Quarks’ spins dictate their location in the proton

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 up direction will congregate in the left half of the proton, while down-spinning quarks hang out on the right. The research also confirms that scientists are on track to the first-ever three-dimensional inside view of the proton.

The proton lies at the heart of every atom that builds our visible universe, yet scientists are still struggling to obtain a detailed picture of how it is composed of its primary building blocks: quarks and gluons. Too small to see with ordinary microscopes, protons and their quarks and gluons are instead illuminated by particle accelerators. At Jefferson Lab, the CEBAF accelerator directs a stream of electrons into protons, and huge detectors then collect information about how the particles interact.

According to Harut Avakian, a Jefferson Lab staff scientist, these observations have so far revealed important basic information on the proton’s structure, such as the number of quarks and their momentum distribution. This information comes from scattering experiments that detect only whether a quark was hit but do not measure the particles produced from interacting quarks.

“If you sum the momenta of those quarks, it can be compared to the momentum of the proton. What scientists were doing these last 40 years, they were investigating the momentum distribution of quarks along the direction in which the electron looks at it – a one-dimensional picture of the proton,” he explains.

Now, he and his colleagues have used a new experimental method that can potentially produce a full three-dimensional view of the proton.

The new method measures neutral pions, made of one quark and one antiquark, as they are produced in collisions of fast-moving electrons with protons.

In addition to the momentum distribution, this method allows one to infer the spatial position of the quark as it was hit – how far the quarks are away from the proton’s center and if their spins are pointing in the up or down direction. It projects a spatial image of the proton’s quark content in the plane transverse to the electron beam.

“It is the transverse space distribution. And so the one-dimensional picture is extended to a three-dimensional image that allows us to understand how those little quarks are distributed in the space. That is, we learn at the same time how far they are from the center and what are their momenta,” Avakian says.

To make the measurement, the researchers needed to thwhack a number of quarks with electrons just hard enough for the quarks to absorb energy from the electrons and then give it away again, without ever breaking up the protons.

“This is the method of exclusive electron scattering, where you don’t destroy the proton, you just touch a single quark,” explains JLab Theorist Christian Weiss. “The electron hits the quark, and this quark shakes off a pion. The quark returns to the proton, and the proton remains intact and recoils. You measure the pion and the recoiling proton in addition to the scattered electron. This method gives you much more control than traditional inclusive scattering, where you do not detect the produced particles.”

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