Moving Quarks Help Solve Proton Spin Puzzle

September 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: Latest results from the LHC experiments are presented in Vienna

Related Stories

Exploring the Higgs boson's dark side

July 21, 2015

In 2012 CERN's Large Hadron Collider (LHC) discovered the Higgs boson, the 'missing piece' in the jigsaw of particles predicted by the Standard Model.

Quarks' spins dictate their location in the proton

April 2, 2013

A successful measurement of the distribution of quarks that make up protons conducted at DOE's Jefferson Lab has found that a quark's spin can predict its general location inside the proton. Quarks with spin pointed in the ...

Recommended for you

New blow for 'supersymmetry' physics theory

July 27, 2015

In a new blow for the futuristic "supersymmetry" theory of the universe's basic anatomy, experts reported fresh evidence Monday of subatomic activity consistent with the mainstream Standard Model of particle physics.

Rogue wave theory to save ships

July 29, 2015

Physicists have found an explanation for rogue waves in the ocean and hope their theory will lead to devices to warn ships and save lives.

Researchers build bacteria's photosynthetic engine

July 29, 2015

Nearly all life on Earth depends on photosynthesis, the conversion of light energy into chemical energy. Oxygen-producing plants and cyanobacteria perfected this process 2.7 billion years ago. But the first photosynthetic ...

3 comments

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

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.