News · W&M News · 2003 archive · Armstrong delivers 2003 distingu

Strange things are happening inside us. More specifically, strange things are happening inside the protons and neutrons responsible for 99.9 percent of our weight. Just how strange are we? David Armstrong, associate professor of physics, estimated average individuals could have about 35 pounds of strangeness in them.

“That may or may not be a surprise,” Armstrong said jokingly during the fifth annual Distinguished Faculty Lecture he delivered recently.

But it is particles, called strange quarks, which constitute the kind of strangeness Armstrong suggests we have inside. In his lecture, he discussed them, explaining in plain English what they are, what they do, and what physicists know and are trying to find out about them.

David Armstrong talks about our 35 pounds of inner strangeness.

 

Quarks, the tiny particles inside protons and neutrons, come in six varieties, named “up,” “down,” “strange,” “charm,” “top” and “bottom.” Up and down quarks make up the average proton or neutron at its simplest level. The proton is made up of two up quarks and one down quark, while the neutron contains two down quarks and one up quark. Of course, the insides of protons and neutrons can be much more complex, Armstrong said, thanks in part to other “quarky” characteristics.

“They’re very social creatures,” he said. “We have never seen, and theory suggests we never will see, a single quark in isolation. They always come in groups.” Groups can include the simple three-quark setup, or a quark/antiquark pair. Antiquarks are the antiparticle to the quark. These particle/antiparticle pairs can “annihilate” to form pure energy, Armstrong explained.

But the opposite is also true—pure energy can convert into a particle and antiparticle. This is where quarks get strange.

Four forces work inside and between particles (strong, weak, electromagnetic and gravity), and quarks feel them all. Since each force is transmitted by a “force carrier,” quarks are always interacting with things like gluons. The theory that explains the interaction between quarks and gluons is called Quantum Chromo Dynamics (QCD). When quarks are close together, QCD is a “feeble” force, but when quarks are relatively far apart, the force becomes incredibly strong.

“The fact that the force gets bigger when the quarks are separated explains why quarks are so social,” Armstrong said. “As I stretch two quarks away from each other, I build up energy in that force field. I build up so much energy, that it’s energetically favorable to create a quark/antiquark pair from that energy, and suddenly, instead of two, I’ve got two pairs of quarks.”

Thanks to gluons transmitting this strong force and the resulting energy inside the proton, there is, occasionally, enough mass produced to create a quark/antiquark pair. This process is ongoing as quark/antiquark pairs are produced and then annihilated to create more gluons.

“You should think of this as a rolling boiling cauldron of activity happening all at once,” Armstrong said.

Strange quarks and strange anti-quarks show up in the mix quite often. But what impact does this constant creation-annihilation cycle have on the proton?

Armstrong, with a team of physicists including William and Mary professors Todd Averett and J. Michael Finn, set out to examine the impact of strange quarks on the shape of the proton. Using the electron beam at Jefferson Lab in Newport News, the team investigated the strange quark effect using a process called electron scattering. The high powered beam of electrons was fired at a target full of protons. When the electrons slammed into the target, their charges interacted with the protons and the quarks inside those protons. The pattern of how many electrons bounced off the protons indicated how the charge is distributed inside the proton.

But electron scattering can’t explain the impact of the strange quarks, since they have the same electric charge as down quarks. However, quarks are sensitive to the weak force, and electrons are not. That interaction depends on a different set of properties than the electromagnetic reaction used in electron scattering. That property is called the weak charge, which is a different value for the down quark and strange quark. Using the weak charge, Armstrong and his team found that the strange quark effect on the shape of the proton is relatively small—about 15 parts per million. What’s so strange about that?

“It could be that these strange quarks, even though we know that a large fraction are being produced inside the proton, are living in such a way that the strange quarks and strange anitquarks are appearing spatially with the same distribution of the proton.”

Armstrong said. Another explanation would be that the strange quark effect is large at one spot, and large at another inside the proton, but those two effects counteract each other. That’s what follow-up experiments hope to determine, Armstrong said.

Physicists plan to fire electrons at different angles to see if the effects are different. “When these experiments produce results, we should have a definitive picture of just how strange the proton is. And all I can say, at this stage, is stay tuned,” Armstrong said.

by Tim Jones