maiani had realized that the contribution from the s-quark exchange interferes destructively with...
  21. H

    Leptonic lifetimes

    In the electroweak theory, the electron is a stable particle. No decay exists. Thus, its lifetime is infinite and nothing else.
  22. H

    Feynman diagram for weak interaction.

    A correction: this is only exactly true for massless particles. Strictly speaking, it's the chirality (and not the helicity) of a fermion which determines its coupling to the weak interactions. With increasing mass of a particle, this makes an increasing difference. This is the reason why you...
  23. H

    Feynman diagram for weak interaction.

    We are talking about helicity, i.e. the spin projection in the direction of the particle's momentum. Particles having spin parallel to momentum are called right-handed and in opposite direction left-handed. Under a "purely right handed fermion" I understand a particle having a sharp spin value...
  24. H

    Feynman diagram for weak interaction.

    I would put it this way: the strength of the weak interaction (the probability of the process to happen) goes with the left handed projection of the lepton or quark wave function. So, a purely right handed fermion has zero probability to decay via weak interaction. However, of course the...
  25. H

    Feynman rules and decay process

    This is certainly wrong: the tau is a lepton and leptons do not interact via strong force. IMHO, this decay is possible via electroweak interactions: c and c-bar annihilate into a virtual Z-boson or photon, which in turn decays into the pair of leptons. This is kinematically forbidden...
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