The proton is not strange. This is the conclusion drawn from the initial results of an experiment at the Jefferson Laboratory, Newport News, Virginia, which cast new light on the deep interior of nuclear particles.
Classically, the proton is understood to contain three “valence” quarks two “up” and one “down”. However, in 1987 the European Muon Collaboration experiment at CERN revealed that only about 30% of the intrinsic angular momentum (spin) of the nucleon is carried by the quarks. Ever since, physicists have been searching for the missing spin-carrying component.
All subnuclear particles contain a skeleton of valence quarks along with a fluid “sea” of transient quarks and antiquarks, and gluons, which transmit information between quarks.
In the case of the proton, the composition of this sea would not necessarily be limited to light up and down quarks but could also contain some of the heavier and more exotic quarks, particularly the “strange” quark, which makes up particles that are not normally found in our world.
One possibility is that this fluid sea contributes to the missing spin of the parent particle. To investigate this, the Hall A Proton Parity Experiment (HAPPEX) was commissioned at the Jefferson Laboratory CEBEF electron accelerator. The idea was to scatter a beam of high-energy polarized (spin-oriented) electrons and look at the spin distribution of the scattered electrons.
When electrons “bounce” off protons in this way, the bounce can be mediated either by an electromagnetic photon, or by the neutral Z carrier of the weak force. The delicate interference of these two effects depends on the way in which the electrons interact with the quarks in the proton. This produces a characteristic leftright asymmetry in the scattered beam. In particular, any strange quarks should contribute to this asymmetry in a different way from the more prominent up and down quarks.
HAPPEX used a 100 µA continuous beam of 3.356 GeV polarized electrons, which were derived from a laser-driven semiconductor photocathode. To measure the asymmetries expected to be at the level of parts per million any correlation between the beam’s spin alignment and intensity was avoided by using a sophisticated feedback system.
The asymmetry measured by HAPPEX is 14.5 ± 2.2 ppm, which does not indicate a significant contribution from strange quarks. However, to reach a definitive conclusion, more data are needed to isolate the charge and magnetic contributions to the asymmetry.
HAPPEX completed a new run last summer that will decrease the present experimental uncertainty by a factor of two. The experimenters say that the success of this experiment bodes well for the future use of these techniques as an incisive probe of subtle effects in nucleon and nuclear structure. The high intensity, together with the high polarization of the electron beam (up to 70%), is a major achievement.