HAPPEx II reveals proton isn't very strange
The most precise measurement yet of strange quarks in the proton, performed at the Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab) by the second phase of the Hall A Proton Parity Experiment (HAPPEx II), shows that the strange quark contribution to the proton's overall charge distribution and magnetic moment is small, indeed: it's so small, at less than 1 percent of the proton's charge distribution and no more than 4 percent of its magnetic moment, that it may be zero. This result firmly pins down strange quark contributions at one specific length scale to the proton's charge distribution and magnetic moment, constraining the world data to less than half of its previous range.
The result and a discussion of its significance will be presented by Paul Souder, professor of physics at Syracuse University, at the APS April Meeting 2006 Plenary Session I9 on Sunday, April 23. Also present will be Krishna Kumar, associate professor of physics at the University of Massachusetts, Amherst, and a co-spokesperson for HAPPEx.
It may seem strange that strange quarks are thought to be integral in determining the properties of the proton. Unlike up and down quarks, strange quarks typically aren't permanent residents of the proton. However, the strange quark may appear as part of the "quark-gluon sea" - a seething mass of particles that constantly blink into and out of existence in the proton due to strong force energy.
"And so, it's been an open question over the last 20 years whether or not these sea quarks actually affect many of the nucleon's properties, such as the mass, the spin and the charge and magnetic moment distribution," Kumar says.
HAPPEx II measured the electromagnetic and the neutral weak forces between a longitudinally polarized beam of electrons and unpolarized target nuclei at a length scale of around one femtometer (roughly the size of a proton). The electromagnetic force is mirror-symmetric (the electrons' spin will not affect the number of electrons scattered), while the weak force is not (electrons polarized one way will interact differently than electrons spinning oppositely). Therefore, the researchers were able to separate the contributions from each of these forces to the measurement by alternating the electron beam's polarization (spin) throughout the experiment and measuring the fractional difference in the number of scattered electrons due to the beam's changing polarization. "This technique is potentially one of the cleanest ways to extract a property purely of the sea quarks," Kumar says.
According to Souder, the result is in line with previous research of the strange quark's influence on the structure of the proton. "We put stringent limits on the fact that the contribution of the sea quarks is small to the magnetic moment of the proton," Souder explains.
HAPPEx II is a parity-violating electron scattering experiment that took data in 2004 and 2005. Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF) provided a polarized electron beam with an energy of 3 GeV (billion electron-volts) and an average of 86 percent polarization. The beam was sent into a 20 centimeter-long cryogenic aluminum target vessel containing either hydrogen (a single proton in the nucleus) or helium-4 (two protons and two neutrons) in Jefferson Lab's Hall A. Electrons that elastically scattered from the nuclei in the target at a forward angle of 6 degrees were measured in Hall A's High Resolution Spectrometers, yielding high-precision data for Q2=.1 (GeV/c)2. After the experiment, a blinding factor was added to the data, and eight months of data analysis followed. The blinding factor was removed just last week, after careful evaluation of the data quality.
The HAPPEx II result agrees with several recent experiments, including SLAC's (the Stanford Linear Accelerator Center) E158, the SAMPLE experiment at MIT-Bates, the A4 experiment at the Mainz Laboratory in Germany, and the G-Zero experiment at Jefferson Lab.
Contact: Kandice Carter
In the news:
Scientists Try to Put Right Spin on Quarks To Understand Matter
(May 19, 2006, Wallstreet Journal)
Particles not particularly useful
(May 13, 2006, Daily Press)
Up and Down, but not Strange
(May 5, 2006, ScienceNOW)
In Protons, Virtual Strange Quarks Less Prevalent than Thought
(May 2, 2006, The AIP Bulletin of Physics News)
Thomas Jefferson National Accelerator Facility’s (Jefferson Lab’s) basic mission is to provide forefront scientific facilities, opportunities and leadership essential for discovering the fundamental structure of nuclear matter; to partner in industry to apply its advanced technology; and to serve the nation and its communities through education and public outreach. Jefferson Lab, located at 12000 Jefferson Avenue, is a Department of Energy Office of Science research facility managed by the Southeastern Universities Research Association.