By Ashley Yeager

This cartoon shows the axis rotation and the intrisic spin, from mass or charge, of the protons and neutrons in a He-3 atom. Credit: Yi Qiang, Jefferson Lab.

It’s good that things spin.

If the Earth didn’t, we wouldn’t have night and day. If wheels didn’t, we wouldn’t have quick ways to get around. And, if the protons and neutrons in atoms didn’t, well, it’s not clear what would happen, but we definitely wouldn’t have medical tests like MRIs.

Even though spinning particles play a large role in our everyday lives, physicists aren’t sure where the rotation comes from. They certainly have some ideas, but actually observing the origin of protons’ and neutrons’ spin has been challenging.

Now, experiments led by physicist Haiyan Gao and her colleagues are providing a more detailed picture of what makes sub-atomic particles turn. The team is using high-energy electrons and a rare form of helium at the Jefferson Lab to look at how quarks, the particles within protons and neutrons, spin.

The experiment seems simple – slam the electrons into helium-3, or He-3, atoms. But, there’s a bit more complexity. The scientists have to carefully manipulate the rotation of all of the particles involved. The neutrons of He-3, for example, must turn perpendicular to the direction of the incoming electrons. And, the electrons have to turn either in the same direction as they are moving or in the exact opposite direction.

The physicists manipulate the direction the particles spin using polarized lasers.

Getting these spins correct is critical to figuring out if the quarks within protons and neutrons spin, and if so, in what direction. In the experiment, the incoming high-energy electrons slam into the cores of the helium-3 atoms and penetrate deep into their neutrons. The electrons strike the quarks inside, knocking them loose.

The physicists found that the struck quarks turn on imaginary axes just like Earth does. And, they turn in the same direction as their parent particle. This is the first successful experiment to measure the alignment between the spin of a quark and the direction its neutron is moving, a discovery that gives scientists hints about where the rotations of neutrons and protons originate, Gao says.

This cartoon shows the rotation of an incoming electron compared to the spin of a helium-3 neutron and the ejected down quark. Credit: Jin Huang, LANL.

Past experiments have shown that the quarks’ intrinsic rotation (from mass or charge) makes up about a third of a proton’s or neutron’s overall spin. The results of the new experiment, which appear in the Feb. 3 Physical Review Letters, suggest that the remaining two-thirds of a parent particle’s rotation comes from orbital motion of the particle’s quarks.

The conclusion, however, is far from certain, Gao cautions. This new experiment is like a snapshot of how the quarks turn. Physicists do not yet have the full picture of the orbital motions of these particles, she says.

To get a more complete picture, she and her team are working on a more sensitive detector and other upgrades to the Jefferson Lab electron beam that they will use to study even more carefully how quarks spin.

And that too, is a good thing, though we may not yet understand – in terms of everyday applications – why.

Citation: “Beam-Target Double-Spin Asymmetry ALT in Charged Pion Production from Deep Inelastic Scattering on a Transversely Polarized 3He Target at 1.4<Q2<2.7 GeV2.” J. Huang et al. (2012). Physical Review Letters. 108: 5.
DOI: 10.1103/PhysRevLett.108.052001