Moving Quarks Help Solve Proton Spin Puzzle

Sep 11, 2008

(PhysOrg.com) -- New theory work at the U.S. Department of Energy’s 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. The result, published in the Sept. 5 issue of Physical Review Letters, agrees with recent experiments and supercomputer calculations.

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 proton's spin. Researchers thus began investigating other sources of the proton’s spin.

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

“Rather than the way the quarks are spinning, it's the way they're moving in orbital motion. In fact, more than half of the spin of the proton is orbital motion of the quarks. That's a really fascinating thing,” Thomas said.

In this paper, Thomas explored the model’s 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 proton’s spin, whereas experiment and lattice QCD point to down quarks.

Thomas said 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 model’s predictions are made at low resolution. However, experiment and supercomputer calculations are made at high resolution.

“In the past, there's 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.”

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.

Article: link.aps.org/abstract/PRL/v101/e102003

Provided by Jefferson Lab

Explore further: And so they beat on, flagella against the cantilever

add to favorites email to friend print save as pdf

Related Stories

White House backs use of body cameras by police

1 hour ago

Requiring police officers to wear body cameras is one potential solution for bridging deep mistrust between law enforcement and the public, the White House said, weighing in on a national debate sparked by the shooting of ...

Study urges 15-year plan for low-carbon growth

1 hour ago

The world can save both financial and environmental costs by shifting toward a low-carbon economy over the next 15 years, a high-level panel said Tuesday ahead of a UN summit.

Recommended for you

And so they beat on, flagella against the cantilever

22 hours ago

A team of researchers at Boston University and Stanford University School of Medicine has developed a new model to study the motion patterns of bacteria in real time and to determine how these motions relate ...

Tandem microwave destroys hazmat, disinfects

Sep 16, 2014

Dangerous materials can be destroyed, bacteria spores can be disinfected, and information can be collected that reveals the country of origin of radiological isotopes - all of this due to a commercial microwave ...

Cornell theorists continue the search for supersymmetry

Sep 16, 2014

(Phys.org) —It was a breakthrough with profound implications for the world as we know it: the Higgs boson, the elementary particle that gives all other particles their mass, discovered at the Large Hadron ...

How did evolution optimize circadian clocks?

Sep 12, 2014

(Phys.org) —From cyanobacteria to humans, many terrestrial species have acquired circadian rhythms that adapt to sunlight in order to increase survival rates. Studies have shown that the circadian clocks ...

User comments : 3

Adjust slider to filter visible comments by rank

Display comments: newest first

dihymo
3.7 / 5 (3) Sep 11, 2008
This isn't surprising actually. Once you let go of the droolgawk-inducing notion that something spins twice before it returns, it's obvious that fractional spin is a shared (perhaps relative) feature.

The other explanations involving spherical rotations or revolving a coffee cup in your hand around twice or even some mechanical components all require at least two pivot points or rotational axes.

But all we had to do was stop saying that we don't know why you have to turn by 720 degrees. There's an extreme to which you can deny quantum mechanics, but if you want to dig deeper you have to avoid taking it for granted to the other extreme as well.

deatopmg
1 / 5 (1) Sep 14, 2008
and are quarks simple made up of electrons and positrons, since they can't be fundimental because they don't wave?
Graeme
not rated yet Sep 14, 2008
I thought that proton spin was quantized in Quantum mechanics, and that they had minimal spin, in which case how can there be less spin in the quarks? The spin would be the same or negative. ie 1 1-1=1