Strange Quarks in the Proton

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 yields no more than 2% of the proton charge radius and less than 4% of its magnetic moment. This result firmly pins down strange quark contributions (at one specific length scale, Q2=0.1 GeV2) to the proton's charge distribution and magnetic form factor, constraining the world experimental data to less than half of its previous range. The HAPPEx II result was a highlight of the Dallas APS Meeting in April 2006.

It is even more significant that the experimental measurement is in excellent agreement with a recent theoretical determination by an international team of physicists from the University of Adelaide, DOE's Thomas Jefferson National Accelerator Facility and the University of Edinburgh. The team applied a unique combination of cutting-edge numerical and analytic approaches to QCD to precisely calculate the strange quark contribution to the charge radius to be less than 1% of the total. (Their earlier result for the magnetic moment was even more precise, at a mere 0.6% of the proton moment.) One of the consequences of the calculation is that the strange quark can never stray too far from its anti-particle. The paper is published in Physical Review Letters.


HAPPEx II news release
Publication in Physical Review Letters