Protons spinning ways are a result of moving quarks

September 13th, 2008 - 2:50 pm ICT by ANI  

Washington, September 13 (ANI): New theory work at the U.S. Department of Energys Thomas Jefferson National Accelerator Facility has shown that more than half of the spin of the proton is the result of the movement of its building blocks: quarks.

It was thought that the spin of the proton would come from its quarks, but experiments beginning with the European Muon Collaboration in the 1980s have established that the quarks spin accounts for only one third of the protons spin.

Researchers thus began investigating other sources of the protons spin.

This research concerns one theoretical model, proposed by Jefferson Lab Chief Scientist Tony Thomas and University of South Carolina Professor Fred Myhrer, which suggests that some of the protons spin is actually generated as orbital angular momentum by its quarks.

Rather than the way the quarks are spinning, its the way theyre moving in orbital motion. In fact, more than half of the spin of the proton is orbital motion of the quarks. Thats a really fascinating thing, Thomas said.

In the new reseacrh, Thomas explored the models predictions further by extracting more detailed information, including how the orbital angular momentum is generated by the different quarks inside the proton, which has two up quarks and one down quark.

He found that the model seemed to contradict experimental results and the results from highly sophisticated supercomputer calculations of quark behavior, called lattice QCD.

The model showed that up quarks carried most of the protons spin, whereas experiment and lattice QCD point to down quarks.

According to Thomas, it turns out that the disagreement is only a matter of resolution.

The only way to relate such models to the underlying theory of quark interactions is to assume the models predictions are made at low resolution.

However, experiment and supercomputer calculations are made at high resolution.

In the past, theres been tremendous success starting with the quark model at some very low scale, and then evolving to a higher scale, where you can compare with experiment, Thomas explained.

If you make that generally accepted assumption, then the resulting high-resolution values are in surprisingly good agreement with state-of-the-art lattice QCD calculations, as well as with recent experiments conducted at Hermes and Jefferson Lab. There is a remarkable degree of consistency, he added.

The next step is to compare the model with results from upcoming, more detailed measurements of the orbital angular momentum of the quarks in the proton. (ANI)

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