30 June 2006

Strange Quarks' Role In Proton Revealed

by Kate Melville

Shedding light on a somewhat controversial subject, physicists from the G-Zero collaboration have found that strange quarks do indeed contribute to the structure of the proton. Their findings indicate that, as previous experiments had hinted, strange quarks in the proton's quark-gluon sea contribute to a proton's properties. The G-Zero collaboration is a multi-year experimental program designed to measure, through the weak force, the strange quark contribution to proton structure.

According to physics dogma, protons, found in the nucleus of the atom, are primarily built of particles called quarks, along with particles called gluons that bind the quarks together. There are three permanent quarks in the proton that come in two "flavors": two "up" and one "down." Up and down quarks are the lightest of the possible six flavors of quarks that appear to exist in the universe. In addition to the proton's three resident quarks, the peculiar rules of quantum mechanics allow other particles to spontaneously appear from time to time. These ghostly particles usually vanish in a fraction of a second, but it was believed possible that their brief existence might influence the structure of the proton.

So the G-Zero physicists set out to catch some of these ghostly particles in the act; targeting the "strange" quark, believing it would be the most likely to have a visible effect.

Doug Beck, physicist and spokesperson for the G-Zero multi-nation collaboration, explained that one way to see these strange quarks is to measure them through the weak interaction. "If we look with photons via the electromagnetic interaction, we see quarks inside the proton. And then, if we do it with the weak interaction, we see a very similar, yet distinctly different view of the quarks. And it's by comparing those pictures that we can get at the strange quark contribution," he said.

Since the hydrogen nucleus consists of a single proton, G-Zero researchers sent a polarized beam of electrons into a hydrogen target. They then watched to see how many protons were "scattered," essentially knocked out of the target, by the electrons. Throughout the experiment, the researchers alternated the electron beam's polarization (spin). "We run the beam with polarization in one direction, and we look to see how many protons are scattered. Then we turn the beam around, in polarization at least, and measure for exactly the same amount of time again and look to see how many protons are scattered. And there will be a different number by about 10 parts per million," Beck explained. That's because the electromagnetic force is mirror-symmetric (the electrons' spin will not affect the number of protons scattered), while the weak force is not (electrons polarized one way will interact slightly differently than electrons spinning oppositely).

"The relative difference in those counting rates tells us how big the weak interaction piece is in this scattering of electrons from protons. We compare it to the strength of the electromagnetic interaction between electrons and protons, and that gives us the answer that we're looking for," Beck said.

The researchers found that strange quarks do indeed contribute to the structure of the proton. In particular, Beck says the researchers found that strange quarks contribute to the proton's electric and magnetic fields - in other words, its charge distribution and magnetization. "All quarks carry charge, and one of the things we measure is where the strange quarks are located in the proton's overall charge distribution," Beck said, "And then there's a related effect. There are these charged quarks inside the protons, and they're moving around. And when charged objects move around, they can create a magnetic field. In G-Zero, we also measure how strange quarks contribute to the proton's magnetization."

The researchers managed to extract a quantity representing the strange quark's contribution to a combination of the proton's charge and magnetization. "The data indicate that the strange quark contributions are non-zero over the entire range of our measurements," Beck said, "And there are a couple of points that overlap other measurements. They agree, so that's a good thing." But by itself, the G-Zero result does not yet allow the researchers to separate the strange quark's contribution to the charge from its contribution to the magnetization. "There's another G-Zero run coming up in December, and that will help us to try to disentangle this combination of the contribution to the charge and the magnetization. So that will give us one more measurement that will allow us to look at those quantities separately," Beck concluded.

Source: Thomas Jefferson National Accelerator Facility