Surprise To Physicists -- Protons Aren't Always Shaped Like A Basketball!

Science Daily ^ | 2003-04-08 | Editorial Source

[Cooter]

JLab, formerly known as CEBAF, is an electron accelerator. I suspect the protons were in a fixed target.

[CyberCowboy777]

mark to read later

[MrLeRoy]

I have never known anyone before who has actually seen an electron.

I have never known anyone who has actually seen the wind.

[RightWhale]

I have never known anyone who has actually seen the wind.

That's more like it. Like my philosophy prof likes it. Like Socrates liked it. I've never seen these three famous dimensions of space, or four if you count time. Never seen time either. Is time hourglass-shaped? Are protons antisymmetric spin sets?

[<1/1,000,000th%]

When we were kids my dad used to rush us home to see Star Trek when it was originally broadcast on network TV (NBC?). My brothers and I all learned the "raised eyebrow". ;)

[NEWwoman]

Oh, yes. It dates me. I remember as a kid seeing Star Trek's first run on our Black and White TV. (Color sets were a luxury.) During college, we gathered in the dorm TV room to watch the reruns on cable and it was in color.

[Right Wing Professor]

Isn't the proton's spin simply the sum of the spins of it's quarks?

If they behave like atoms, there's also an orbital contribution.

[edsheppa]

I didn't understand your post so I searched around the web a little and found this.

When quark-gluon field theory (QCD) corrections are taken into account, the analysis of all published data indicates that quark spins account for only about 30% of the nucleon spin. The origin of the remaining fraction of the spin is not yet understood. Taken at face value, the recent SMC and E143 data indicate a significant contribution to the nucleon spin from the strange quarks and antiquarks with a net polarization opposite to that of the nucleon. This disagrees with the predictions of naive quark models, such as those proposed 20 years ago by J. Ellis and R. Jaffe as a benchmark for the subsequent data.

One possible interpretation is a large contribution from the AdlerBellJackiw "axial anomaly" by which a quantum effect destroys a classically conserved current. This effect is expected to be small unless there is a large gluon polarization. The small quark spin fraction and the polarization of the strange quark sea can alternatively be explained naturally in a class of approximate models of QCD in which the spin-1/2 nucleon corresponds to a sort of "knot" in the field of pions (topological soliton). Thus, to understand the spin structure of the proton, one has to take into account an unexpectedly large effect of either the gluon spins or the orbital angular momentum of quarks and gluons.

That said (and I only understand it roughly), what is mysterious to me is how these disparate bits add to the quark spins to equal exactly +-1/2. If the orbital motion of the quarks increases there must be a ledger balancing change in spin somewhere else.

[SunkenCiv]

Note: this topic is from April 8, 2003. Thanks vannrox (wherever you are).

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