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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

    Speed and mass

    We know it from measuring the rest mass of a free electron - "free" means "unbound". In contrast to the photon, the electron does not move with a "natural velocity". Arbitrary speeds are possible - you should also remember that speed is relative, i.e. it also depends on the state of the...
  4. H

    Speed and mass

    The 511 KeV correspond to the rest mass of the electron. When you accelerate it, it gains additional kinetic energy (1/2)*m*v^2 You may accelerate a particle which is lighter than the electron to a higher speed such that the above term for the kinetic energy will yield the same number...
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    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.
  6. H

    Speed and mass

    KeV is "kilo electron volt": it is the amount of energy which an electron gains through acceleration by a voltage of 1000 V; see e.g. http://en.wikipedia.org/wiki/Electronvolt
  7. H

    Speed and mass

    In principle, magnetic fields are helpful to measure the mass of a charged particle. A magnetic field exerts the Lorentz force on a moving charge; the resulting acceleration may be measured and is inverse to the mass of the electron. In practice, Penning traps are often used...
  8. H

    Speed and mass

    Unfortunately, in the history of physics, the notion of a "relativistic mass" had been introduced. Nowadays, this is rather regarded as a kind of energy which increases inertia. "Relativistic mass" is not a scalar number as we may expect but a tensor: "relativistic mass" would vary with the...
  9. 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...
  10. 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...
  11. H

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

    Getting curious: what exactly do you think is wrong in my post?
  12. 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...
  13. 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...
  14. H

    One dimensional Coulomb potential

    You have to solve the exact equation. Finally, applying the boundary condition X(0) = 0 (because of the unfinite potential step) should deliver the discrete spectrum of solutions.
  15. H

    Faster than the speed of light

    Agreed ... in order to illustrate your comment, I refer to Gell-Mann's book: "The Quark and the Jaguar" (http://www.amazon.com/dp/0805072535/?tag=pfamazon01-20)
  16. H

    Electrostatic force, voltage, electrostatic potential

    Hmm, I can't follow. Above, the energy U is not a function of distance x. The force on a probe charge in electrostatics is usually F = q*E = -q*dU/dx with E(x) for the electric field, q for the charge of the test object and U(x) for the ELECTRIC POTENTIAL (same units as VOLTAGE, not energy). I...
  17. H

    Photon and anti-photon annihilation?

    The experiment is about a scattering process with REAL photons (originating from laser sources). Of course, these photons have positive energies (frequencies).
  18. H

    Photon and anti-photon annihilation?

    I think that this process has in fact been observed - even for real photons: Positron production in multiphoton light-by-light scattering
  19. 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..
  20. 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...
  21. 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...
  22. H

    Relativistic quantum mechanics and causality

    Are you sure ? Afaik, the propagator of a massless particle has a pole on the lightcone, but summing up the contributions due to internal lines in a diagram (by integration over 4-momenta), there are also contributions from propagators connecting events with timelike or even spacelike distance...
  23. 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.
  24. 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...
  25. H

    Leptonic lifetimes

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

    Virtual particles

    The fundamental difference between real and virtual particles is that virtual particles do not have to be on their mass shell, i.e. they do not fulfill the fundamental energy-momentum-mass relation of Special Relativity...
  27. 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...
  28. H

    Faster-than-light newbie question

    According to the principle of relativity, you can of course do this. The rest system of a ship travelling with a constant speed is an inertial coordinate system which can by no experiment be distinguished from any other inertial system. So, you can do your walk there just the same way as in...
  29. 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...
  30. 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...
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