FOR RELEASE: Sunday, April 19, 1998, 11:00 AM EDT
Edward R. Kinney, (Tel. 49-40-8998-4578, Fax 49-40-8998-4034,
University of Colorado & Deutsches Elektronen-Synchrotron
Boulder, Colorado, USA
Ed Kinney's Home Page
Can be reached at Hyatt Regency Columbus, 614-463-1234, April 18-20.
Popular Version of Paper
Sunday, April 19, 1998, 11:00 AM EDT
1998 APS April Meeting, Columbus
For several decades it has become commonly accepted by physicists that the neutron and proton, the particles which make up 99.95% of the matter of which we and our surroundings are made, are themselves made up of smaller objects known as quarks held together by a force field called the "glue." The existence of these quarks has been difficult to establish because they have never been observed singly, but always in clusters of at least two or three. This is now understood to be a result of the glue, rather than the quarks, as this glue is so strong as to be able to create more quarks, rather than allow a quark to remain isolated. So far six different types or "flavors" of quarks have been discovered, but in fact the lightest two flavors, "up" and "down," with a smattering of the heavier "strange" flavor, comprise ordinary matter.
The proton and neutron are thought each to be made up of three quarks most of the time: two ups and a down for the proton and just the inverse for the neutron, two downs and an up. These quarks are bound together by the glue, which once in a while happens to produce a pair of new quarks, one matter and one antimatter, much like a bubble in a pot of boiling soup. This new quark-antiquark pair exists only for a short time however before it "dissolves" back into glue.
If this were not complicated enough, we know that in fact the quarks act like small "spinning" tops and the glue is "swirling." In fact, both the proton and neutron themselves act like they are spinning, so we know that somehow these spinning, swirling constituents of the proton (and don't forget the quark-antiquark pairs appearing and disappearing) are put together in just such a way to give the proton spin, which is constant no less. But to test the understanding of the laws governing these fundamental forces in nature, one needs to understand the way all of these pieces go together.
How do we know these things? Mostly by using the scattering of electrons or neutrinos from the quarks. Electrons and neutrinos are also common parts of the world around us, and they have the virtue of not being affected by the glue field, so they are much simpler to understand. Electron scattering may seem rather esoteric, but in fact we all use this technique to see the world around us. Our eyes detect the scattering of light from objects, and this light in many cases was produced by electrons. However our eyes, even with very powerful microscopes, are generally adapted to see down to distance on the order one micrometer. If we wish to see smaller structures we turn to x-rays (light our eyes cannot detect) and electron microscopes, and instead of only our brain, we use computers to reconstruct the images. Seeing the quarks means seeing objects roughly a billion times smaller that what the light microscope can see, which in practice means building electron microscopes which are miles long. The principle we use to "see" is nonetheless the same.
One of a group of new experiments aimed at trying to understand the structure of the proton's spin is HERMES at the HERA electron accelerator at the German Electron Synchrotron Laboratory (DESY, as in "daisy"). The HERMES experiment uses polarization techniques which allow us to look at just how the proton's quarks and glue are making up the total spin. In the case of the proton, we find that the scattering looks quite different if we point the proton spin in opposite directions, whereas the neutron hardly looks different at all. The HERMES observations confirm earlier measurements at experiments at the European Center for Nuclear Research (CERN) in Geneva and at the Stanford Linear Accelerator (SLAC), but use a completely different type of accelerator and target, thus providing an important cross check of the earlier data.
In addition, HERMES can go further by using information from the debris of the scattering reaction. We have carried out an analysis similar to one at CERN where one looks at particles coming out from the electron scattering in order to locate those which contain the quark which scattered the electron. In this type of enormous electron microscope, the proton is blown to bits by the process of looking for the quark, and we can try to find which bit contains the quark that was hit. Using this additional information, we can find the contributions to the proton spin from the up and down quarks separately as well as the contribution from the quark-antiquark pairs, though we cannot yet break the latter down into different quark flavors. With just our first preliminary data, we can match the precision of the earlier measurements at CERN; significantly more data has already been acquired which will allow a better determination in the next six months.
These types of studies are unraveling the quark-glue structure of the proton in new ways which complement experiments which do not look at the spin part of the structure. The contribution coming from the swirling glue remains almost completely unknown at this point, and a number of future experiments are planned to study this directly. The HERMES experiment will study the quark-antiquark pairs in more detail soon and in a few years be able to produce the world's most precise measurements of the contributions of the different flavors to the quark part of the proton and neutron spin. In addition, techniques are being developed at HERMES which may give us completely new information about the way the quarks and glue make up the proton's and neutron's spin.