Gauge Theories in Particle Physics: Questions Answered

[mxbob468]

i'm reading 'gauge theories in particle physics' by aitchison (http://www.ft.uam.es/personal/hernandez/QFT/Aitchison--Gauge-Theories-in-Particle-Physics-Volume-1.pdf)

and i have some questions:

1. middle of page 23 right before section 1.2.3 heading: he implies because of the spectroscopy of the nucleus, specifically the doublet triplet quartet structure, we could infer that there were two nucleons, 1 uncharged and 1 charged. why?

2. bottom first paragraph 24: the context of figure 1.9 is that it's a scatter plot of energies of electrons scattered off protons but he says that the peak at 1.23GeV is the same as the delta particle. is the delta particle an excited state of the proton? neutron? i guess i could wiki that but what i really want to know is what it means that it comes in "4 charge combinations". charge combinations of what?

3. page 28 figure 1.12: why does the slowly sloping, and scattering angle invariant, part between 3GeV and 8GeV imply that nucleons have "hard constituents". i don't understand his explanation.

4. page 42 right under eqn 2.3: the wording just might be awkward but what does "mutual potential of one ... due to the presence of another..." mean. it seems that it's just like a test charge under the influence of a field produced by a source charge?

5. same page right above eqn 2.6: looking for a solution of the form of a traveling wave is tantamount to treating the problem quantum mechanically? is that then quantization? from undergrad qm i thought quantization was discretization of energy levels due to dirichlet boundary conditions.

any help i could get with any of these would go a long way for me because until next year i don't have access to anything even remotely resembling a university (living in a village in the middle of africa {uganda}).

thanks

 

[Bill_K]

1) The fact that pairs of mirror nuclei exist having all their properties the same except their charge suggests that you can get one from the other by substituting a proton for a neutron or vice versa.

2) The delta can be regarded as an excited state of a nucleon. It comes in four possible charge states, -1, 0, +1 and +2. These are charge combinations of quarks. Up quark = charge +2/3, down quark = charge -1/3. So the possibilities are ddd = charge -1, ddu..., duu..., uuu = charge +2.

3) The surprising fact about the scattering intensity is that it does not go to zero for large momentum transfer, which is the same as large scattering angle. Just as Geiger and Marsden were surprised by large-angle alpha scattering and led to conclude that the atom contained a "hard" central nucleus, one concludes that nucleons contain hard particles within them.

5) This is not a proof of quantization, rather showing what would follow assuming that the n-p potential was due to the exchange of a previously unknown particle.