Re: Is electromagnetic field theory unified?

From: Bjoern Feuerbacher (feuerbac_at_thphys.uni-heidelberg.de)
Date: 03/04/05 

Date: Fri, 04 Mar 2005 11:45:43 +0100

Creighton Hogg wrote:

[snip]

> Okay, but pulling apart a nucleus doesn't directly have anything to do
> with gluon exchange. The force *between* nucleons is given by the
> exchange of colorless combinations of quarks and gluons, for example
> pions. This was how the pion was first predicted I believe, before strong
> interactions were understood as well as they are today. That being said,
> there is still alot of work being done today to connect QCD with observed
> nuclear physics. It's a non-trivial task. However, my point here is that
> the interactions between *nucleons* are not going to exhibit
> hadronization, fragmentation, and confinement because all objects involved
> are colorless combinations of other particles. So let's go a step lower
> in scale.
> There's something called deep inelastic scattering that has been studied
> at HERA, an electron/positron proton collider where Zeus is an experiment.
> Now, what happens is that you have an incoming electron exchanging a
> virtual photon with one of the quarks in the proton. This photon is
> exchanging a very large momentum between the two, so it is very short
> wavelength. In essence, it doesn't see the proton as a solid object, but
> as a bag of valence quarks, gluons, and quark anti-quark pairs. Now, when
> this photon is absorbed by one of the quarks, it kicks it out hard and
> fast from the proton. Now, this is where confinement and all that stuff
> like the string model come in. The quark that was kicked out is still
> interacting via gluon exchange with the rest of the proton, and you can
> model this by a string of *constant* tension connecting the quark with one
> of the other particles in the proton. As this string gets longer, the
> total energy contained in the string grows, since the energy contained is
> essentially the tension times the length of the string. Eventually, if
> the quark was kicked out hard enough, it will become energetically
> favorable for the string to "snap" and create a particle anti-particle
> pair. So what you get now is two shorter strings with a quark on each end
> of them. This is the basic picture of fragmentation. Let this process
> keep repeating until eventually the strings are short enough and the
> momenta similar enough that all the particles connected by strings are
> going to recombine in a process called hadronization. The end result of
> this process is something called a hadron jet. Jets are very important to
> measurements in modern particle physics.
> Now, what was the point of this entire digression? Well, my point is that
> the idea of "elasticity" may work on the level of a whole nucleus, I'm
> not sure really, but on the level of the individual protons and neutrons
> it is going to fail in high energy interactions. Also, gluon behavior is
> not elastic. Elastic implies a force growing over distance, which isn't
> right. What you're seeing in the production of jets is the effect of
> gluon screening, essentially it's because gluons themselves carry color
> charge and interact with themselves.
>
> (Franz and Bjoern, if you have anything to add or correct please do so
> with my blessing.)

Everything you said was quite correct (on a pop-science level). Nicely
done!

Bye,
Bjoern

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