Atom Smasher Sheds New Light On Proton Spin Mystery
¬† Quarks also have a spin of one half. Physicists originally assumed that two of the proton‚Äôs three quarks were always spinning in opposite directions, canceling one another out, leaving the remaining one half as the proton‚Äôs total spin. ‚ÄúThat was the na√Įve idea 25 years ago,‚ÄĚ says Daniel de Florian of the University of Buenos Aires, leader of one of the new papers, which was published July 2 in Physical Review Letters. ‚ÄúBy the end of the ‚Äė80s it was possible to measure the contribution of the spin of the quarks to the spin of the proton, and the first measurement showed it was 0 percent. That was a very big surprise.‚ÄĚ Later measurements actually suggested quarks can contribute up to 25 percent of the proton‚Äôs total spin, but that still leaves the lion‚Äôs share unaccounted for. ¬†
Gluons are also present inside protons as the representatives of the strong nuclear force, a fundamental interaction that binds the quarks together. Gluons each have a spin of 1, and depending on which direction it is they could add up to make most of rest of the proton‚Äôs spin. Measuring their contribution is a tricky task. RHIC is the only experiment that can address the question, because it is the only particle accelerator built to collide ‚Äúspin-polarized‚ÄĚ protons, meaning that the particles are all spinning in a certain direction when they crash. (At the more powerful Large Hadron Collider in Switzerland, the particles‚Äô spins are not aligned.) ¬† When two protons slam together, their interaction is controlled by the strong force, so gluons are intimately involved.
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Quarks are the subatomic particles that make up larger particles such as protons, while gluons are the 'glue' that holds them together. The term spin here is somewhat of a misnomer here, however. It does not describe the process of actual spinning, like a ball rotating, but it refers to what is basically 'quantum spinning', also called 'nuclear...
( via huffingtonpost.com )
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